14. ASSIGNMENT TOPICS WITH MATERIALS

UNIT-I 1. Need for the development of unconventional machining methods

Unconventional manufacturing processes is defined as a group of processes that remove

excess material by various techniques involving mechanical, thermal, electrical or chemical

energy or combinations of these energies but do not use a sharp cutting tools as it needs to be

used for traditional manufacturing processes.

Extremely hard and brittle materials are difficult to machine by traditional machining

processes such as turning, drilling, shaping and milling. Nontraditional machining processes,

also called advanced manufacturing processes, are employed where traditional machining

processes are not feasible, satisfactory or economical due to special reasons as outlined

below.

Very hard fragile materials difficult to clamp for traditional machining

When the work piece is too flexible or slender

When the shape of the part is too complex

Material removal processes once again can be divided into two groups

(a) Conventional Machining Processes

(b) Non-Traditional Manufacturing Processes or Unconventional Machining processes

Conventional Machining Processes mostly remove material in the form of chips by applying

forces on the work material with a wedge shaped cutting tool that is harder than the work

material under machining condition.

The major characteristics of conventional machining are:

Generally macroscopic chip formation by shear deformation

Material removal takes place due to application of cutting forces energy domain can

be classified as mechanical

Cutting tool is harder than work piece at room temperature as well as under machining

conditions

Need for unconventional machining processes

Extremely hard and brittle materials or difficult to machine material are difficult to

machine by traditional machining processes.

When the work piece is too flexible or slender to support the cutting or grinding forces

when the shape of the part is too complex.

2. Difference between conventional and unconventional machining processes

Conventional Manufacturing

Processes

Unconventional Manufacturing Processes

1. Generally macroscopic chip

formation by shear

deformation.

2. There may be a physical tool

present. for example a cutting

tool in a Lathe Machine.

3. Cutting tool is harder than

work piece at room

temperature as well as under

machining conditions.

4. Material removal takes place

due to application of cutting

forces energy domain can

be classified as mechanical

5. Conventional machining

involves the direct contact of

tool and work piece

6. Lower accuracy and surface

finish.

7. Suitable for every type of

material economically.

8. Tool life is less due to high

surface contact and wear.

9. Higher waste of material due

to high wear.

10. Noisy operation mostly

causes sound pollutions.

1. Material removal may occur with chip formation

or even no chip formation may take place. For

example in AJM, chips are of microscopic size

and in case of Electrochemical machining

material removal occurs due to electrochemical

dissolution at atomic level.

2. There may not be a physical tool present. For

example in laser jet machining, machining is

carried out by laser beam. However in

Electrochemical Machining there is a physical

tool that is very much required for machining.

3. There may not be a physical tool present. For

example in laser jet machining, machining is

carried out by laser beam. However in

Electrochemical Machining there is a physical

tool that is very much required for machining.

4. Mostly NTM processes do not necessarily use

mechanical energy to provide material removal.

They use different energy domains to provide

machining. For example, in USM, AJM, WJM

mechanical energy is used to machine material,

whereas in ECM electrochemical dissolution

constitutes material removal.

5. Whereas unconventional machining does not

require the direct contact of tool and work piece.

6. Higher accuracy and surface finish.

7. Not Suitable for every type of material

11. Lower capital cost.

12. Easy set-up of equipment.

13. Skilled or un-skilled operator

may require.

14. Generally they are manual to

operate.

15. They cannot be used to

produce prototype parts very

efficiently and economically.

economically

8. Tool life is more

9. Lower waste of material due to low or no wear.

10. Quieter operation mostly no sound pollutions are

produced.

11. Higher capital cost

12. Complex set-up equipment.

13. Skilled operator required.

14. Generally they are fully automated process.

15. Can be used to produce prototype parts very

efficiently and economically.

3. Working principle of Ultrasonic Machining (USM) The term ultrasonic refers to the frequency range above the audible range and is above 16

KHZ (i.e; 16KC/S). Ultrasonic machining (USM) is a mechanical metal removal process (for

brittle materials, application for ductile materials is limited) in which material is removed by

high frequency oscillations of a shaped tool using an abrasive slurry.

The transducer (device for converting any type of energy into ultrasonic waves, eg.

mechanical energy or electrical energy converted into mechanical vibrations) generates the

high frequency vibrations of the order of 20-30 KHZ with amplitude of the order of 0.02 mm.

This vibration is transmitted to the tool made of soft material through a mechanical coupler

(known as tool holder). The tool shape is a close complimentary shape of the final surface to

be generated.

The tool while oscillating would be pressed against the work piece with a load of few

kilograms and fed down wards continuously as the cavity (hollow space) is cut in the work.

The tool is shaped as the approximate mirror image of the configuration of the cavity desired

in the work.

The slurry which is made of abrasive particles (grains) suspended in a liquid, is fed into the

cutting zone under pressure. The slurry is about 30% in concentration from the point of view

of pump design and of achieving adequate penetration.

The material removal rates in USM are relatively small, but in material which is brittle; this is

the only way to produce economically complex cavities without breaking the work piece.

Since there is no direct contact between work piece and the tool, fragile workpieces can be

conveniently used in USM.

4. Elements and characteristics of USM The elements of USM Process are: Work material: Applicable for only brittle material.

Tool material: It should be selected in such a way that

Energy utilization is optimum

It should also have adequate strength to withstand stresses at the nodal plane.

Common examples are titanium, low carbon steel, stainless steel, etc.

Tool Size:

The shape of the tool and its dimensions are governed by the size of the abrasive used.

The length of the tool should be short, since massive tools absorb the vibration energy,

reducing the efficiency of machining. Long tools cause over stressing of the tool and the

brazed point.

Abrasive Slurry: The abrasive selected should be harder than the work material being

machined. Common examples are aluminium oxide, silicon carbide and boron nitride etc.

The most common used liquids are water, benzens etc.

Transducer: The transducer mainly consists of a cylinder which is made up of piezoelectric

ceramic. It converts electrical energy into mechanical vibration. Transducer then vibrates

sonotrode at low amplitude and high frequency.

Sonotrode (Horn): It is made of low carbon steel. One end of it is connected with the

transducer and other end contains tool. The sonotrode vibrates at low amplitude and high

frequency and removes material from the w/p by abrasion where it contacts it.

Control Unit: The control unit consists of an electronic oscillator which produces an

alternating current at high frequency. The frequency produced is usually in between 18 kHz

to 40 kHz in ultrasonic range.

The transducer and sonotrode is attached to control unit with a cable.

The control unit has an electronic oscillator that produces an alternating current with

high ultrasonic frequency ranges in between 18 kHz to 40 kHz.

This high frequency alternating current is supplied to the transducer. The transducer

converts this alternating current into mechanical vibration and transmits this mechanical

vibration to the sonotrode attached to it.

The sonotrode is vibrated by the transducer with low amplitude and high frequency.

When this vibrating sonotrode strikes the surface of the w/p, it removes the material

form it. The slurry flows in between the tool and work-piece and helps in the removal of

the material from the surface.

The slurry used in the ultrasonic machining contains 20 % to 60% of water by volume,

aluminum oxide, boron Carbide and silicon carbide particles.

This is how ultrasonic machining works.

5. Economic considerations and recent developments in USM Economic considerations:

The process has the advantages of machining hard and brittle materials to complex shapes

with good accuracy and reasonable surface finish. Considerable economy results from USM

of hard alloy press tools, dies and wire drawing equipment on account of the high wear

resistance of tools made of these alloys.

The machines have no high speed moving parts. Working on machines is not hazardous,

provided care is taken to shield ultrasonic radiations from falling on the body.

The power consumption of ultrasonic machining is 0.1 W-h/mm3 for glass and about 5 W-

h/mm3 for hard alloys. The cost of the manufacture and use of the tools, particularly if they

have complicated contours, is very high. Another item adding to the cost of ultrasonic

machining is abrasive. The abrasive slurry has to be periodically replaced because during use

the particles are eventually broken and blunted.

Ultrasonic machines are not yet completely reliable; failure sometimes occurs on account of

faults in acoustic head, pump or generator.

It is probable that with more research in the near future on techniques and machines, the

process will have more economic advantages.

Recent development:

Mullard Research Laboratories, USA, have developed a process that combines

electrochemical reaction with ultrasonic abrasion. Using a 60W ultrasonic drill and abrasive

suspended in an alkaline electrolyte, Mullard researchers have reported that tool steel can be

machined nine times faster than by ultrasonics alone.

Engis Limited of England has developed the Disonic Die Ripper in which the diamond-plated

tool oscillates at ultrasonic speed as well as rotates at high speed (5000 rpm) in a liquid to

rapidly remove material from tungsten carbide dies. The use of diamond-plated tool

eliminates the need of abrasive slurry and frequent re-grinding of steel tools. The oscillating-

cum-rotational system is claimed to increase the material removal rate several times.

UNIT-II 1. Principle, working, elements, applications, advantages and limitations of Abrasive

Jet Machining process

Principle:

Abrasive jet machining (AJM) uses a stream of fine grained abrasive mixed with air or some

other carrier gas at high pressure. This stream is directed by means of a suitably designed

nozzle on to the work surface to be machined. Metal removal occurs due to erosion caused by

the abrasive particles impacting the work surface at high speed.

Working:

Dry air or gas is filtered and compressed by passing it through the filter and compressor. A

pressure gauge and a flow regulator are used to control the pressure and regulate the flow rate

of the compressed air. Compressed air is then passed into the mixing chamber. In the mixing

chamber, abrasive powder is fed. A vibrator is used to control the feed of the abrasive

powder. The abrasive powder and the compressed air are thoroughly mixed in the chamber.

The pressure of this mixture is regulated and sent to nozzle. The nozzle increases the velocity

of the mixture at the expense of its pressure. A fine abrasive jet is rendered by the nozzle.

This jet is used to remove unwanted material from the work-piece.

Construction of Abrasive Jet Machining (AJM):

The constructional requirements of Abrasive Jet Machining (AJM) are listed and described

below:

Abrasive jet: It is a mixture of a gas (or air) and abrasive particles. Gas used is carbon

dioxide or nitrogen or compressed air. The selection of abrasive particles depends on

the hardness and Metal Removal Rate (MRR) of the work-piece. Most commonly,

aluminium oxide or silicon carbide particles are used.

Mixing chamber: It is used to mix the gas and abrasive particles.

Filter: It filters the gas before entering the compressor and mixing chamber.

Compressor: It pressurizes the gas.

Hopper: Hopper is used for feeding the abrasive powder.

Pressure gauges and flow regulators: They are used to control the pressure and

regulate the flow rate of abrasive jet.

Vibrator: It is provided below the mixing chamber. It controls the abrasive powder

feed rate in the mixing chamber.

Nozzle: It forces the abrasive jet over the work-piece. Nozzle is made of hard and

resistant material like tungsten carbide.

Operations that can be performed using Abrasive Jet Machining:

The following are some of the operations that can be performed using Abrasive Jet

Machining:

Drilling

Boring

Surface finishing

Cutting

Cleaning

Deburring

Etching

Trimming

Advantages of Abrasive Jet Machining:

Surface of the work-piece is cleaned automatically.

Smooth surface finish can be obtained.

Equipment cost is low.

Hard materials and materials of high strength can be easily machined.

Limitations of Abrasive Jet Machining:

Metal removal rate is low

In certain circumstances, abrasive particles might settle over the work-piece.

Nozzle life is less. Nozzle should be maintained periodically.

Abrasive Jet Machining cannot be used to machine soft materials.

2. Working principle, elements and process variables and applications of Water Jet

Machining

Working principle:

The key element in water jet machining is a water jet, which travels at velocities as high as

900m/s. When the stream strikes a work-piece surface the erosive force of water removes the

material rapidly. The water, in this case, acts like a saw and cuts a narrow groove in the

work-piece material.

The elements of the machining system:

1. Hydraulic pump: The hydraulic pump is powered from a 30 Kilowatt electric motor and

supplies oil at pressure as high as 117bars in order to drive a reciprocating plunger pump

termed as intensifier.

2. Intensifier: The intensifier accepts the water and expels it through the accumulator at

higher pressures of 3800bar. The intensifier converts the energy from the low-pressure

hydraulic fluid into ultrahigh-pressure water.

3. Accumulator: The accumulator maintains the continuous flow of the high pressure water

and eliminates pressure fluctuations. It relies on the compressibility of water (12 percent at

3800bar) in order to maintain a uniform discharge pressure and water jet velocity.

4. High- Pressure tubing: High-pressure tubing transports pressurized water to the cutting

head. Typical tube diameters are 6 to 14mm.

5. Jet Cutting nozzle: The nozzle provides a coherent water jet stream for optimum cutting

of low density, soft material that is considered unmachinable by conventional methods.

Nozzles are normally made from synethic sapphire.

6. Catcher: The catcher acts as a reservoir for collecting the machining debris entrained in

the water jet.

Process variables:

(i) Process characteristics:

Pressure Higher value Can cut thicker materials

Nozzle Diameter

Traverse Rate Decreased Value for Thicker parts

Stand-off-Distance: 3mm-25mm

(ii) Process performance:

Material cut

Porous, Fibrous, Granular, Soft

Corrugated Board (3m/s), Aluminium (0.0025m/s) etc.

No Predrilled Hole is required Any direction and Location but Accessible for the

water jet

Too thick parts Cut in more than one pass Energy consumption/ unit length is less

Machined surface: No Burrs, No thermal Damage & Good surface Finish

: Tolerance, Straightness of cut edges & Finish =

(Work-piece Thickness & Cutting Speed)

To cut insulation of cables 69-200MPa 5-10s / Cable

(iii) Applications:

Cutting of Asbestos (Minimizes Airborne Dust)

Carbide Grit Safety Walks

Fibre Glass & Polyethylene Automotive Parts

High Speed Cutting of Corrugated Box

3. Principle of Electrochemical Machining process and influences of process

parameters in machining output.

Electrochemical machining is one of the latest and potentially the most useful of the non-

tradition of machining processes. The basic principles of the process are not new but

applications of the process as a metal working tool are definitely new. Extensive development

of the process has taken place in recent years mainly due to (i) the need to machine harder

and tougher materials, (ii) the increasing cost of manual labour and (iii) the need to machine

configurations beyond the capability of conventional machining methods.

Principle:

Michael Faraday discovered that if two electrodes are placed in a bath containing a

conductive liquid and D.C. potential is applied across them metal can be deplated from the

anode and plated on the cathode. This is the (process) principle was in use for a long time in a

process called electroplating. With certain modifications, ECM is the reverse of electro-

plating i.e. work-piece is made the anode.

The objective of electrolysis principle is also to deposit metal on the work-piece, but in ECM

the objective is to remove metal the work-piece is connected to the positive and the tool to

the negative terminal. i.e. work-piece is made anode and tool as cathode.

The whole process consists of a work-piece and a suitably-shaped tool, the gap between the

tool and the work being full of a suitable electrolyte. When the current is passed dissolution

of the anode occurs. However, the dissolution rate is more where the gap is less and vice

versa as the current density is inversely proportional to the gap. Now if the tool is given a

downward motion the work surface tends to take the same shape as that of the tool and at a

steady state, the gap is uniform as shown in figure. Thus the shape of the tool is reproduced in

the job.

Process parameters:

i) Cathode tool: The accuracy of the tool shape directly affects the work-piece accuracy,

since the configuration of cavity produced cannot be more accurate than the tool that

produces it. The same is applicable to the surface finish of the tool. The materials that find

wide applications in the manufacturing of tools are aluminium, brass, bronze, copper,

stainless steel etc.

ii) Anode work-piece: The work material must be a good conductor of electricity. The

material removal rate is proportional to the atomic weight and inverse of the valency of work

material. The fixtures for holding the work are made of some insulating material (such as

epoxy resins, glass, fibre resins perpex and pvc). They should have good thermal stability and

low moisture absorption properties.

iii) D.C. power and control system: The process needs low voltages of the order of 2 to 20 v

and in rare cases up to 30v. Normal current requirements are as high as 800amp/cm2 of the

work-piece area to be machined. Three phase 440v A.C. power supply available from mains

is converted to low voltage D.C. by a step-down transformer and a rectifier.

iv) Electrolyte: The electrolyte used in ECM performs the following operations:

a) Completing the electric circuit between the tool and the work-piece.

b) Allowing desirable machining reactions to occur.

c) Carrying away heat generated during the chemical reactions.

d) Carrying away products of reaction from the zone of machining.

4. Tool design and economic aspects in Electro Chemical Machining process

Tool design in ECM:

(i) Tool and fixtures are required to operate for long periods in a corrosive environment of

electrolyte and stray electric currents. In order to avoid the rapid corrosion of each tool,

the selection of proper material is very important. Generally, stainless steel, copper,

brass, bronze, monel, reinforced plastics or copper-tungsten alloy are used. Parts that

have anode potential corrode rapidly and, therefore, the number of parts in electrical

contact with the work piece is limited. Non-metallic materials may be useful as these

are electrically non-conductive and chemically corrosion resistant.

(ii) All electrolyte ducts need to be made of non-corrosive materials, as one of the major

process requirements is that no particles of corrosion should enter the electrolyte flow in

the tool-work gap. To prevent overheating, there is a limit to the minimum cross-section

of the current-carrying parts. For 1000amp, it is about 6cm2 for copper, 25cm2 for

bronze and brass, and 250cm2 for stainless steel. Smaller areas are permissible for metal

surfaces cooled by rapidly flowing electrolyte.

(iii) Fixtures and tools should be rigid enough to avoid vibration or deflection under the high

hydraulic forces which they are subjected to. Proper alignment between the tool and

work fixture is essential and is best achieved with removable setting pieces.

(iv) Electrical joints should be strictly limited as these are sources of power loss and at times

may fail under the wet corrosive conditions in the work enclosure.

Economic aspects of ECM:

(i) Fixed costs of ECM installations are quite high as compared to its operating costs.

Overhead costs are the same as for other conventional machining methods. Some costs

are unique, such as those of high power, electrode tooling and electrolyte.

(ii) ECM needs power of high current capacity. In localities where power is sufficiently

cheap, this factor can be over looked.

(iii) Electrode or tooling cost is a fixed cost because there is little wear of the ECM tool.

There occurs, however, a negligible abrasion wear of electrode due to electrolyte flow

across the gap. With regard to actual tooling cost, it is not very different from

conventional machine tooling.

(iv) Electrolyte is not as costly as one might think to be. The most widely used electrolyte is

sodium chloride (salt) and it is quite cheap. The normal price of the salt seldom exceeds

Re.0.50 per kg when purchased in large quantities.

(v) Cost of work piece fixtures are not very high. The cost per piece will, however, depend

on the number of work pieces finished.

(vi) On the shop floor, ECM installations need not be operated by very skilled engineers and

the operation of the machinery can be learnt easily.

(vii) The economic success of ECM, in fact, depends largely on the choice of applications. If

an operation is simple or if the material can be easily machined by other methods, the

high cost of the ECM plant cannot be justified.

5. Electrochemical Deburring Process

In almost all forming and machining operations very fine burrs of metal are invariably left on

the work piece. These burrs are undesirable, particularly for precision components, as they

may break, loose and disturb a delicately balanced mechanism. They are also dangerous for

the fingers. These burrs have been successfully removed through electrochemical means by a

process called electrochemical deburring.

In this process the tool and the work-piece are placed in a fixed relative position with a gap of

0.1 to 1.0mm (i.e. there is no relative movement of the tool with respect to the work piece).

The tool which is positioned near the base of the burr is designed so that only that portion of

the work piece containing burrs is exposed to tool material. The remaining portion of the tool

is insulated.

The current levels in ECD are of the order of 6A/cm of linear edge length at 7 to 25 v DC

supply. The electrolyte which is generally sodium nitrate is circulated at a pressure of 0.1 to

0.4 N / mm2 to give flow rate of 5 to 20 lit / min for a 100A electrolyzing current.

Advantages:

(i) Both external and internal burrs which may be inaccessible can be removed.

(ii) The equipment is simple to operate and easy to maintain.

(iii) The system is very fast (5 to 40 times faster than hand deburring) and the operating

costs are low.

(iv) Burrs may be removed even after heat treatment.

(v) No stresses or embrittlement caused by ECD.

(vi) No tool wear and tool designs are fairly simple.

(vii) Electrolytic system is small and simple to maintain.

(viii) A simple power supply system can be used.

(ix) ECD can be included in transfer lines.

Applications:

Manufacturing of automobile connecting rods, gear teeth, blanking dies, valve ports and

nozzle holes.

UNIT-III 2. Principle, working and applications of Electric Discharge Machining process

Principle:

Electric discharge machining is a process of metal removal based on the principle of erosion

of metals by an interrupted electric spark discharge between the electrode tool and the

workpiece. Fundamentally, the electric erosion effect is understood by the breakdown of

electrode material accompanying any form of electric discharge. The discharge is usually

through a gas, liquid or any cases through solids. A necessary condition for producing a

discharge is ionization of the dielectric i.e. splitting up of its molecules into ions and

electrons.

Working:

The main components are (i) Power supply (ii) dielectric medium (iii) Work piece and the

tool (iv) Servo control.

The work-piece and the tool are electrically connected to a D.C. electric power supply. The

work-piece is connected to the positive terminal of the electric source, so that it becomes the

anode and the tool is the cathode. A gap known as the spark gap ranges of 0.005 to 0.05 mm

is maintained between the work-piece and the tool and suitable dielectric slurry is forced

through this gap at pressure of 2kgf/cm2 or less. When a suitable voltage range of 50-450v is

applied, the dielectric breaks down and electrons are emitted from cathode and the gap is

ionized. In fact a small ionized fluid column is formed owing to formation of electrons in the

spark gap where the process of ionization collision takes place. When more electrons collect

in the gap the resistance drops causing electric spark to jump between work surface and tool.

Each electric discharge or spark causes a focused stream of electrons to move with a very

high velocity and acceleration from the cathode towards the anode and creates compression

shock waves on both the electrode surface, closest to the tool. The generation of compression

shock waves develops a local rise in temperature. The whole sequence of operation occurs

within a few-microseconds. However the temperature of spot hit by electrons is of order of

10,0000C. This temperature is sufficient to melt a part of the metals. The forces of electric

and magnetic fields caused by the spark produce a tensile force and tear-off molten particles

from this spot in the work piece. A part of the metal may vaporize and fill up the gap. The

metal is thus removed in this way from the work piece. The electric and magnetic fields on

the heated metal cause a compressive force to act on the cathode tool so that metal removal

from the tool is at a slower rate than that from the work-piece.

The current density in the discharge of channel is of the order of 10000A/cm2, power density

of the order of 500 MW/cm2.

Electro-hydraulic servo control is usually preferred. The servo gets its input signal from the

difference between a selected reference voltage and the actual voltage across the gap. The

signal is amplified and the tool as it wears a little is advanced by hydraulic control. A short

circuit across the gap causes the servo to reverse the motion of the tool until the correct gap is

stabilized.

Applications:

(i) Hardened steel dies, stamping tools, wire drawing and extrusion dies, header dies,

forging dies, intricate mould cavities and such parts are made by the EDM process.

(ii) The process is widely used for machining of exotic materials that are used in aerospace

and automotive industries.

(iii) EDM being a non-contact type of machining process, it is very well suited for making

fragile parts that cannot take the stress of machining.

(iv) Ex: washing machine agitators, electronic components, printer parts and difficult to

machine features such as the honeycomb shapes.

(v) Deep cavities, slots and ribs can be easily made by EDM.

(vi) Micro-EDM process can successfully produce micro-pins, micro-nozzles and micro-

cavities.

3. Process of Wire cut EDM and list its advantages, applications, and limitations.

EDM, primarily, exists commercially in the form of die-sinking machines and wire process, a

slowly moving wire travels along a prescribed path and removes material from the work-

piece. Wire EDM uses electro-thermal mechanisms to cut electrically conductive materials.

The material is removed by a series of discrete discharges between the wire electrode and the

work-piece in the presence of dielectric fluid, which creates a path for each discharge as the

fluid becomes ionized in the gap. The area where discharge takes place is heated to extremely

high

temperature, so that the surface is melted and removed. The removed particles are flushed

away by the flowing dielectric fluids.

The wire EDM process can cut intricate components for the electric and aerospace industries.

This non-traditional machining process is widely used to pattern tool steel for die

manufacturing cutting machines (Wire EDM). The concept of wire EDM is shown in Figure.

In this the wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or

brass coated wires are also used extensively in this process. The wire used in this process

ngth and good electrical conductivity. Wire EDM can also

employ to cut cylindrical objects with high precision.

This process is usually used in conjunction with CNC and will only work when a part is to be

cut completely through. The melting temperature of the parts to be machined is an important

parameter for this process rather than strength or hardness. The surface quality and MRR of

the machined surface by wire EDM will depend on different machining parameters such as

applied peak current, and wire materials.

The wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or brass

coated wires are also used extensively in this process. The wire used in this process should

DM can also employ to

cut cylindrical objects with high precision.

Advantages:

(i) Wire EDM has more effective metal-cutting capabilities than laser, flame-cut, plasma,

or die cutting.

(ii) The most intricate parts and delicate shapes, including small or odd angles, sharp

corners, contours, cavities, and external or internal tapers can be cut.

(iii) Wire EDM machines cut to very tight tolerances +/- .0001" (.0025mm).

(iv) Since the entire process is computer- and robotics-controlled, we can create duplicate

parts that are virtually identical

Applications:

(iii) Wire EDM has been employed for making various types of dies. It is possible to control

tolerances effectively.

(iv) The process is also used for fabrication of press tools and electrodes for use in other

areas of EDM.

Limitations:

(iii) Only able to machine conductive materials.

(iv) More expensive process than conventional process

4. Factors to be considered in the selection of tool electrode material and dielectric

fluid used in EDM.

The spark erosion process is basically a copying process and the shape and accuracy of the

machined part will therefore depend primarily on the shape and accuracy of the tool or

cutting electrode. The spark erosion machine should ensure that under identical conditions,

the maximum accuracy in size and shape is obtained.

It has been observed that in the EDM erosion process, both the work piece as well as the

electrode get eroded. Hence, the accuracy of the machined part obtained depends on the

electrode wear.

The main factors determine the suitability of a material for use as an electrode are:

(i) It should be a good conductor of electricity and heat.

(ii) It should be easily machinable to any shape at a reasonable cost.

(iii) It should produce efficient material removal rates from the work pieces

(iv) It should resist the deformation during the erosion process

(v) It should exhibit low electrode wear rates

(vi) It should be available in a variety of shapes

Various electrode materials used are graphite, copper, copper graphite, brass, zinc alloys,

steel, copper tungsten, silver-tungsten, tungsten etc.

For dielectric fluids to be used in the EDM process, it is essential that they should:

(i) Remain electrically non-conductive until the required breakdown voltage is reached,

that is, they should have high dielectric strength.

(ii) Breakdown electrically in the shortest possible time once the breakdown voltage has

been reached.

(iii) Quench the spark rapidly or deionize the spark gap after the discharge has occurred.

(iv) Provide an effective cooling medium.

(v) Be capable of carrying away the swarf particles in suspension, away from the working

gap.

(vi) Have a good degree of fluidity

(vii) Be cheap and easily available.

(viii) Provide insulation between the electrode and the work piece.

The common dielectric fluids that can be used are transformer oil, paraffin oil, kerosene,

lubricating oils or various petroleum distillate fractions. Recently distilled water has also

been used in place of dielectric fluid and this has been found to permit very high metal

removal rates.

5. Characteristic of spark eroded surface and the factors needed for machine tool

selections in EDM.

In EDM, the material removal is due to thermal phenomenon and local temperature in the

region (8000 to 120000C), this temperature will have an effect on the structure and the

mechanical properties of machined surfaces. The effect may or may not be significant,

depending upon the type of work material and the working conditions employed. Sometimes

tiny micro-cracks can be observed, particularly in the machining of tungsten carbide or other

hard materials. The size of micro-cracks depends on the type of material and the electrical

parameters, such as the pulse energy and duration. Generally speaking the crack depth

increases with pulse duration and energy.

Surface finish:

The surface produced by the EDM process consists of a multitude of small craters randomly

distributed all over the machined face. The CLA value of the surface finish in this case ranges

between 2 and 4 . The quality of surface mainly depends upon the energy per spark. If the

energy content is high deeper craters will result leading to a poor surface. The surface

roughness has also been found to be inversely proportional to the frequency of discharge.

Machine tool selection:

A variety of EDM machines ranging from small machines to large units are now

commercially available. The factors that have to be considered in their selection are the

i)number of parts to be machined ii) accuracy required iii) size of the work piece iv) depth of

the cavity v) orientation of the cavity

Equipment must be versatile and accurate for tool room work where a variety of work piece

configuration is encountered, EDM machine tool design and construction is a function of the

accuracy required. In cases where the positioning accuracy need not be held closer than 0.025

or 0.050mm, a conventional coordinate table can be used to obtain the position read-out from

the lead screw via the hand wheel dial. For higher accuracy an optical read out independent

of the lead screw is desirable.

Large sized jobs require machines with high rigidity to avoid excessive deflection. High

rigidity is also essential whilst working with large sized electrodes. The electrode holding

column must be made rigid enough to support the weight of the electrode and also to

withstand the coolant back pressure a peculiarity of this process.

6. Principle, working and process of metal removal in Electric Discharge Grinding

Principle:

Electrical discharge grinding (EDG) is a non-traditional thermal process for machining

difficult to machine hard and brittle electrically conductive materials. EDG has been

developed by replacing the stationary electrode used in electrical discharge machining

(EDM) with rotating electrode. In EDG process, material is removed melting and

vaporization as same as EDM process. But there are ample differences with EDM instead of

mechanism of material. In EDG process, an electrically conductive wheel is used as a tool

electrode instead of stationary tool electrode used in EDM. There is no contact with

workpiece and tool electrode (rotating wheel) except during electric discharge. Due to the

rotational motion of wheel electrode, the peripheral speed of wheel transmitted to the

stationary dielectric into gap between workpiece and wheel resulting flushing efficiency of

process is enhanced. Therefore, the molten material is effectively ejected from gap and no

debris accumulation take place into gap while in EDM debris accumulation is major problem

which adverse effect on performances of process. Due to the enhanced in flushing, higher

material removal and better surface finish is obtained as compare to the conventional EDM

process. At the same machining condition, EDG gives better performances than EDM and it

is machined extremely hard materials faster (2-3 times) as compare to the conventional

grinding. The high speed of wheel is not always beneficial and after a certain value of speed,

the spark becomes instable and produces adverse effect on performance. There is no physical

contact between work-piece and wheel, so that the process becomes more advantageous for

machining thin and fragile electrically conductive materials.

Working and process of metal removal:

The detail of EDG process has been illustrated in figure and wheel-workpiece interaction is

shown in. In this process, a rotating eclectically conductive metallic wheel is used which is

known as grinding wheel. The grinding wheel used in this process, having no any abrasive

particles and rotates its horizontal axis. Due to the similarities of process with conventional

grinding and material is removed due to the electrical discharge, it is known as electrical

discharge grinding (EDG). In this process, the spark is generated between rotating wheel and

work-piece. The rotating wheel and work-piece both are separated by dielectric fluid and

during machining both (work-piece and wheel) are continuously deeped into dielectric fluid.

The dielectric fluids are mainly Kerosene oil, Paraffin oil, Transformer oil or de-ionized

water. The main purpose of dielectric is to make a conductive channel during ionization when

suitable breakdown voltage is applied. The servo control mechanism utilized to maintain the

constant gap between work-piece and wheel in range of 0.013-0.075 mm. A pulse generator

is used for maintaining the DC pulse power supply in ranges of voltage, current and

frequency are 30-400V, 30-100A and 2-500 kHz respectively. When pulse power supply is

applied, the spark takes place into gap due to the ionization and striking of ions and electrons

at their respective electrodes. Due to spark, high temperature generated between ranges of

8000°C to 12000°C or as so high up to 200000C by each spark resulting material is meted

from both the electrodes. Simultaneously DC pulse power supply switch is deactivated

resulting the breakdown of spark occurs and fresh dielectric fluid entering into gap. Due to

the high flushing efficiency, the molten materials flush away in form of micro debris from

gap and formed the crater on work surface.

In EDG without abrasive particle, the wheel is made of graphite which rotates on is

horizontal axis but instead of graphite wheel, some other materials are used for making wheel

for EDG process such as copper, brass and mild steel. Due to the high wear resistance, the

mild steel wheel gives low wheel wear as compare to the copper and brass wheel. The main

developments in EDG without abrasives are: electro-discharge grinding and electro-discharge

milling (ED milling). In EDG process with abrasive particle, the rotating wheel replaced with

metal bonded abrasive wheel or and such types of wheel is known as composite wheel. In

composite wheel, the main purposes of abrasive particles are: to enhanced the material

removal, to achieve better surface finish and requirement of low grinding forces. Electro-

discharge abrasive grinding (EDAG) is the main development of EDG process with abrasive

wheel. It is further developed in three different grinding configurations such as electro-

discharge abrasive cut-off grinding, electro-discharge abrasive face grinding and electro-

discharge abrasive surface grinding.

UNIT-IV 1. Electron Beam Machining with the process parameters

Electron Beam Machining (EBM) is a thermal process. Here a steam of high speed

electrons impinges on the work surface so that the kinetic energy of electrons is

transferred to work producing intense heating. Depending upon the intensity of heating

the work-piece can melt and vaporize. The process of heating by electron beam is used

for annealing, welding or metal removal. During EBM process electrons with very high

velocities can be obtained by using enough voltage of 1, 50,000 V can produce velocity

of 228,478 km/sec and it is focused on 10 200 µm diameter. Power density can go up

to 6500 billion W / sq.mm. Such a power density can vaporize any substance

immediately. Complex contours can be easily machined by maneuvering the electron

beam using magnetic deflection coils. To avoid a collision of the accelerating electrons

with the air molecules, the process has to be conducted in vacuum. So EBM is not

suitable for large work pieces. Process is accomplished with vacuum so no possibility of

contamination. No effects on work piece because about 25-50 µm away from machining

spot remains at room temperature and so no effects of high temperature on work.

Process parameters:

The process parameters, which directly affect the machining characteristics in Electron Beam

Machining, are:

The accelerating voltage

The beam current

Pulse duration

Energy per pulse

Power per pulse

Lens current

Spot size

Power density

In EBM the gun is operated in pulse mode. This is achieved by appropriately biasing the

biased grid located just after the cathode. Switching pulses are given to the bias grid so as to

Beam current is directly related to the number of electrons emitted by the cathode or

-amp to 1 amp.

Increasing the beam current directly increases the energy per pulse. Similarly increase in

pulse duration also enhances energy per pulse. High-energy pulses (in excess of 100 J/pulse)

can machine larger holes on thicker plates.

The energy density and power density is governed by energy per pulse duration and spot size.

Spot size, on the other hand is controlled by the degree of focusing achieved by the

electromagnetic lenses. A higher energy density, i.e., for a lower spot size, the material

removal would be faster though the size of the hole would be smaller.

The plane of focusing would be on the surface of the work-piece or just below the surface of

the work-piece.

2. Process capability of EBM and its influences on machining quality.

mm, i.e., with a l/d ratio of around 10. In a typical hole drilled by electron beam the hole can

be tapered along the depth or barrel shaped. By focusing the beam below the surface a

reverse taper can also be obtained. Typically there would be an edge rounding at the entry

point along with presence of recast layer. Generally burr formation does not occur in EBM.

A wide range of materials such as steel, stainless steel, Ti and Ni super-alloys, aluminium as

well as plastics, ceramics, leathers can be machined successfully using electron beam. As the

mechanism of material removal is thermal in nature as for example in electro-discharge

machining, there would be thermal damages associated with EBM. However, the heat-

affected zone is rather narrow due to shorter pulse duration in EBM. Typically the heat-

Some of the materials like Al and Ti alloys are more readily machined compared to steel.

Number of holes drilled per second depends on the hole diameter, power density and depth of

the hole as well as material type as mentioned earlier. Fig. depicts the variation in drilling

speed against volume of material removed for steel and aluminium alloy.

EBM does not apply any cutting force on the work-pieces. Thus very simple work holding is

required. This enables machining of fragile and brittle materials by EBM. Holes can also be

drilled at a very shallow angle of as less as 20 to 300.

3. Comparison of thermal and non-thermal processes of EBM and the advantages,

limitations and applications.

Thermal type: In this type of EBM process the surface of the thermo electronic cathode is

heated to high temperature that the electrons acquire speed to escape out to the shape around

the cathode. As a result the work piece is heated by the bombardment of these electrons in a

localized area to melt and vaporize at the point of bombardment.

Non thermal type: In this type of EBM process the electron beam is used to cause a

chemical reaction.

Advantages:

(i) This process is not dependent on the work piece material properties.

(ii) This process can be applied to hard and soft materials

(iii) This process suitable for cutting delicate shapes.

(iv) No mechanical distortion

Limitations:

(i) High capital cost of the equipment and necessary regular maintenance applicable for

any equipment using vacuum system.

(ii) Need of auxiliary backing material.

(iii) Low MRR

(iv) Very high specific energy consumption.

(v) Heat affected zone is rather less in EBM but recast layer formation cannot be avoided.

Applications:

(i) Suitable for drilling fine holes.

(ii) Cutting integrated circuit board

(iii) Apply to soft as well as hard materials

4. Working of a Laser Beam Machining

As the name implies in Laser Beam Machining the source of energy is the LASER

(Light Amplification by Simulated Emission of Radiation). The laser beam focuses optical

energy on the surface of the work-piece. A laser beam can be so powerful when used with

lens system that it can melt and vaporize diamond as the energy density can be of the order of

105 kW / cm2. This huge amount of energy is released due to some specific atoms having

higher energy levels and particular frequency.

Different types of lasers are used in Laser beam machining (LBM). Common example are

solid state, gas and semiconductor. At times high power lasers are required for machining and

welding and in those cases only solid state lasers can provide such power levels.

Ruby-laser or crystalline aluminium oxide or saphire is the most commonly used solid state

laser. Generally these lasers are fabricated in into rods having length about 150 mm. Their

ends are well furnished to close optical tolerances. Figure below shows a schematic view of

laser beam machining process.

A small amount of chromium oxide is added to dope the ruby crystal. A flash of high

intensity light, generally Xenon-filled flash lamp is used to pump the laser. To fire the xenon

lamp a large capacitor is required to be discharged through it and 250 to 1000 watts of

electric power is needed to do this. The intense radiation discharged from the lamp excites the

fluorescent impurity atoms (chromium atoms) and these atoms reach a higher energy level.

After passing through a series of energy levels when the atoms fall back to original energy

level, an intense beam of visible light emission is observed. This beam is reflected back from

the coated rod ends and makes more and more atoms excited and stimulated and return to

ground level. A stimulated avalanche of light is obtained which is transmitted through the

coated part (~80% reflective). This light which is highly coherent in time and space has a

very narrow frequency band, is highly in phase and quite parallel. If this light is focused in

association with ordinary lenses on the desired spot of the w/p, high energy density is gained

which helps to melt and vaporize the metal.

5. Mechanism of metal removal, advantages and limitations of laser beam machining

process.

Laser beam machining (LBM) is a non-traditional subtractive manufacturing process, a form

of machining, in which a laser is directed towards the work piece for machining. This process

uses thermal energy to remove material from metallic or nonmetallic surfaces. The laser is

focused onto the surface to be worked and the thermal energy of the laser is transferred to the

surface, heating and melting or vaporizing the material. Laser beam machining is best suited

for brittle materials with low conductivity, but can be used on most materials.

Advantages:

(xiii) No tool wear as there is no direct contact between tool and work-piece.

(xiv) Metal and non-metals (e.g. plastics and rubbers) irrespective of their brittleness and

hardness can be machined.

(xv) Laser beam can go through a long distance as a result LBM can be used to weld, drill or

cut areas which are difficult to reach.

(xvi) Laser beam welding gives the opportunities to weld/cut magnetic as well as heat treated

materials without losing their properties. (some change in the properties is observed in

the heat affected zone).

(xvii) Any environment is suitable for laser beam machining through transparent medium

and magnetic fields.

(xviii) Very little distortion is observed and tow materials can be easily joined together.

(xix) Difficult-to-machine or refractory materials can be drilled.

(xx) Micro sized holes can created in all types of materials.

(xxi) Energy obtained is of high density as a result high heat is obtained.

(xxii) Beam configuration and size of exposed area is easily controllable.

(xxiii) Precise location of the spot is ensured.

(xxiv) By applying unidirectional multiple pulses deep holes of very short diameter can be

drilled.

Limitations:

(viii) The initial cost is very high and lifespan of the flash lamp is short.

(ix) The safety procedures are needed to be followed very strictly.

(x) Material removal rate is not up to the mark.

(xi) While machining some plastics bum or char is noticed.

(xii) Too deep holes are not possible to drill.

(xiii) Machined holes are not round shaped or straight.

(xiv) Overall efficiency is very low (0.3 ~0.5 %).

UNIT-V

1. Principle of operation and process details of Plasma Arc Machining

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt

and displace material in its path called PAM, this is a method of cutting metal with a plasma-

arc, or tungsten inert-gas-arc, torch. The torch produces a high velocity jet of high

temperature ionized gas called plasma that cuts by melting and removing material from the

work piece. Temperatures in the plasma zone range from 20,000° to 50,000° F (11,000° to

28,000° C). It is used as an alternative to oxy-fuel gas cutting, employing an electric arc at

very high temperatures to melt and vaporize the metal.

Principle of operation:

PAM is a thermal cutting process that uses a constricted jet of high-temperature plasma gas to

melt and separate metal. The plasma arc is formed between a negatively charged electrode

inside the torch and a positively charged work piece. Heat from the transferred arc rapidly

melts the metal, and the high-velocity gas jet expels the molten material from the cut.

2. Process details of PAM

Plasma Gun

Gases are used to create plasma like, nitrogen, argon, hydrogen or mixture of these gases.

The plasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given

negative polarity and nozzle of the gun is given positive polarity. Supply of gases is

maintained into the gun. A strong arc is established between the two terminals anode and

cathode. There is a collision between molecules of gas and electrons of the established arc.

As a result of this collision gas molecules get ionized and heat is evolved. This hot and

ionized gas called plasma is directed to the work-piece with high velocity. The established

arc is controlled by the supply rate of gases.

Power Supply and Terminals

Power supply (DC) is used to develop two terminals in the plasma gun. A tungsten electrode

is inserted to the gun and made cathode and nozzle of the gun is made anode. Heavy potential

difference is applied across the electrodes to develop plasma state of gases.

Cooling Mechanism

As we know that hot gases continuously comes out of nozzle so there are chances of its over

heating. A water jacket is used to surround the nozzle to avoid its overheating.

Tooling

There is no direct visible tool used in PAM. Focused spray of ho0t, plasma state gases works

as a cutting tool.

Work-piece

Work-piece of different materials can be processed by PAM process. These materials are

aluminium, magnesium, stainless steels and carbon and alloy steels. All those material which

can be processed by LBM can also be processed by PAM process.

3. Applications of plasma in manufacturing industries

Industrial

Plasma cleaning and Plasma activation in manufacturing of industrial products has become a

reliable tool in the optimization of process specifications.

differentiating your product from your competition with quality leads to customer retention,

cost reduction, and growth. The ability to consistently provide a stronger bond and a cleaner

surface in manufacturing processes provides a more reliable, defect free part to your

customer.

The application of plasma surface treatment can be used in industrial applications when a

requirement for increased bonding strength is needed in the area of painting, adhesive

assembly or bonding. Plasma processing can also be used to clean just about any surface

imaginable.

Plasma Science and Industry

The application of Gas Plasma to the everyday manufacturing of industrial products is

growing at an enormous rate. The need in industry to optimize every aspect of a product has

become necessary to remain competitive. No industry is untouched by the application of Gas

Plasma in the area of optimized product performance. Some of the markets that have seen

the most growth in application of Gas Plasma are aerospace, automotive, electronics, food

packaging, glass, marine, medical, military, optics, packaging, paint, paper, plastics, and

textiles.

own ability to innovate, the need to drive our product market and apply the technology at

hand.

Industrial Manufacturing Cleaning and Adhesion Optimization

The optimization of materials in the area of product performance is critical to being

Unfortunately the best materials for the product are not

always the easiest to assemble, bond or keep clean. Gas Plasma provides solutions to those

difficult manufacturing problems providing clean, activated surfaces that promote and

optimize a reliable bond for product performance. Gas Plasma is a clean, environmentally

friendly alternative to solvents and complicated, heavily regulated clean lines.

Some of the useful applications are:

Single runs autogenously and multi-run circumferential pipe welding.

In tube mill applications.

Welding cryogenic, aerospace and high temperature corrosion resistant alloys.

Nuclear submarine pipe system (non-nuclear sections, sub assemblies).

Welding steel rocket motor cases.

Welding of stainless steel tubes (thickness 2.6 to 6.3 mm).

Welding of carbon steel, stainless steel, nickel, copper, brass, monel, inconel,

aluminium, titanium, etc.

Welding titanium plates up to 8 mm thickness.

Welding nickel and high nickel alloys.

Melting, high melting point metals.

Plasma torch can be applied to spraying, welding and cutting of difficult to cut metals

and alloys.

4. Working principle of chemical machining

Chemical machining (CHM) process is a controlled chemical dissolution (CD) of a work-

piece material by contact with strong reagent (etchant). Special coatings called maskants

protect areas from which the metal is not to be machined. It is one of the non-conventional

machining processes as shown in figure.

The advancement of technology causes to the development of many hard-to-machine

materials: stainless steel, super alloys, ceramics, refractories and fiber-reinforced composites

due to their high hardness, strength, brittleness, toughness and low machinability properties.

Sometimes, the machined components require high surface finish and dimensional accuracy,

complicated shape and special size, which cannot be achieved by the conventional machining

processes. Moreover, the rise in temperature and the residual stresses generated in the work-

piece due to traditional machining processes may not be acceptable. These requirements have

led to the development of non- traditional machining (NTM) processes. In these processes,

the conventional cutting tools are not employed instead, energy in its direct form is utilized.

Chemical energy is used in chemical machining. This process is the precision contouring of

metal into any size, shape or form without the use of physical force, by a controlled chemical

reaction. Material is removed by microscopic electrochemical cell action, as occurs in

corrosion or chemical dissolution of a metal.

This controlled chemical dissolution will simultaneously etch all exposed surfaces even

though the penetration rates of the etch may be only 0.0005 0.0030 in./min. The basic

process takes many forms: chemical milling of pockets, contours, overall metal removal,

chemical blanking for etching through thin sheets; photochemical machining (PCM) for

etching by using of photosensitive resists in microelectronics; chemical or electrochemical

polishing where weak chemical reagents are used (sometimes with remote electric assist) for

polishing or deburring and chemical jet machining where a single chemically active jet is

used.

Chemical machining offers virtually unlimited scope for engineering and design ingenuity, to

gain the most from its unique characteristics, chemical machining should be approached with

the idea that this industrial tool can do jobs not practical or possible with any other metal

working methods. Chemical machining will likely prove to be of considerable value in the

solution of problems that are constantly arising as the result of the introduction of new

materials.

5. Process details and applications of CHM

Process details:

Cleaning

The first step of the process is a cleaning of work-piece, i.e. it is required to ensure that

material will be removed uniformly from the surfaces to be processed.

Masking

Masking is similar to masking action is any machining operation. This is the action of

selecting material that is to be removed and another that is not to be removed. The material

which is not to be removed is applied with a protective coating called maskant. This is made

of a materials are neoprene, polyvinylchloride, polyethylene or any other polymer. Thinkers

of maskent are maintained up to 0.125 mm. The portion of work-piece having no application

of maskent is etched during the process of etching.

Etching

In this step the material is finally removed. The work-piece is immersed in the enchant where

the material of work-piece having no protective coating is removed by the chemical action of

enchant. Enchant is selected depending on the work-piece material and rate of material

removal; and surface finish required. There is a necessity to ensure that maskant and enchant

should be chemically in active. Common enchants are H2SO4, FeCL3, HNO3. Selection of

enchant also affects MRR. As in CHM process, MRR is indicated as penetration rates

(mm/min).

Demasking

After the process is completed demasking is done. Demasking is an act of removing maskent

after machining.

Applications

Nontraditional machining processes are widely used to manufacture geometrically complex

and precision parts for aerospace, electronics and automotive and many other industries.

There are different geometrically designed parts, such as deep internal cavities, miniaturized

microelectronics and nontraditional machining processes may only produce fine quality

components. All the common metals including aluminum, copper, zinc, steel, lead, and nickel

can be chemically machined. Many exotic metals such as titanium, molybdenum, and

zirconium, as well as nonmetallic materials including glass, ceramics, and some plastics, can

also be used with the process. CHM applications range from large aluminum alloy airplane

wing parts to minute integrated circuit chips. The practical depth of cut ranges between 2.54

to 12.27 mm. Shallow cuts in large thin sheets are of the most popular application especially

for weight reduction of aerospace components. Multiple designs can be machined from the

same sheet at the same time. CHM is used to thin out walls, webs, and ribs of parts that have

been produced by forging, casting, or sheet metal forming.

Further process applications related to improving surface characteristics include the

following:

(i) Elimination of alpha case from titanium forgings and super plastic formed parts.

(ii) Exclusion of the decarburized layer from low alloy steel forgings.

(iii) Taking away of the recast layer from parts machined by EDM (Electro Discharge

Machining).

(iv) Removal of sharp burrs from conventionally machined parts of complex shapes.

(v) Removal of a thin surface from forgings and castings prior to penetration inspection

below the surface (required for the detection of hidden defects).

Chemical machining is an effective method for the machining of shallow holes and

depressions, for profiling of the edges of sheet-metal work-pieces, and for machining of

shallow cavities of large surface areas (particularly in light alloys).

15. TUTORIAL TOPICS AND

QUESTIONS: NIL

16. UNIT-WISE QUESTION BANK

UNIT-I

Two Marks Questions with Answers

1. List out the limitations of traditional machining processes and important

characteristics of unconventional machining processes.

Ans.:

Limitations:

(i) Rapid improvement in the properties of materials.

(ii) Metals and non metals

(iii) Tool material hardness and work-piece hardness

(iv) Complex shapes

(v) Low tolerances

(vi) Better surface integrity and high surface finish

Important characteristics:

(i) Performance is independent of strength barrier

(ii) Use difference kind of energy in different form

(iii) In general low mrr but better quality products

(iv) Comparatively high initial investment cost.

2. Explain the elements and mechanics of metal removal in USM.

Ans.:

Elements of Process are:

(i) Work material: Applicable for only brittle material.

(ii) Tool material: t should be selected in such a way that energy utilization is optimum

(iii) Tool Size: The shape of the tool and its dimensions are governed by the size of the

abrasive used.

(iv) Abrasive Slurry: The abrasive selected should be harder than the work material being

machined.

(v) Transducer: The transducer in USM is utilized to convert the electrical energy to

vibratory motion either the piezoelectric or magneto strictive principles.

Material removal during USM due to cavitation under the tool and chemical corrosion due

to slurry media are considered insignificant. Hence, material removal due to these two factors

has been ignored. Contributions to the material removal by abrasive particles due to

3. ist the applications and limitations of USM.

Ans.:

The term ultrasonic refers to the frequency range above the audible range and is above 16

KHZ (i.e; 16KC/S). Waves are usually classified as shear waves and longitudinal waves.

High velocity longitudinal waves can easily propagate in solids, liquids and gases. They are

normally used in ultrasonic applications.

Applications:

(i) Suitable for application on to both non-conductive and conductive materials.

(ii) Tungsten and other hard carbides are being successfully machined by this method

(iii) The process is particularly suited to make holes with a curved axis of any shape that can

be made on the tool.

(iv)The smallest hole can be cut by USM 0.05mm in diameter and largest diameter in some

applications is 100mm in diameter.

Limitations:

(i) Metal removal rates are low.

(ii) Depth of hole produced is limited.

(iii) Tool wear is high and sharp corners cannot be produced.

(iv) Flat surfaces cannot be produced at the bottom of the cavity because of ineffective slurry

distribution.

4. Explain the functions of horn in USM.

Ans.:

An ultrasonic horn (also known as acoustic horn, is a tapering metal bar commonly used

for augmenting the oscillation displacement amplitude provided by an

ultrasonic transducer operating at the low end of the ultrasonic frequency spectrum

(commonly between 15 and 100 kHz). The device is necessary because the amplitudes

provided by the transducers themselves are insufficient for most practical applications of

power ultrasound.

5. Briefly explain the economic considerations in USM.

Ans.:

Economic considerations:

The process has the advantages of machining hard and brittle materials to complex shapes

with good accuracy and reasonable surface finish. Considerable economy results from USM

of hard alloy press tools, dies and wire drawing equipment on account of the high wear

resistance of tools made of these alloys.

The machines have no high speed moving parts. Working on machines is not hazardous,

provided care is taken to shield ultrasonic radiations from falling on the body.

The power consumption of ultrasonic machining is 0.1 W-h/mm3 for glass and about 5 W-

h/mm3 for hard alloys. The cost of the manufacture and use of the tools, particularly if they

have complicated contours, is very high. Another item adding to the cost of ultrasonic

machining is abrasive. The abrasive slurry has to be periodically replaced because during use

the particles are eventually broken and blunted.

Ultrasonic machines are not yet completely reliable; failure sometimes occurs on account of

faults in acoustic head, pump or generator.

It is probable that with more research in the near future on techniques and machines, the

process will have more economic advantages.

6. Explain the recent development of USM.

Ans.:

Mullard Research Laboratories, USA, have developed a process that combines

electrochemical reaction with ultrasonic abrasion. Using a 60W ultrasonic drill and abrasive

suspended in an alkaline electrolyte, Mullard researchers have reported that tool steel can be

machined nine times faster than by ultrasonics alone. Engis Limited of England has

developed the Disonic Die Ripper in which the diamond-plated tool oscillates at ultrasonic

speed as well as rotates at high speed (5000 rpm) in a liquid to rapidly remove material from

tungsten carbide dies. The use of diamond-plated tool eliminates the need of abrasive slurry

and frequent re-grinding of steel tools. The oscillating-cum-rotational system is claimed to

increase the material removal rate several times.

Three Marks Questions with Answers

1. Quote the necessity for unconventional machining process and explain how non-

traditional machining processes are classified?

Ans.:

With the development of technology, more and more challenging problems are faced by the

scientists and technologists in the field of manufacturing. The difficulty in adopting the

traditional manufacturing processes can be attributed mainly to the following three basic

sources.

(i) New materials with a low machinability.

(ii) Dimensional and accuracy requirements.

(iii) A higher production rate and economy.

Classification:

Mechanical processes:

Ultrasonic Machining

Abrasive Jet Machining

Water Jet Machining

Abrasive Water Jet Machining

Electrochemical and chemical processes:

Electrochemical Machining

Electrochemical Grinding

Electrochemical Deburring

Electrochemical Honing

Chemical Machining

Thermal metal removal processes:

Electric Discharge Machining

Plasma Arc Machining

Electron Beam Machining

Laser Beam Machining

2. Explain the mechanics of metal removal in USM.

Ans:

USM is mechanical material removal process or an abrasive process used to erode holes or

cavities on hard or brittle work piece by using shaped tools, high frequency mechanical

motion and an abrasive slurry. USM offers a solution to the expanding need for machining

brittle materials such as single crystals, glasses and polycrystalline ceramics, and increasing

complex operations to provide intricate shapes and work piece profiles. It is therefore used

extensively in machining hard and brittle materials that are difficult to machine by traditional

manufacturing processes. The hard particles in slurry are accelerated toward the surface of

the work piece by a tool oscillating at a frequency up to 100 KHz - through repeated

abrasions, the tool machines a cavity of a cross section identical to its own.

particles (suspended in carrier) move under the high frequency vibrating tool. There are two

possibilities when the tool hits an abrasive particle. If the size of the particle is small and the

gap between the bottom of the tool and work surface is large enough, then the particle will be

thrown by the tool, to hit the work surface (throwing model). Under the reverse conditions,

the particle will be hammered over the work surface. In the both cases, a particle after hitting

the work surface generated a crater of depth and radius.

For an effective cutting operation, the following parameters need to be carefully considered:

The machining tool must be selected to be highly wear resistant, such as high-carbon

steels.

The abrasives (25- -based, up to 40% solid volume) slurry

Includes: Boron carbide, silicon carbide and aluminum oxide.

Mechanics of metal removal in USM

3. What are the constituents of slurry used in USM? Name the characteristics of good

suspension media.

Ans:

The abrasive slurry contains fine abrasive grains.The most common abrasives are Boron

Carbide (B4C), Silicon Carbide (SiC), Corrundum (Al2O3), Diamond and Boron silicarbide.

B4C is the best and most efficient among the rest but it is expensive. SiC is used on glass,

germanium and most ceramics. Cutting time with SiC is about 20-40% more than that with

B4C. Diamond dust is used only for cutting diamond and rubies. Water is the most

commonly used fluid although other liquids such as benzene, glycerol and oils are also used.

Liquid Media:

Acts as an acoustic bond between the work-piece and the vibrating tool.

Helps efficient transfer of energy between the work-piece and the tool.

Acts as the coolant

Provides a medium to carry the abrasive to the cutting zone.

Helps to carry away the worn abrasive and swarf

The characteristics of good suspension media are:

Density, approximately equal to the abrasive.

Good wetting properties to wet the tool, work and abrasive.

High thermal conductivity and specific heat for efficient removal of heat from the

cutting zone.

Low viscosity to carry the abrasive down the sides of the hole between the tool and the

work-piece.

Non corrosive properties to avoid corrosion of the work-piece and the tool.

Water is frequently used as the liquid carrier since it satisfies most of the requirements. Some

inhibitor is generally added to the water.

4. Briefly explain the economic considerations and recent developments in USM.

Ans:

Economic considerations:

The process has the advantages of machining hard and brittle materials to complex shapes

with good accuracy and reasonable surface finish. Considerable economy results from USM

of hard alloy press tools, dies and wire drawing equipment on account of the high wear

resistance of tools made of these alloys.

The machines have no high speed moving parts. Working on machines is not hazardous,

provided care is taken to shield ultrasonic radiations from falling on the body.

The power consumption of ultrasonic machining is 0.1 W-h/mm3 for glass and about 5 W-

h/mm3 for hard alloys. The cost of the manufacture and use of the tools, particularly if they

have complicated contours, is very high. Another item adding to the cost of ultrasonic

machining is abrasive. The abrasive slurry has to be periodically replaced because during use

the particles are eventually broken and blunted.

Ultrasonic machines are not yet completely reliable; failure sometimes occurs on account of

faults in acoustic head, pump or generator.

It is probable that with more research in the near future on techniques and machines, the

process will have more economic advantages.

Recent development:

Mullard Research Laboratories, USA, have developed a process that combines

electrochemical reaction with ultrasonic abrasion. Using a 60W ultrasonic drill and abrasive

suspended in an alkaline electrolyte, Mullard researchers have reported that tool steel can be

machined nine times faster than by ultrasonics alone.

Engis Limited of England has developed the Disonic Die Ripper in which the diamond-plated

tool oscillates at ultrasonic speed as well as rotates at high speed (5000 rpm) in a liquid to

rapidly remove material from tungsten carbide dies. The use of diamond-plated tool

eliminates the need of abrasive slurry and frequent re-grinding of steel tools. The oscillating-

cum-rotational system is claimed to increase the material removal rate several times.

5. Write the applications, limitations, dimensional accuracy and surface quality of

USM process.

Ans.:

Applications:

(i) Suitable for application on to both non-conductive and conductive materials.

(ii) Tungsten and other hard carbides are being successfully machined by this method

(iii) The process is particularly suited to make holes with a curved axis of any shape that

can be made on the tool.

(iv) The smallest hole can be cut by USM 0.05mm in diameter and largest diameter in some

applications is 100 mm in diameter.

Limitations:

(i) Metal removal rates are low.

(ii) Depth of hole produced is limited.

(iii) Tool wear is high and sharp corners cannot be produced.

(iv) Flat surfaces cannot be produced at the bottom of the cavity because of ineffective

slurry distribution.

Dimensional accuracy:

(i) Generally the form accuracy of machined parts suffers from the following disturbing

factors which causes oversize conicity and out of roundness.

(ii) Side wear of the tool

(iii) Abrasive wear

(iv) Inaccurate feed of the tool holder

(v) Form error of the tool

(vi) Incorrect grain size of the abrasives

Surface quality:

The surface finish is closely related to the machining rate in USM, i.e. the larger the grit size

the faster the cutting but the coarser the surface finish. The larger the grit number, the

smoother becomes the produced surface. However, other factors such as tool surface,

amplitude of tool vibrations and material being machined also affect the surface finish.

Five Marks Questions with Answers

1. Discuss the reasons for the development of unconventional machining methods.

Write the circumstances under which individual process will have advantage over

others.

Ans.:

Manufacturing processes can be broadly divided into two groups:

(a) Primary manufacturing processes: Provide basic shape and size

(b) Secondary manufacturing processes: Provide final shape and size with tighter control on

dimension, surface characteristics

Material removal processes once again can be divided into two groups:

(a) Conventional Machining Processes

(b) Non-Traditional Manufacturing Processes or Unconventional Machining processes

Conventional Machining Processes mostly remove material in the form of chips by applying

forces on the work material with a wedge shaped cutting tool that is harder than the work

material under machining condition.

The major characteristics of conventional machining are:

Generally macroscopic chip formation by shear deformation

Material removal takes place due to application of cutting forces energy domain can

be classified as mechanical

Cutting tool is harder than work piece at room temperature as well as under machining

conditions

Non-conventional manufacturing processes is defined as a group of processes that remove

excess material by various techniques involving mechanical, thermal, electrical or chemical

energy or combinations of these energies but do not use a sharp cutting tools as it needs to be

used for traditional manufacturing processes. Material removal may occur with chip

formation or even no chip formation may take place. For example in AJM, chips are of

microscopic size and in case of Electrochemical machining material removal occurs due to

electrochemical dissolution at atomic level.

Need for unconventional machining processes

Extremely hard and brittle materials or difficult to machine material are difficult to

machine by traditional machining processes.

When the work piece is too flexible or slender to support the cutting or grinding forces

when the shape of the part is too complex.

2. What do you understand by the

Differentiate between conventional and unconventional machining processes.

Ans.:

The conventional manufacturing processes in use today for material removal primarily rely

on electric motors and hard tool materials to perform tasks such as sawing, drilling and

broaching. Conventional forming operations are performed with the energy from electric

motors, hydraulics and gravity. Likewise, material joining is conventionally accomplished

with thermal energy sources such as burning gases and electric arcs.

In contrast, non-traditional manufacturing processes harness energy sources considered

electrochemical reaction, high temperature plasmas and high-velocity jets of liquids and

abrasives. Materials that in the past have been extremely difficult to form, are now formed

with magnetic fields, explosives and the shock waves from powerful electric sparks.

Material-joining capabilities have been expanded with the use of high-frequency sound

waves and beams of electrons and coherent light. During the last 55 years, over 20 different

non-traditional manufacturing processes have been invented and successfully implemented

into production.

Conventional Manufacturing

Processes

Unconventional Manufacturing Processes

1. Generally macroscopic chip

formation by shear

deformation.

2. There may be a physical tool

present. for example a cutting

tool in a Lathe Machine.

3. Cutting tool is harder than

work piece at room

temperature as well as under

machining conditions.

4. Material removal takes place

1. Material removal may occur with chip formation

or even no chip formation may take place. For

example in AJM, chips are of microscopic size

and in case of Electrochemical machining

material removal occurs due to electrochemical

dissolution at atomic level.

2. There may not be a physical tool present. For

example in laser jet machining, machining is

carried out by laser beam. However in

Electrochemical Machining there is a physical

tool that is very much required for machining.

due to application of cutting

forces energy domain can

be classified as mechanical

5. Conventional machining

involves the direct contact of

tool and work piece

6. Lower accuracy and surface

finish.

7. Suitable for every type of

material economically.

8. Tool life is less due to high

surface contact and wear.

9. Higher waste of material due

to high wear.

10. Noisy operation mostly

causes sound pollutions.

11. Lower capital cost.

12. Easy set-up of equipment.

13. Skilled or un-skilled operator

may require.

14. Generally they are manual to

operate.

15. They cannot be used to

produce prototype parts very

efficiently and economically.

3. There may not be a physical tool present. For

example in laser jet machining, machining is

carried out by laser beam. However in

Electrochemical Machining there is a physical

tool that is very much required for machining.

4. Mostly NTM processes do not necessarily use

mechanical energy to provide material removal.

They use different energy domains to provide

machining. For example, in USM, AJM, WJM

mechanical energy is used to machine material,

whereas in ECM electrochemical dissolution

constitutes material removal.

5. Whereas unconventional machining does not

require the direct contact of tool and work piece.

6. Higher accuracy and surface finish.

7. Not Suitable for every type of material

economically

8. Tool life is more

9. Lower waste of material due to low or no wear.

10. Quieter operation mostly no sound pollutions are

produced.

11. Higher capital cost

12. Complex set-up equipment.

13. Skilled operator required.

14. Generally they are fully automated process.

15. Can be used to produce prototype parts very

efficiently and economically.

3. Ultrasonic Machining (USM).

Ans.:

The term ultrasonic refers to the frequency range above the audible range and is above 16

KHZ (i.e; 16KC/S). Ultrasonic machining (USM) is a mechanical metal removal process (for

brittle

materials, application for ductile materials is limited) in which material is removed by high

frequency oscillations of a shaped tool using an abrasive slurry.

A schematic diagram of USM set-up is shown in figure. The transducer (device for

converting any type of energy into ultrasonic waves, eg. mechanical energy or electrical

energy converted into mechanical vibrations) generates the high frequency vibrations of the

order of 20-30 KHZ with amplitude of the order of 0.02 mm. This vibration is transmitted to

the tool made of soft material through a mechanical coupler (known as tool holder). The tool

shape is a close complimentary shape of the final surface to be generated.

The tool while oscillating would be pressed against the work piece with a load of few

kilograms and fed down wards continuously as the cavity (hollow space) is cut in the work.

The tool is shaped as the approximate mirror image of the configuration of the cavity desired

in the work.

The slurry which is made of abrasive particles (grains) suspended in a liquid, is fed into the

cutting zone under pressure. The slurry is about 30% in concentration from the point of view

of pump design and of achieving adequate penetration.

The material removal rates in USM are relatively small, but in material which is brittle; this is

the only way to produce economically complex cavities without breaking the work piece.

Since there is no direct contact between work piece and the tool, fragile workpieces can be

conveniently used in USM.

4. Briefly explain the elements and its characteristics of USM.

Ans.:

The elements of USM Process are:

Work material: Applicable for only brittle material.

Tool material: It should be selected in such a way that

Schematic representation of USM

Energy utilization is optimum

It should also have adequate strength to withstand stresses at the nodal plane.

Common examples are titanium, low carbon steel, stainless steel, etc.

Tool Size:

The shape of the tool and its dimensions are governed by the size of the abrasive used.

The length of the tool should be short, since massive tools absorb the vibration energy,

reducing the efficiency of machining. Long tools cause over stressing of the tool and the

brazed point.

Abrasive Slurry: The abrasive selected should be harder than the work material being

machined. Common examples are: aluminium oxide, Silicon carbide and boron nitride etc.

The most common used liquids are water, benzens etc.

Transducer: The transducer mainly consists of a cylinder which is made up of piezoelectric

ceramic. It converts electrical energy into mechanical vibration. Transducer then vibrates

sonotrode at low amplitude and high frequency.

Sonotrode (Horn): It is made of low carbon steel. One end of it is connected with the

transducer and other end contains tool. The sonotrode vibrates at low amplitude and high

frequency and removes material from the w/p by abrasion where it contacts it.

Control Unit: The control unit consists of an electronic oscillator which produces an

alternating current at high frequency. The frequency produced is usually in between 18 kHz

to 40 kHz in ultrasonic range.

The transducer and sonotrode is attached to control unit with a cable.

The control unit has an electronic oscillator that produces an alternating current with

high ultrasonic frequency ranges in between 18 kHz to 40 kHz.

This high frequency alternating current is supplied to the transducer. The transducer

converts this alternating current into mechanical vibration and transmits this mechanical

vibration to the sonotrode attached to it.

The sonotrode is vibrated by the transducer with low amplitude and high frequency.

When this vibrating sonotrode strikes the surface of the w/p, it removes the material

form it. The slurry flows in between the tool and work-piece and helps in the removal of

the material from the surface.

The slurry used in the ultrasonic machining contains 20 % to 60% of water by volume,

aluminum oxide, boron Carbide and silicon carbide particles.

This is how ultrasonic machining works.

5. Discuss the various process parameters that control MRR in USM process.

Ans:

The various process Parameters that control MRR in USM process are:

(i) Frequency: The material removal rate (mrr) increases linearly with the frequency but in

actual characteristic is not exactly linear. The mrr tends to be somewhat lower than the

theoretically-predicted value.

(ii) Amplitude: When the amplitude of vibration is increased, the mrr is expected to increase

but tends to increase the surface roughness but the effect is minimal.

(iii) Static loading (feeding force): With an increase in static loading (i.e. feed force) in mrr

tends to increase however, in practice, it tends to decrease beyond a certain critical value of

the force as the grains start getting crushed.

(iv)Hardness ratio of the tool and the work-piece: The ratio of the work-piece hardness

and the tool hardness affects the mrr quite significantly. Clearly a more brittle material is

machined more rapidly.

(v) Grain Size:

(vi)Concentration of abrasive in the slurry: The mrr is expected to depend the

concentration of abrasive grains in the slurry i.e. proportional to the concentration.

Abrasives: (material size, shape, and concentration):

Al2O3 , SiC, B4C, Boronsilicarbide, Diamond

C

Tool and tool holder: (material, frequency of vibration and amplitude of vibration):

ao) 15

f) 19 25 kHz

F) related to tool dimensions

Work piece: (hardness):

terial

Multiple Choice Questions with Answers

1. Non-Traditional machining is recommended when we need which of the following

features?

a) Complex shapes

b) High surface quality

c) Low-rigidity structures

d) All of the mentioned

2. Non-Traditional machining can also be called as?

a) Contact Machining

b) Non-contact machining

c) Partial contact machining

d) Half contact machining

3. In which of the following industries, Non-traditional machining methods play an

important role?

a) Automobile

b) Aerospace

c) Medical

d) All of the mentioned

4. Different classifications of Non-traditional machining based on source of energy are?

a) Mechanical

b) Thermal

c) Chemical and electro-chemical.

d) All of the mentioned

5. Which of the following process comes under mechanical machining?

a) USM

b) EDM

c) LBM

d) PAM

6. Sources used in thermal machining are?

a) Ions

b) Plasma

c) Electrons

d) All of the mentioned

7. What is the full form of USM in advanced machining process?

a) Ultrasound manufacturing

b) Ultrasonic machining

c) UV spectrum manufacturing

d) Ultra sonar machining

8. Which is softer material in USM?

a) Tool

b) Work piece

c) Both of them

d) None of the mentioned

ranges between?

a) 5-10 kHz

b) 10-15 kHz

c) 18-20 kHz

d) 25-50 kHz

10. Amplitude of oscillation of tool in USM ranges between?

a) 0.1-

b) 10-

c) 50-

d) 100-

11. The machining system of USM contains which of the following components?

a) Magnetostrictor

b) Concentrator

c) Tools and slurry

d) All of the mentioned

12. State whether following statement is true or false.

m

a) True

b) False

13. In Ultrasonic Machining, magnetostrictor converts magnetic energy into which type

of energy?

a) Mechanical energy

b) Electrical energy

c) Thermal energy

d) None of the mentioned

14. What is the value of the amplitude obtained without mechanical amplifier?

a) 0.0001 0.001 µm

b) 0.001 0.1 µm

c) 1 10 µm

d) 10 100 µm

15. State whether the following statement is true or false.

a) True

b) False

16. At what rate slurry is pumped through nozzle in USM?

a) 10 L/min

b) 25 L/min

c) 50 L/min

d) 75 L/min

Answer: 1. (d), 2. (b), 3.(d), 4.(d), 5.(a), 6.(d), 7.(d), 8.(a), 9.(c), 10.(b), 11.(d), 12.(a), 13.(a),

14.(b), 15.(b), 16.(b)

Fill in the Blanks with Answers

1. In USM machining, material is removed by _____.

2. USM removes materials using ____ tool.

3. In USM Magnetostrictor converts magnetic energy into ____ type of energy.

4. In USM dominant in material removal is ____.

5. The rate of material removal in USM depends on ____.

6. Machinability rate of glass by USM is ____.

7. ____ can be used as abrasive carrying medium.

8. As the tool area increases, MRR ____.

9. When the static pressure of the feed is increases, MRR ____.

10. USM accuracy levels are limited to ____ value.

Answer: 1. Erosion, 2. Axially oscillating, 3.Mechanical, 4. Hammering, 5. Frequency,

6. 100%, 7. Air, 8. Decreases, 9. Increase up to a limiting condition, 10. ± 0.05 mm

UNIT-II

Two Marks Questions with Answers

1. Identify the transfer medium, types of abrasives and list few desirable properties

of carrier gas in AJM.

Ans.:

The transfer medium is the mixture of a gas (or air) and abrasive particles. Gas used is

carbon-di-oxide or nitrogen or compressed air. The selection of abrasive particles depends on

the hardness and Metal Removal Rate (MRR) of the work-piece. Most commonly, aluminium

oxide or silicon carbide particles are used.

Properties of carrier gas:

(i) Carrier gas, to be used in abrasive jet machining, must not flare excessively when

discharged from the nozzle into the atmosphere.

(ii) Further the gas should be nontoxic, cheap, easily available and capable of being dried

and cleaned without difficulty.

(iii) Air is most widely used owing to easy availability and little cost.

(iv) The gases that can be used are air, carbon dioxide or nitrogen.

2. Describe a few unique benefits of AWJM.

Ans.:

(i) Water is cheap, non-toxic, readily available, and can be disposed easily.

(ii) The jet approaches the ideal single point tool. This facilitates designing an efficient

automated system.

(iii) A very narrow cut in some materials reduces wastage and lowers cost.

(iv) Any contour can be cut and the process gives a clean cut .Accurate cutting enables

intricate shapes including sharp corners. Further, the operation is possible in both

horizontal and vertical positions.

(v) The method does not generate heat. Hence there is no possibility of rewelding of the

edges of lamination being stack-cut. This occurs frequently in cutting plastics with

conventional methods.

3. List the principle and functions of electrolyte in Electrochemical Machining.

Ans.:

Electrochemical Machining (ECM) is based upon Faraday's law of electrolysis. Faraday's law

states that the mass of a metal altered by the electrode is proportional to the quantity of

electrical charges transferred to that electrode.

In ECM the removal of metal is controlled by the anodic dissolution in

the electrolyte.

In ECM:

The work-piece acts as the anode

The tool act as cathode.

The electrodes should be placed closely with a gap of about 0.5 mm.

The anodes and cathodes should be immersed into electrolyte.

The electrolyte used in ECM performs the following operations:

Completing the electric circuit between the tool and the work-piece.

Allowing desirable machining reactions to occur.

Carrying away heat generated during the chemical reactions.

Carrying away products of reaction from the zone of machining.

4. Describe the tool materials used and process parameters of ECM.

Ans.:

(i) The accuracy of the tool shape directly affects the work-piece accuracy, since the

configuration of cavity produced cannot be more accurate than the tool that produces it.

(ii) The same is applicable to the surface finish of the tool.

(iii) The materials that find wide applications in the manufacturing of tools are aluminium,

brass, bronze, copper, stainless steel etc.

Process parameters:

(i) Power Supply:

Type direct current

Voltage 2 to 35 V

Current 50 to 40,000 A

Current density 0.1 A/mm2 to 5 A/mm2

(ii) Electrolyte:

Material NaCl and NaNO3

Temperature 20oC 50oC

Flow rate 20 lpm per 100 A current

Pressure 0.5 to 20 bar

Dilution 100 g/l to 500 g/l

Working gap 0.1 mm to 2 mm

Overcut 0.2 mm to 3 mm

Feed rate 0.5 mm/min to 15 mm/min

Electrode

(iii) Material Copper, brass, bronze

(iv) Surface Roughness Ra 0.2 to 1.5

5. Point out the application of Electro Chemical Honing.

Ans.:

(i) ECH can be employed to produce precision finishing and improved surface

integrity.

(ii) As in ECH, most of the material is removed by ECM action, the process keeps the

work-piece cool, free of heat distortion and produces burr and stress free surfaces.

(iii) The rotating and reciprocating honing motion correct shape deviations of cylindrical

work-pieces such as circularity, taper, bell-mouth hole, barrel-shaped hole, axial

distortion, and boring tool marks.

(iv) Cast tool steels, high-alloy steels, carbide, titanium alloys, Incoloy, Stainless steel,

Inconel, etc. are typical list of materials that can be processed by ECH.

(v) This process is an ideal choice for increasing the lifecycle of the critical components

such as bushings and sleeves, rollers, petrochemical reactors, moulds and dies, gun

barrels, pressure vessels, etc. which are made of very hard and/or tough, wear-resistant

materials, most of which are prone to heat distortions and as a result, ECH has wide

application area including automobile, avionics, petrochemical, power generation,

machine tool and fluid power industries.

6. What are the parameters that affect the material removal rate in Electro Chemical

Grinding?

Ans.:

The parameters that affect the material removal rate in ECG are:

Most of the material removal in electrolytic grinding is by electrochemical action. Some of

the metal is mechanically removed by the abrasive which is in contact with the work. The

main functions of the abrasive particles are:

(i) They act as insulators to maintain a small gap between the wheel and work-piece.

(ii) They remove electrolysis products that may be formed on the work-piece.

(iii) To cut chips if the wheel should contact the work-piece particularly in the event of

power failure.

When the process is started, about 90% of metal is removed by electrolytic action and only

about 10% by abrasive grinding. The abrasive particles rub against the workpiece, scrubbing

off the electrolysis products, thus allowing good dimensional control.

In general practical metal removal rates with ECG are of the order of 1.0cm3/min/100A.

However, 0.5cm3/min/100A is often used for approximation.

7. Explain the surface finish and accuracy produced by ECG.

Ans.:

(i) Surface finish produced by ECG on tungsten carbides can be expected to range from

0.2-0.4 microns for plunger grinding, and 0.4-0.5 microns for surface or traverse

grinding. In case of steels and various alloys surface finish up to 0.4-0.6 microns can be

easily obtained. Generally speaking, the higher the hardness of alloy, the better is the

finish.

(ii) Practical tolerances using ECG are the order of 0.01mm. Better accuracies than this can

be achieved by making a full depth cut in one pass. There is a tendency, however, for a

slightly rounded edge to be produced. If higher accuracies are essential, the majority of

stock can be removed by ECG and a final pass of 0.01mm-0.1mm can be taken

conventionally, with the same wheel, by merely turning off the power supply.

8. List the advantages and applications of ECD process.

Ans.:

Advantages:

(i) Both external and internal burrs which may be inaccessible can be removed.

(ii) The equipment is simple to operate and easy to maintain.

(iii) The system is very fast (5 to 40 times faster than hand deburring) and the operating

costs are low.

(iv) Burrs may be removed even after heat treatment. No stresses or embrittlement caused

by ECD.

(v) No tool wear and tool designs are fairly simple.

(vi) Electrolytic system is small and simple to maintain.

(vii) A simple power supply system can be used.

(viii) ECD can be included in transfer lines.

Applications:

Automobile connecting rods, gear teeth, blanking dies, valve ports, nozzle holes etc.

Three Marks Questions with Answers

1. Explain the elements and material removal rate in AJM.

Ans.:

The elements of AJM are:

(i) Carrier Gas: Carrier gas, to be used in abrasive jet machining, must not flare excessively

when discharged from the nozzle into the atmosphere. Further the gas should be nontoxic,

cheap, easily available and capable of being dried and cleaned without difficulty. The gases

that can be used are air, carbon dioxide or nitrogen. Air is most widely used owing to easy

availability and little cost.

(ii) Type of Abrasives: The choice of abrasive depends on the type of machining operation,

for example, .roughing, finishing etc. work material and cost. The abrasive should have a

sharp and irregular shape and be fine enough to remain suspended in the carrier gas and

should also have excellent flow characteristics. The abrasives used for cutting are aluminium

oxide and silicon carbide whereas sodium bicarbonate, dolomite, glass beads etc. are used for

cleaning, etching, deburring and polishing. Reuse of abrasives is not recommended because

not only dies its cutting ability decrease but contamination also clogs the orifice of the nozzle.

(iii) Grain Size: The rate of metal removal depends on the size of the abrasive grain. Finer

grains are less irregular in shape, and hence, posses lesser cutting ability. Moreover, finer

grains tend to stick together and choke the nozzle. The most favourable grain sizes range

from 10 to 50 . Coarse grains are recommended for cutting whereas finer grains are useful in

polishing, deburring etc.

(iv) Jet Velocity: The kinetic energy of the abrasive jet is utilized for metal removal by

erosion. For the erosion of glass by silicon carbide (grain size 25 ) the minimum jet velocity

has been found to be around 150m/s. The jet velocity is a function of the nozzle pressure,

nozzle design, abrasive grain size and the mean number of abrasives per unit volume of the

carrier gas.

(v) Mean Number of Abrasive Grains per Unit Volume of the Carrier Gas:

An idea about the mean number of abrasive grains per unit volume of the carrier gas can be

obtained from the mixing ratio M. It is defined as

M =

A large value of M should result in higher rates of metal removal but a large abrasive flow

rate has been found to adversely influence jet velocity, and may sometimes even clog the

nozzle.

(vi) Stand-off Distance (SOD):

Stand-off is defines as the distance between the face of the nozzle and the working surface of

the work. SOD has been found to have considerable effect on the rate of metal removal as

well as accuracy. A large SOD results in the flaring up of the jet which leads to poor

accuracy.

(vii) Nozzle Design:

The nozzle has to withstand the erosive action of abrasive particles, and hence, must be made

of materials that can provide high resistance to wear. The common materials for the nozzle

are sapphire and tungsten carbide.

The accuracy of machining is also dependent upon the shape of cut. It may not be possible to

machine components with sharp corners due to stray cutting in this process.

MATRIAL REMOVAL RATE (MRR) in AJM:

Taking into consideration the fact that metal removal is due to the chipping of the work

surface brought about by the impacting abrasive particles.

Q = KNd3v3/2( )3/4 ...................................................... (i)

K = A constant

N = Number of abrasive particles taking cut at a time.

d = Size of abrasive particle; (or, mean dia of abrasive particles) m ( )

v = velocity of the abrasive grains, m/sec

= Density of abrasive material, Kgm-3

= Yield stress of material, N-m-2

From equation (i) it can be seen that

Q Nv3/2

2. Discuss process parameters, process performance and applications of Abrasive

Water Jet Machining.

Ans.:

1. Process parameters:

WATER Flow rate and Pressure

ABRASIVES Type, Size, and Flow rate

Water nozzles and abrasive jet nozzle

Cutting parameters Feed rate and stand-off-distance

W/P Material

Mixing Tube Diameter & Length

Angle of Cutting

Traverse Speed

Number of passes

Water jet pressure:

Critical Pressure (Pc) Below this pressure no cutting Different for different

workpiece materials

Above a definite jet pressure Machined depth tends to stabilize

Relationship between jet pressure and machined depth Steeper with higher abrasive

flow rate

Increased Pressure Higher nozzle wear rate, Higher cost of Pump maintenance,

Lower efficiency

Water flow rate:

WATER Propelling fluid enables high abrasive flow rate (up to 5 kg/min)

ABRASIVE VELOCITY Up to 300m/s

AWJs Coherent hence more suitable for cutting

Abrasive flow rate:

Machined Depth

Above mc Reduced Machined Depth

Increase in m Wear of Mixing Nozzle Mixing Efficiency

Abrasive particle size:

Optimum Particle Size

Finer Particles For shallow depth of cut; Coarse Particles For high depth of cut

Different Abrasive sizes for different depths of cut

Abrasive materials:

Machined depth (Type of Abrasive)

Traverse speed:

Overcut decreases with an increase in traverse speed

Traverse speed vs Area generation rate has an optimum

Number of passes :

Multiple Passes : Single water jet with multiple passes; multiple tandem jets with

single pass

+ Increase in number of passes Cumulative depth

+ KERF ACTS as a local mixing chamber

Stand-off-distance:

Stand-off-Distance Machined depth

REBOUND PARTICLES

Beyond Upper value of SOD No cutting

Smaller SOD Deeper cut

Visual examination:

Visual examination using movie camera:

Two modes of material removal

i) Cutting mode Shallow angle of impact

ii) Deformation Mode Larger angle of impacts.

Penetration rate and depth of cut Function of time

2. Process performance:

Can cut thick materials upto 200mm.

KERF width decreases as W/P hardness increases

Machined surfaces No Thermal/ Mechanical damage

Machining of Glass Stray cutting leads to frost surface

3. Applications

Metals & Non-metals both

Omni-Directional cutting with no burrs

Industries aerospace, Nuclear, Foundries, Construction, etc.

Steel components cut Plates, Tubes, Corrugated structures, etc.

3. Explain with neat sketch the electrochemical grinding (ECG) and write its

advantages over conventional method.

Ans.:

Schematic representation of ECG

Principle:

In ECG method, the work is machined by the combined action of electro-chemical effect and

conventional grinding operation. The majority of the metal removal results from the

electrolytic action. A schematic view of electrolytic grinding is shown in figure. Here the tool

electrode is a rotating metal bonded diamond or aluminium oxide wheel and it acts as

cathode. The work acts as anode and hence current flows between the work and wheel. A

constant gap of about 0.255mm is maintained between work and wheel, and the region

between the two (i.e. work & wheel) is flooded with electrolyte. The electrolyte is carried

past the work surface at high speed by the rotary action of the grinding wheel. The grinding

wheel runs at speeds of 900 to 1800m/min.

The power supply is DC voltage of 5 to 20v, current density of 100 to 200 A/cm2. Bushes are

used on the grinder spindle for the supply of current into the spindle from which it then flows

to the grinding wheel.

The commonly used electrolytes are: Aqueous solutions of sodium silicate, borax sodium

nitrate and sodium nitrite etc.

Material removal:

Most of the material removal in electrolytic grinding is by electrochemical action. Some of

the metal is mechanically removed by the abrasive which is in contact with the work. The

main functions of the abrasive particles are:

i) They act as insulators to maintain a small gap between the wheel and work-piece.

ii) They remove electrolysis products that may be formed on the work-piece.

iii) To cut chips if the wheel should contact the work-piece particularly in the event of power

failure.

When the process is started, about 90% of metal is removed by electrolytic action and only

about 10% by abrasive grinding. The abrasive particles rub against the work-piece, scrubbing

off the electrolysis products, thus allowing good dimensional control.

In general practical metal removal rates with ECG are of the order of 1.0cm3/min/100A.

However, 0.5cm3/min/100A is often used for approximation.

Surface finish:

Surface finish produced by ECG on tungsten carbides can be expected to range from 0.2-0.4

microns for plunger grinding, and 0.4-0.5 microns for surface or traverse grinding. In case of

steels and various alloys surface finish up to 0.4-0.6 microns can be easily obtained.

Generally speaking, the higher the hardness of alloy, the better is the finish.

Accuracy:

Practical tolerances using ECG are the order of 0.01mm. Better accuracies than this can be

achieved by making a full depth cut in one pass. There is a tendency, however, for a slightly

rounded edge to be produced. If higher accuracies are essential, the majority of stock can be

removed by ECG and a final pass of 0.01mm-0.1mm can be taken conventionally, with the

same wheel, by merely turning off the power supply.

Advantages:

The advantages of the process over conventional grinding are:

i) Increased material removal rates

ii) Reduced cost of grinding

iii) Reduced heating of work-piece and therefore less risk of thermal damage.

iv) Abrasive of burrs on the finished surface

v) Improved surface finish with no grinding scratches.

vi) Reduced pressure of work against the wheel.

Disadvantages:

i) Higher cost of grinding wheel.

ii) Higher cost of maintenance.

iii) Tolerances achieved are rather low (of the order of 0.025mm). Due to this the workpieces

need final abrasive machining.

iv) Difficult optimization, due to the complexity of the process.

Applications:

i) The most common application of ECG is the grinding of tungsten carbide tool inserts.

ii) ECG is particularly useful for grinding fragile parts such as honey-comb, thin walled

tubes, and skins hypodermic needles etc.

iii) High production rates can be achieved when grinding hard, tough, stringy, work hard

enable or less heat-sensitive materials.

4. Explain the process Electro Chemical Honing (ECH), with a neat sketch.

Ans.:

Schematic representation of ECH

Principle:

In ECH process, the stock removal capabilities of ECM are combined with the accuracy

capabilities of honing. The basic principles of the process regarding voltage, current,

electrolyte and materials processed are the same as those described under ECM. The process,

as shown in figure consists in rotating and reciprocating the tool inside the cylindrical

components. The electrolyte is fed under pressure through holes in the tools that at every

point there is uniform flow and velocity. The gap between the tool and the work piece is

usually adjusted by the use of an expanding tool. At the start of the operation, the gap is

approximately 1mm and increases during the cycle.

Bonded abrasive honing stones are forced out with equal pressure in all directions from slots

in the tool. These stones are nonconductive and assist in electrochemical action. They also

abrade the residue left by the electrochemical action. A clean surface thus generated is ready

for further chemical attack. If the work piece is not round but tapered or wavy, the stones cut

away all high areas and remove the geometric error.

As in conventional honing, the abrasive should be self dressing. If the stones glaze, the stock

removal rate is reduced. If they dress too rapidly the operating cost becomes high. The design

of the tool and the adjusting head mechanism is important for proper abrasive application.

Parameters:

The direct current power source is a 3000A, 24v rectifier, current density is of the order of

15to 40 A/cm2 is used.

Accuracy and Surface Finish:

Size tolerance of 0.01mm on the diameter can be obtained and roughness can be maintained

at less than 0.005mm. Surface roughness of the order of 0.1-0.5 microns CLA is obtained by

this method but if it is required to have a specified roughness, the stones are put to work for a

few seconds after the power is switched off. The surface finish obtained in this manner is

dependent upon the size of abrasive grains speed of rotation and reciprocation and duration of

Advantages:

Increased metal removal rate particularly on hard materials, burr-free action, less pressure

required between stones and work reduced noise and distortion when honing thin-walled

tubes, cooler action leading to increased accuracy with less material damage.

Applications:

The process is easily adaptable to cylindrical parts for trueing the inside surfaces. The size of

the cylinder that can be processed by this method is limited only by the current and

electrolyte that can be supplied and distributed. Any surface roughness compatible with the

material being cut is duplicated over a number of component parts.

5. Why burrs are removed from the work-piece after machining? Explain the

electrochemical deburring processes in detail.

Ans.:

In almost all forming and machining operations very fine burrs of metal are invariably left on

the work piece. These burrs are undesirable, particularly for precision components, as they

may break, loose and disturb a delicately balanced mechanism. They are also dangerous for

the fingers. These burrs have been successfully removed through electrochemical means by a

process called electrochemical deburring.

Process:

In this process the tool and the work-piece are placed in a fixed relative position with a gap of

0.1 to 1.0mm (i.e. there is no relative movement of the tool with respect to the work piece).

The tool which is positioned near the base of the burr is designed so that only that portion of

the work piece containing burrs is exposed to tool material. The remaining portion of the tool

is insulated.

The current levels in ECD are of the order of 6A/cm of linear edge length at 7 to 25 v DC

supply. The electrolyte which is generally sodium nitrate is circulated at a pressure of 0.1 to

0.4 N/mm2 to give flow rate of 5 to 20 lit/min for a 100A electrolyzing current.

Schematic representation of ECD

Advantages:

i) Both external and internal burrs which may be inaccessible can be removed.

ii) The equipment is simple to operate and easy to maintain.

iii) The system is very fast (5 to 40 times faster than hand deburring) and the operating costs

are low.

iv) Burrs may be removed even after heat treatment.

v) No stresses or embrittlement caused by ECD.

vi) No tool wear and tool designs are fairly simple.

vii) Electrolytic system is small and simple to maintain.

viii) A simple power supply system can be used.

ix) ECD can be included in transfer lines.

Applications:

Manufacturing of automobile connecting rods, gear teeth, blanking dies, valve ports, nozzle

holes etc.

Five Marks Questions with Answers

1. Explain the principle and working of an Abrasive Jet Machining process with a

neat sketch.

Ans.:

Principle:

Abrasive jet machining (AJM) uses a stream of fine grained abrasive mixed with air or some

other carrier gas at high pressure. This stream is directed by means of a suitably designed

nozzle on to the work surface to be machined. Metal removal occurs due to erosion caused by

the abrasive particles impacting the work surface at high speed.

Working:

Dry air or gas is filtered and compressed by passing it through the filter and compressor.A

pressure gauge and a flow regulator are used to control the pressure and regulate the flow rate

of the compressed air. Compressed air is then passed into the mixing chamber. In the mixing

chamber, abrasive powder is fed. A vibrator is used to control the feed of the abrasive

powder. The abrasive powder and the compressed air are thoroughly mixed in the chamber.

The pressure of this mixture is regulated and sent to nozzle. The nozzle increases the velocity

of the mixture at the expense of its pressure. A fine abrasive jet is rendered by the nozzle.

This jet is used to remove unwanted material from the work-piece.

Construction of Abrasive Jet Machining (AJM):

The constructional requirements of Abrasive Jet Machining (AJM) are listed and described

below:

Schematic representation of AJM

Abrasive jet: It is a mixture of a gas (or air) and abrasive particles. Gas used is carbon

dioxide or nitrogen or compressed air. The selection of abrasive particles depends on

the hardness and Metal Removal Rate (MRR) of the workpiece. Most commonly,

aluminium oxide or silicon carbide particles are used.

Mixing chamber: It is used to mix the gas and abrasive particles.

Filter: It filters the gas before entering the compressor and mixing chamber.

Compressor: It pressurizes the gas.

Hopper: Hopper is used for feeding the abrasive powder.

Pressure gauges and flow regulators: They are used to control the pressure and

regulate the flow rate of abrasive jet.

Vibrator: It is provided below the mixing chamber. It controls the abrasive powder

feed rate in the mixing chamber.

Nozzle: It forces the abrasive jet over the work-piece. Nozzle is made of hard and

resistant material like tungsten carbide.

Operations that can be performed using Abrasive Jet Machining:

The following are some of the operations that can be performed using Abrasive Jet

Machining:

Drilling

Boring

Surface finishing

Cutting

Cleaning

Deburring

Etching

Trimming

2. Quote the process variables, advantages, limitations and applications of AJM.

The following are some of the important process parameters of abrasive jet machining:

(i) Abrasive mass flow rate:

Mass flow rate of the abrasive particles is a major process parameter that influences the metal

removal rate in abrasive jet machining. In AJM, mass flow rate of the gas (or air) in abrasive

jet is inversely proportional to the mass flow rate of the abrasive particles. Due to this fact,

when continuously increasing the abrasive mass flow rate, Metal Removal Rate (MRR) first

increases to an optimum value (because of increase in number of abrasive particles hitting the

work-piece) and then decreases. However, if the mixing ratio is kept constant, Metal

Removal Rate (MRR) uniformly increases with increase in abrasive mass flow rate.

(ii) Nozzle tip distance:

Nozzle Tip Distance (NTD) is the gap provided between the nozzle tip and the work-piece.

Upto a certain limit, Metal Removal Rate (MRR) increases with increase in nozzle tip

distance. After that limit, MRR remains constant to some extent and then decreases. In

addition to metal removal rate, nozzle tip distance influences the shape and diameter of cut.

For optimal performance, a nozzle tip distance of 0.25 to 0.75 mm is provided.

(iii) Gas pressure:

Air or gas pressure has a direct impact on metal removal rate. In abrasive jet machining,

metal removal rate is directly proportional to air or gas pressure.

(iv) Velocity of abrasive particles:

Whenever the velocity of abrasive particles is increased, the speed at which the abrasive

particles hit the work-piece is increased. Because of this reason, in abrasive jet machining,

metal removal rate increases with increase in velocity of abrasive particles.

(v) Mixing ratio:

Mixing ratio is a ratio that determines the quality of the air-abrasive mixture in Abrasive Jet

Machining (AJM). It is the ratio between the mass flow rate of abrasive particles and the

mass flow rate of air (or gas). When mixing ratio is increased continuously, metal removal

rate first increases to some extent and then decreases.

(vi) Abrasive grain size:

Size of the abrasive particle determines the speed at which metal is removed. If smooth and

fine surface finish is to be obtained, abrasive particle with small grain size is used. If metal

has to be removed rapidly, abrasive particle with large grain size is used.

Advantages:

(i) Because AJM is a cool machining process, it is best suited for machining brittle and

heat-sensitive materials like glass, quartz, sapphire and ceramics.

(ii) It is not reactive with any work-piece material.

(iii) No tool changes are required.

(iv) Intricate parts of sharp corners can be machined.

Limitations:

(i) The removal rate is low

(ii) Stray cutting cannot be avoided

(iii) The tapering effect may occur especially when drilling in metals.

(iv) The abrasive may get impeded in the work surface

(v) Suitable dust collecting systems should be provided

Applications:

(i) Drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing and etc.

(ii) Micro-deburring by hypodermic needles

(iii) Frosting glass and trimming of circuit boards hybrid circuit resistors, capacitors,

gallium and silicon.

(iv) Removal of films and delicate cleaning of irregular surfaces.

3. Describe the working principle, metal removal rate, process principles and

applications of Water Jet Machining with neat sketch

Ans.:

Working principle:

The key element in water jet machining is a water jet, which travels at velocities as high as

900m/s. When the stream strikes a work-piece surface the erosive force of water removes the

material rapidly. The water, in this case, acts like a saw and cuts a narrow groove in the

work-piece material.

Schematic representation of WJM

The elements of the machining system:

(i) Hydraulic pump: The hydraulic pump is powered from a 30 Kilowatt electric motor and

supplies oil at pressure as high as 117bars in order to drive a reciprocating plunger pump

termed as intensifier.

(ii) Intensifier: The intensifier accepts the water and expels it through the accumulator at

higher pressures of 3800bar. The intensifier converts the energy from the low-pressure

hydraulic fluid into ultrahigh-pressure water.

(iii) Accumulator: The accumulator maintains the continuous flow of the high pressure

water and eliminates pressure fluctuations. It relies on the compressibility of water (12

percent at 3800bar) in order to maintain a uniform discharge pressure and water jet velocity.

(iv) High- Pressure tubing: High-pressure tubing transports pressurized water to the cutting

head. Typical tube diameters are 6 to 14mm.

(v) Jet Cutting nozzle: The nozzle provides a coherent water jet stream for optimum cutting

of low density, soft material that is considered un-machinable by conventional methods.

Nozzles are normally made from synethic sapphire.

(vi) Catcher: The catcher acts as a reservoir for collecting the machining debris entrained in

the water jet.

Process principles:

Process characteristics:

Pressure Higher value Can cut thicker materials

Nozzle Diameter

Traverse Rate Decreased Value for Thicker parts

Stand-off-Distance: 3mm-25mm

Process performance:

Material cut

Porous, Fibrous, Granular, Soft

Corrugated Board (3m/s), Aluminium (0.0025m/s) etc.

No Predrilled Hole is required Any direction and Location but Accessible for the

water jet

Too thick parts Cut in more than one pass Energy consumption/ unit length is less

Machined surface: No Burrs, No thermal Damage & Good surface Finish

: Tolerance, Straightness of cut edges & Finish =

(Work-piece Thickness & Cutting Speed)

To cut insulation of cables 69-200MPa 5-10s/Cable

Applications:

Cutting of Asbestos (Minimizes Airborne Dust)

Carbide Grit Safety Walks

Fibre Glass & Polyethylene Automotive Parts

High Speed Cutting of Corrugated Box

4. Explain the working principle and various elements of Abrasive Water Jet

Machining with neat sketch.

Ans.:

Working principle:

Abrasive water jet machining (AWJM) or abrasive flow machining is a process which uses

abrasives with water for machining of materials. In AJM, air driven abrasive jet strikes the

work-piece and removes the material while in USM, abrasive grins in liquid slurry strike the

work-piece surface at ultrasonic frequency and cut the material at low material removal rate.

Recent developments have witnessed improvements in jet cutting technology by using

abrasive water jets where water is used as carrier fluid. In principle, this process is similar to

abrasive jet machining except that in this case water is used as a carrier fluid in place of gas.

These processes offer advantage of cutting electrically non-conductive as well as difficult-to-

machine materials comparatively more rapidly and efficiently than other processes. Other

advantages claimed for this process may be listed as: practically no dust, high cutting speed,

multidirectional cutting capacity, no fire hazards, no thermal or deformation stresses, high

quality of machined edge, easy adaptation for remote control, recycling of abrasive particles,

low power requirement, almost no delamination, and reduced striations.

Schematic representation of AWJM

Elements of the System

(i) Pumping System

(ii) Abrasive Feed System

(iii) Abrasive Water Jet Nozzle

(iv) Catcher

(i) Pumping System:

Intensifier 415M Pa, 75 HP Motor

High Velocity Jet

(ii) Abrasive Feed System:

Delivers Dry Abrasives

To control flow rates Control orifice Diam

Cannot supply abrasives over long distances Use directly slurry To feed over a

long distance More power required

Water jet nozzle diam 75 to 635

For long life of a nozzle Sapphire

(iii) Abrasive Jet Nozzle:

Functions: Mixing of Abrasives & Water

Forming High Velocity Jet

Materials: WC, Boron carbide, Sapphire

Types: (a) Single jet side feed nozzle

Simple to make

Rapid Wear of Exit Part

Non-Optimal Mixing Efficiency

(b) Multiple jets central feed nozzle

Centrally located abrasive feed system

Surrounded by multiple water jets Converging Annulus

Higher Nozzle Life & Better Mixing

Difficult & Costly to Fabricate

(iv) Catcher:

Stationary nozzle & moving work-piece + Long Narrow Tube placd under the point

of cut

Moving Nozzle & Stationary W/P + A water filled settling tank underneath the W/P

+ Transfer of high pressure water

5. Quote the principle of Electrochemical Machining process with sketch and

discuss influences of process parameters in machining output.

Ans.:

Electrochemical machining is one of the latest and potentially the most useful of the non-

tradition of machining processes. The basic principles of the process are not new but

applications of the process as a metal working tool are definitely new. Extensive development

of the process has taken place in recent years mainly due to (i) the need to machine harder

and tougher materials, (ii) the increasing cost of manual labour and (iii) the need to machine

configurations beyond the capability of conventional machining methods.

Principle:

Michael Faraday discovered that if two electrodes are placed in a bath containing a

conductive liquid and D.C. potential is applied across them metal can be deplated from the

anode and plated on the cathode. This is the (process) principle was in use for a long time in a

process called electroplating. With certain modifications, ECM is the reverse of electro-

plating i.e. work-piece is made the anode.

Also the objective of electrolysis principle is to deposit metal on the work-piece. But since in

ECM the objective is to remove metal the work-piece is connected to the positive and the tool

to the negative terminal. i.e. work-piece is made anode and tool as cathode.

Fig. shows the schematic representation of the principle of ECM, which consists of a work-

piece and a suitably-shaped tool, the gap between the tool and the work being full of a

suitable electrolyte. When the current is passed dissolution of the anode occurs. However, the

dissolution rate is more where the gap is less and vice versa as the current density is inversely

proportional to the gap. Now if the tool is given a downward motion the work surface tends

to take the same shape as that of the tool and at a steady state, the gap is uniform as shown in

figure. Thus the shape of the tool is reproduced in the job.

Schematic representation of ECM

Process parameters:

i) Cathode tool: The accuracy of the tool shape directly affects the work-piece accuracy,

since the configuration of cavity produced cannot be more accurate than the tool that

produces it. The same is applicable to the surface finish of the tool. The materials that find

wide applications in the manufacturing of tools are aluminium, brass, bronze, copper,

stainless steel etc.

ii) Anode work-piece: The work material must be a good conductor of electricity. The

material removal rate is proportional to the atomic weight and inverse of the valency of work

material. The fixtures for holding the work are made of some insulating material (such as

epoxy resins, glass, fibre resins perpex and pvc). They should have good thermal stability and

low moisture absorption properties.

iii) D.C. power and control system: The process needs low voltages of the order of 2 to 20 v

and in rare cases up to 30v. Normal current requirements are as high as 800amp/cm2 of the

workpiece area to be machined. Three phase 440v A.C. power supply available from mains is

converted to low voltage D.C. by a step-down transformer and a rectifier.

iv) Electrolyte: The electrolyte used in ECM performs the following operations:

a) Completing the electric circuit between the tool and the work-piece.

b) Allowing desirable machining reactions to occur.

c) Carrying away heat generated during the chemical reactions.

d) Carrying away products of reaction from the zone of machining.

6. (i) What is the essential properties and selection of an electrolyte?

(ii) Derive an expression for metal removal rate in ECM. Calculate the machining

rate and the electrode feed rate when iron is electrochemically machined, using

copper electrode and sodium chloride solution (specific resistance = 5.0 ohm cm).

The power supply data of the ECM machine used are:

Supply voltage: 18v D.C.

Current: 5000 amp

Ans.:

(i) The essential properties and selection of an electrolyte are:

a) High electrical conductivity

b) Low viscosity and high specific heat

c) Chemical stability

d) Resistance to formation of passivating film on work surface

e) Non corrosive and non toxic in nature

f) Inexpensive and readily available commonly used electrolysis are Sodium chloride solution

in water, Sodium nitrate solution, Potassium nitrate, Sodium sulphate, Sodium hydroxide,

Sodium fluoride and potassium chloride etc.

(ii) Expression for metal removal rate:

Theoretically, the metal removal rate can be estimated as follows:

Let

h- -

A- Area of the current path (m2)

Specific resistance of electrolyte (ohm-m)

E- Machining voltage (volt)

I- Current flowing through gap (amp)

t- time for which current flows(s)

Current density (amp/m2) S is given by

S = = =

electrolysis is proportional to the quantity of the electricity passed and to the electrochemical

equivalent of the anode material, that is,

m I.t.(N/n)

= (1/96500) I .t .(N/n) . (1/d) .

where N is the atomic weight of work material, n the valency of work material, d the density

is the current efficiency which may be defined as the efficiency of the

current in removing metal from the work piece. The current efficiency is close to 100 per cent

when sodium chloride is used as the electrolyte, but for nitrate and sulphate solutions, it is

somewhat lower.

Specific metal removal rate(s) can be written as

s = (1/96500) .(N/n) . (1/d) . (m3/ amp s)

and feed rate of electrode

f = S s

= [ ][N/n] [1/d] [1/96500] (m/s)

Solution:

The current efficiency can be taken as 100 per cent with sodium chloride electrolyte.

For iron (anode), atomic weight N = 56

valency n = 2

density d = 7.87 106 g/m3

Specific metal removal rate

s = (1/96500).(N/n) . (1/d)

= (1/96500) .(56/2) . (1/7.87 106)

= 3.67 10-11 m3/amp/s

Metal removal rate =s .I

= 3.67 10-11 5000

= 1.835 10-7m3/s

Electrode feed rate = . s

= m/s

= 1.585 mm/min (Ans)

7. What are the tool design and economic aspects in Electro Chemical Machining

process? Explain.

Ans.:

Tool design in ECM:

(i) Tool and fixtures are required to operate for long periods in a corrosive environment of

electrolyte and stray electric currents. In order to avoid the rapid corrosion of each tool,

the selection of proper material is very important. Generally, stainless steel, copper,

brass, bronze, monel, reinforced plastics or copper-tungsten alloy are used. Parts that

have anode potential corrode rapidly and, therefore, the number of parts in electrical

contact with the work piece is limited. Non-metallic materials may be useful as these

are electrically non-conductive and chemically corrosion resistant.

(ii) All electrolyte ducts need to be made of non-corrosive materials, as one of the major

process requirements is that no particles of corrosion should enter the electrolyte flow in

the tool-work gap. To prevent overheating, there is a limit to the minimum cross-section

of the current-carrying parts. For 1000amp, it is about 6cm2 for copper, 25cm2 for

bronze and brass, and 250cm2 for stainless steel. Smaller areas are permissible for metal

surfaces cooled by rapidly flowing electrolyte.

(iii) Fixtures and tools should be rigid enough to avoid vibration or deflection under the high

hydraulic forces which they are subjected to. Proper alignment between the tool and

work fixture is essential and is best achieved with removable setting pieces.

(iv) Electrical joints should be strictly limited as these are sources of power loss and at times

may fail under the wet corrosive conditions in the work enclosure.

Economic aspects of ECM:

(i) Fixed costs of ECM installations are quite high as compared to its operating costs.

Overhead costs are the same as for other conventional machining methods. Some costs

are unique, such as those of high power, electrode tooling and electrolyte.

(ii) ECM needs power of high current capacity. In localities where power is sufficiently

cheap, this factor can be over looked.

(iii) Electrode or tooling cost is a fixed cost because there is little wear of the ECM tool.

There occurs, however, a negligible abrasion wear of electrode due to electrolyte flow

across the gap. With regard to actual tooling cost, it is not very different from

conventional machine tooling.

(iv) Electrolyte is not as costly as one might think to be. The most widely used electrolyte is

sodium chloride (salt) and it is quite cheap. The normal price of the salt seldom exceeds

Re.0.50 per kg when purchased in large quantities.

(v) Cost of work-piece fixtures are not very high. The cost per piece will, however, depend

on the number of work pieces finished.

(vi) On the shop floor, ECM installations need not be operated by very skilled engineers and

the operation of the machinery can be learnt easily.

(vii) The economic success of ECM, in fact, depends largely on the choice of applications. If

an operation is simple or if the material can be easily machined by other methods, the

high cost of the ECM plant cannot be justified.

8. (a)Write the factors that influence the surface finish and accuracy in ECM.

(b) List the advantages, limitations and applications of ECM process.

Ans.:

(a) The factors that influence the surface finish and accuracy in ECM process are:

The material removal rates with ECM are sufficiently large and comparable with that of the

conventional methods.

i) Excellent surface finish of the order of 0.4 can be obtained with tolerances of the order

of 0.02mm or less.

ii) The repeatability is also good. This is possible because the tool wear is almost non-

existent.

The process parameters that have a control on the performance of the ECM process are given

in the following lines:

(a) Feed rate: A high feed rate results in a higher material removal rate. It also decreases the

equilibrium machining gap resulting in improvement of the surface finish and tolerance

control.

(b) Voltage: Low voltage decreases the equilibrium machining gap and results in a better

surface and finer tolerance control.

(c) Current: Increased current leads to electrolyte heating, the limiting condition being the

boiling point of the electrolyte. The reaction of more metallic ions with the electrolyte causes

higher hydrogen evolution. It also leads to polarized ionic layers forming at the electrodes

causing voltage drops.

(d) Electrolyte concentration: Low concentration of electrolyte decreases the machining gap

and results in a better surface and finer tolerance control.

(e) Electrolyte temperature: Low temperature of electrolyte is conductive to a better surface

finish and tolerances.

(b) Advantages

(i) Complex three dimensional surfaces can be machined accurately

(ii) Since there is no cutter marks surface finish is higher.

(iii) The tool wear is practically nil which results in a large number of components produced

per tool.

(iv) The ECM process does not thermally affect the work-piece.

Limitations:

(i) Non-conductive materials cannot machined.

(ii) Use of corrosive media as electrolytes makes it difficult to handle.

(iii) Sharp interior edges and corners (<0.2 mm radius) are difficult to handle.

(iv) It is a very expensive process.

Applications:

(i) Facing and turning of complex three dimensional shapes.

(ii) Die sinking particularly deep narrow slots and holes.

(iii) Profile and any odd shape contouring.

(iv) Multiple hole drilling.

(v) Broaching, deburring, grinding, honing and cutting off etc.

Multiple Choice Questions with Answers

1. Which of the following, are the processes and applications in which Abrasive jet

machining can be applied?

a) Drilling

b) Cutting

c) Deburring

d) All of the mentioned

2. In Abrasive jet machining, intricate shapes and holes are machined on which type of

materials?

a) Brittle

b) Thin

c) Difficult to machine

d) All of the mentioned

3. What is the amount of material utilizes when we machine parts using Abrasive jet

machining?

a) Very low

b) Low

c) Medium

d) High

4. After how much time tool has to be changed in AJM?

a) 1 hr

b) 2 hrs

c) 5 hrs

d) No tool change required

5. By using Abrasive jet machining, how much amount of hardening does the materials

experience?

a) No hardening

b) Very less hardening

c) Average hardening

d) High hardening

6. Which type of materials cannot be machined using Abrasive jet machining?

a) Soft materials

b) Hard materials

c) Difficult to machine materials

d) None of the mentioned

7. Which of the following materials in Abrasive jet machining can be a health hazard?

a) Abrasive grains

b) Air carrier

c) Silica dust

d) None of the mentioned

8. What is the percentage of the abrasives and water in the mixture?

a) 20% water and 80% abrasives

b) 80% water and 20% abrasives

c) 30% water and 70% abrasives

d) 70% water and 30% abrasives

9. What are the materials used for abrasives in Abrasive water jet machining?

a) SiC

b) Corundum

c) Glass beads

d) All of the mentioned

10. What is the grain size of abrasive particles, which are often used for Abrasive water

jet machining?

a) 0.01 0.50 µm

b) 10 150 µm

c) 200 500 µm

d) 500 1000 µm

11. How is the material removed in Abrasive water jet machining?

a) Vaporization

b) Electron transfer

c) Corrosion

d) Erosion

12. Which of the following is not a process parameter of Abrasive water jet machining?

a) Frequency of vibration

b) Orifice diameter

c) Pressure

d) Stand-off distance

13. Which of the following come under the process parameters of the Abrasive water jet

machining?

a) Abrasive size

b) Machine impact angle

c) Traverse speed

d) All of the mentioned

14. Which of the following material removal mechanisms is implemented by ECM?

a) Mechanical abrasion

b) Electrochemical dissolution

c) Chemical corrosion

d) Mechanical erosion

15. Electrolysis occurs when which of the following takes place between electrodes?

a) Electric current flow

b) Electron flow

c) All of the mentioned

d) None of the mentioned

16. Amount of mass dissolved is directly proportional to which of the following

quantities?

a) Amount of electricity

b) Frequency of vibrations

c) Amplitude of oscillations

d) All of the mentioned

17. What are the values of voltages used in ECM?

a) 1 to 8 V

b) 10 to 30 V

c) 40 to 80 V

d) 90 to 110 V

18. How does the current pass between the two electrodes in ECM?

a) Electrolytic solution

b) Direct contact of electrodes

c) All of the mentioned

d) None of the mentioned

19. Feed control system is responsible for which action in ECM?

a) Giving feed to tool

b) Electrolyte supply

c) Power supply

d) None of the mentioned

20. When local metal removal rates are high how will be the current density and current

efficiency?

a) High

b) Medium

c) Low

d) Very low

21. State whether following statement is true or false regarding the electrolytes in ECM.

a) True

b) False

22. When the electrolyte flow is low, what happens to the current efficiency?

a) Increases

b) Decreases

c) Remains same

d) Increase and then decrease

23. In Electrochemical machining, larger grain size causes which type of finish?

a) Smoother

b) Rougher

c) Finer

d) All of the mentioned

24. Which type of gap width is necessary for higher degree of accuracy?

a) Very small

b) Small

c) Medium

d) High

25. Which of the following electrolytes are used for machining purpose in ECG?

a) Sodium nitrate

b) Hydrochloric acid

c) Nitric acid

d) Potassium permanganate

26. What is the main mechanism of material removal in Electro chemical grinding?

a) Mechanical erosion of material

b) Electro chemical dissolution

c) Melting and vaporization

d) Electron removal from material

27. Removal rates of ECG process are how may time to that of the conventional

machining processes?

a) 2 times

b) 3 times

c) 4 times

d) 5 times

28. How much amount of burr is produce in Electro chemical grinding process?

a) Less amount

b) Moderate amount

c) High amount

d) No burrs are produced

29. Which of the following electrolytes is used for ECH process?

a) Sodium chloride

b) Sodium nitrate

c) Hydrochloric acid

d) Sulphuric acid

30. State whether following statement is true or false about ECH process.

a) True

b) False

Answer: 1.(d), 2.(d), 3.(d), 4.(d), 5.(a), 6.(a), 7.(c), 8.(d), 9(d), 10.(d), 11.(d), 12.(a), 13.(d),

14.(b), 15.(c), 16.(a), 17.(b), 18.(a), 19.(a), 20.(a), 21.(b), 22(b), 23.(b), 24.(b), 25.(a), 26.(b),

27.(c), 28.(d),29 .(b), 30.(b),

Fill in the Blanks with Answers

1. The system which consists of electrolytic solution and electrodes can be referred as

____.

2. Electrolytic solution should ensure ____type of anodic dissolution?

3. For a better surface finish, ____ type of current distribution is required?

4. Fine dimensional control can be obtained if throwing power of electrolyte is ____.

5. In ECDB process, rotating and feeding the tool electrode _____ the deburring process.

6. Electro chemical grinding is _______ Electro chemical machining.

7. The value of gap voltage maintained in ECG process is _______.

8. The grinding wheel is _____ and the work piece is ____ in ECG process.

9. ECH has _____ material removal rates than conventional honing or the internal

cylindrical grinding.

10. ECH employs_____ type of abrasive stone for removal of material?

Answer: 1. Electrolytic cell, 2. Uniform, 3. Even, 4. Low, 5. Enhances, 6. Similar to, 7. 4 to

40 V, 8. Cathode, anode , 9. Higher, 10. Reciprocating

UNIT-III

Two Marks Questions with Answers

1. Explain the purpose of dielectric in EDM process.

Ans.:

Electro Discharge Machining (EDM) is an electro-thermal non-traditional machining process,

where electrical energy is used to generate electrical spark and material removal mainly

occurs due to thermal energy of the spark.

EDM is mainly used to machine difficult-to-machine materials and high strength temperature

resistant alloys. EDM can be used to machine difficult geometries in small batches or even on

job-shop basis. Work material to be machined by EDM has to be electrically conductive.

Functions of dielectric:

(i) Breakdown electrically in the shortest possible time once the breakdown voltage has

been reached.

(ii) Quench the spark rapidly or deionize the spark gap after the discharge has occurred.

(iii) Provide an effective cooling medium.

(iv) Be capable of carrying away the swarf particles in suspension, away from the working

gap.

(v) Provide insulation between the electrode and the work piece.

2. Explain the principle of operation of wire-cut EDM process.

Ans.:

EDM, primarily, exists commercially in the form of die-sinking machines and wire process, a

slowly moving wire travels along a prescribed path and removes material from the work-

piece. Wire EDM uses electro-thermal mechanisms to cut electrically conductive materials.

The material is removed by a series of discrete discharges between the wire electrode and the

work-piece in the presence of dielectric fluid, which creates a path for each discharge as the

fluid becomes ionized in the gap. The area where discharge takes place is heated to extremely

high temperature, so that the surface is melted and removed. The removed particles are

flushed away by the flowing dielectric fluids.

The wire EDM process can cut intricate components for the electric and aerospace industries.

This non-traditional machining process is widely used to pattern tool steel for die

manufacturing cutting machines.

3. Write the advantages and applications of electric discharge grinding.

Ans.:

Advantages:

(i) There is no physical contact between the tool and the work-piece and hence no cutting

forces act on the work-piece. Even fragile work-pieces or jobs can be machined using

this process.

(ii) This process can be applied to electrically conductive materials. Physical and

metallurgical properties of the work material such as strength, toughness,

microstructure are no barrier to its application.

(iii) Since metal removal is due to thermal effects, there is no heating (or thermal damage)

to the work-piece material.

(iv) Complicated die contours in hard materials can be produced to a high degree of

accuracy and surface finish.

(v) The overall production rate compares well with the conventional process because it can

dispense with operations like grinding.

(vi) The process can be automated easily requiring very little attention from the machine

operator.

(vii) The actual surface produced by EDM consists of small craters, which help in the

retention of lubricants.

Applications:

(i) Manufacture of tools having complicated profiles and for a number of other

components.

(ii) For making stamping tools, wire drawing and extrusion dies, forging dies, intricate

mould cavities etc.

4. Quote the factors that affect the surface finish and machining accuracy in EDM.

Ans.:

Machining accuracy:

Taper:The holes produced by this process are usually tapered due to the presence of a frontal

spark accompanied by a side spark. The taper at any section of the work piece has been found

to be proportional to d2.

Overcut: Overcut in EDM is due to side sparks and is dependent on the gap length and crater

dimensions. The overcut O can be expressed by the relationship

O = A .C1/3 + B

where A and B are constants the values of which depend upon the tool work pair.

Dependence of the overcut on the capacitance of the R-C circuit.

Surface finish:

The surface produced by the EDM process consists of a multitude of small craters randomly

distributed all over the machined face. The CLA value of the surface finish in this case ranges

between 2 and 4 . The quality of surface mainly depends upon the energy per spark. If the

energy content is high deeper craters will result leading to a poor surface. The surface

roughness has also been found to be inversely proportional to the frequency of discharge.

5. Write the factors for selection of machine tool in EDM.

Ans.:

A variety of EDM machines ranging from small machines to large units are now

commercially available. The factors that have to be considered in their selection are :

(i) number of parts to be machined

(ii) accuracy required

(iii) size of the work-piece

(iv) depth of the cavity

(v) orientation of the cavity.

Equipment must be versatile and accurate for tool room work where a variety of work

piece configuration is encountered, EDM machine tool design and construction is a function

of the accuracy required. In cases where the positioning accuracy need not be held closer than

0.025 or 0.050 mm, a conventional coordinate table can be used to obtain the position read-

out from the lead screw via the hand wheel dial. For higher accuracy, an optical read-out

independent of the lead screw is desirable.

Large sized jobs require machines with high rigidity to avoid excessive deflection. High

rigidity is also essential whilst working with large sized electrodes. The electrode holding

column must be made rigid enough to support the weight of the electrode and also to

withstand the coolant back pressure, a peculiarity of this process.

6. What do you mean by wear ratio? List the factors that influence it.

The amount of erosion suffered by the electrode compared with that by the work piece is

referred to as wear ratio and depends on:

(i) The physical and chemical properties of both the electrode and work piece material and

(ii) The environmental conditions.

Wear ratio WR =

Apart from the these melting point ratio, other factors which also influence the wear ratio are:

a. Metal removal rate: Generally the wear ratio increases with cutting rate.

b. Cross-Sectional area of the electrodes: Wear ratio is reduced with increase in cross-

sectional area.

c. Work piece material: Sintered hard-metals induce considerably higher wear ratio.

d. Configuration of the electrode: A form with slender projections or a narrow section will

exhibit a higher tool wear than one of a similar cross-section which is more compact.

Thus a star section will show more wear than a circular section

e. Depth: Deep cavities forms will provide a higher electrode tool wear than shallow ones.

7. Differentiate between Wire Cut and Conventional EDM.

Ans.:

Conventional EDM, as described above, uses a tool to disperse the electric current. This tool,

the cathode, runs along the metal piece, the anode, and the electrical current reacts to melt or

vaporize the metal. As a result of the dielectric fluid, what little debris produced washes away

from the piece.

Wire cut EDM discharges the electrified current by means of a taut thin wire, which acts as

the cathode and is guided alongside the desired cutting path, or kerf. A dielectric fluid

submerges the wire and work-piece, filtering and directing the sparks. The thin wire allows

precision cuts, with kerfs as wide as three inches and a positioning accuracy of +/-

This heightened precision allows for complex, three dimensional cuts, and produces highly

Accurate punches, dies and stripper plates.

Wire cut EDM equipment is run by computer numerically controlled (CNC) instruments,

which can control the wire on a three-dimensional axis to provide greater flexibility whereas

conventional EDM cannot always produce tight corners or very intricate patterns, wire

increased precision allows for intricate patterns and cuts. Additionally, wire EDM is

able to cut metals as thin as

At a certain thickness, wire EDM will simply cause the metal to evaporate, thereby

eliminating potential debris. The wire of a WCEDM unit emits sparks on all sides, which

means the cut must be thicker than the wire itself. In other words, because the wire is

surrounded by a ring of current, the smallest and most precise cutting path possible is the

added diameter of the ring and wire; technicians easily account for this added dimension.

Manufacturers continue to produce thinner and thinner wires to allow for smaller kerfs and

even finer precision.

Three Marks Questions with Answers

1. Discuss the factors to be considered in the selection of tool electrode material and

dielectric fluid used in EDM.

Ans.:

The spark erosion process is basically a copying process and the shape and accuracy of the

machined part will therefore depend primarily on the shape and accuracy of the tool or

cutting electrode. The spark erosion machine should ensure that under identical conditions,

the maximum accuracy in size and shape is obtained.

It has been observed that in the EDM erosion process, both the work piece as well as the

electrode gets eroded. Hence, the accuracy of the machined part obtained depends on the

electrode wear.

The main factors determine the suitability of a material for use as an electrode are:

(ii) It should be a good conductor of electricity and heat.

(iii) It should be easily machinable to any shape at a reasonable cost.

(iv) It should produce efficient material removal rates from the work pieces

(v) It should resist the deformation during the erosion process

(vi) It should exhibit low electrode wear rates

(vii) It should be available in a variety of shapes

Various electrode materials used are graphite, copper, copper graphite, brass, zinc alloys,

steel, copper tungsten, silver-tungsten, tungsten etc.

For dielectric fluids to be used in the EDM process, it is essential that they should:

(i) Remain electrically non-conductive until the required breakdown voltage is reached,

that is, they should have high dielectric strength.

(ii) Breakdown electrically in the shortest possible time once the breakdown voltage has

been reached.

(iii) Quench the spark rapidly or deionize the spark gap after the discharge has occurred.

(iv) Provide an effective cooling medium.

(v) Be capable of carrying away the swarf particles in suspension, away from the working

gap.

(vi) Have a good degree of fluidity

(vii) Be cheap and easily available.

(viii) Provide insulation between the electrode and the work piece.

The common dielectric fluids that can be used are transformer oil, paraffin oil, kerosene,

lubricating oils or various petroleum distillate fractions. Recently distilled water has also

been used in place of dielectric fluid and this has been found to permit very high metal

removal rates.

2. Explain briefly the characteristic of spark eroded surface and explain the factors

needed for machine tool selections in EDM.

Ans.:

Characteristics of spark eroded surfaces:

In EDM, the material removal is due to thermal phenomenon and local temperature in the

region (8000 to 120000C), this temperature will have an effect on the structure and the

mechanical properties of machined surfaces. The effect may or may not be significant,

depending upon the type of work material and the working conditions employed. Sometimes

tiny micro-cracks can be observed, particularly in the machining of tungsten carbide or other

hard materials. The size of micro-cracks depends on the type of material and the electrical

parameters, such as the pulse energy and duration. Generally speaking the crack depth

increases with pulse duration and energy.

Sparked eroded extrusion dies

Surface finish:

The surface produced by the EDM process consists of a multitude of small craters randomly

distributed all over the machined face. The CLA value of the surface finish in this case ranges

between 2 and 4 . The quality of surface mainly depends upon the energy per spark. If the

energy content is high deeper craters will result leading to a poor surface. The surface

roughness has also been found to be inversely proportional to the frequency of discharge.

Machine tool selection:

A variety of EDM machines ranging from small machines to large units are now

commercially available. The factors that have to be considered in their selection are the

i)number of parts to be machined ii) accuracy required iii) size of the work piece iv) depth of

the cavity v) orientation of the cavity

Equipment must be versatile and accurate for tool room work where a variety of work piece

configuration is encountered, EDM machine tool design and construction is a function of the

accuracy required. In cases where the positioning accuracy need not be held closer than 0.025

or 0.050mm, a conventional coordinate table can be used to obtain the position read-out from

the lead screw via the hand wheel dial. For higher accuracy an optical read out independent

of the lead screw is desirable.

Large sized jobs require machines with high rigidity to avoid excessive deflection. High

rigidity is also essential whilst working with large sized electrodes. The electrode holding

column must be made rigid enough to support the weight of the electrode and also to

withstand the coolant back pressure a peculiarity of this process.

3. Briefly explain the process characteristics, surface finish and machining accuracy

in EDM process.

Process characteristics:

Material removal in EDM mainly occurs due to intense localized heating almost by point heat

source for a rather small time frame. Such heating leads to melting and crater formation as

shown in Fig.1

The metal removal rates depend upon the following parameters:

(i) Current in each spark

(ii) Frequency of the discharge

(iii) Electrode material

(iv) Work piece material

(v) Dielectric flushing condition

The followings are the product quality issues in EDM

(i) Surface finish

(ii) Overcut

(iii) Tapercut

As shown in Fig. 2 sparks take place side by side. They occur completely randomly so that

over time one gets uniform average material removal over the whole tool cross section. But

for the sake of simplicity, it is assumed that sparks occur side by side as shown in Fig. 2.

The amount of metal removed is dependent upon the current in the spark. The metal removed

by the spark can be assumed to be a crater as shown in Fig. 2. The amount removed therefore

will depend upon the crater depth, which is directly proportional to the current. Thus as

shown schematically in figure the material removed increases and at the same time the

surface finish decreases.

However, decreasing the current in the spark and increasing its frequency improves the

surface finish in view of the small crater size, but at the same time the material removal rate

can be maintained by increasing the frequency as well as increase with forced circulation of

dielectric fluid.

MACHINING ACCURACY:

Taper: The holes produced by this process are usually tapered due to the presence of a

frontal spark accompanied by a side spark as shown in Fig.3. The taper at any section of the

work piece has been found to be proportional to d2 (d=diameter of the tool).

Fig. 1 Schematic representation of crater formation in EDM process

Fig. 2 Schematic representation of the sparks in EDM process.

Work-piece

Taper cut and over cut Taper cut prevention

Fig. 3 Schematic depiction of taper cut and over cut and control of taper cut

Tool

Overcut: Overcut in EDM is due to side sparks and is dependent on the gap length and crater

dimensions. The overcut O can be expressed by the relationship

O = A . + B

where A and B are constants the values of which depend upon the tool work pair, C is the

capacitance .

Thus it may be noted that surface roughness in EDM would increase with increase in spark

energy and surface finish can be improved by decreasing working voltage, working current

and pulse on time.

In EDM, the spark occurs between the two nearest point on the tool and work-piece. Thus

machining may occur on the side surface as well leading to overcut and taper-cut as depicted

in Fig. 3. Taper cut can be prevented by suitable insulation of the tool. Overcut cannot be

prevented as it is inherent to the EDM process. But the tool design can be done in such a way

so that same gets compensated.

5. Discuss the analysis of R-C circuit in EDM and prove that for maximum power

delivery, the discharge voltage should be 72% of supply voltage.

Ans.:

The relaxation circuit shown in figure can be considered to be made up of the

(i) Charging circuit

(ii) Discharging circuit

c by

the D.C. source of voltage V0 a heavy current ic will flow in the circuit. The voltage across

Elementary R-C circuit

the gap (which is almost the same as that across the capacitor) Vc varies with the time

according to the relation

Vc = V0 [1- ] --------(i)

If Rc×C, C=( Rc)

Vc= V0[1-e-tc

/ ] --------(ii)

The charging current(ic) can be specified by ic=( V0 e-tc

/ )/ Rc --------(iii)

The energy delivered per spark

P=(1/2)(CVc2)

=(1/2) ( Rc) [V0 {1- }]2

P=( V02 Rc)[(1/2)- e-t

c / +(1/2) e-2t

c / ]-----------(iv)

Average power is given by

Pave=(P/tc) =[( V02 (Rc×tc)][(1/2)- e-t

c / +(1/2) e-2t

c / ]-----------(v)

For maximum power delivery through the circuit where x c)

i.e. and (V02/Rc)=Constant

(V02/Rc) [ e-1/x e-2/x] =0

The solution of this equation is x=0.793

c)=0.793

(tc

Substituting this value in equation (ii)

Vc= V0[1-e-1.26]=0.716 V0 V0

Hence Vc V0

Thus for maximum power delivery the discharge voltage (Vc) should be 72%of the supply

voltage.

8. Explain the advantages and applications of Electric Discharge Grinding (EDG)

over conventional grinding.

Ans.:

Advantages:

(i) Material removal rates ranges from 0.16 to 2.54 cm3/min and surface finishes in the

range of 1.6 to 3.2 µm Ra are possible. Three layers must be removed or modified in

case of highly stressed applications.

(ii) The process produces good surface finish, accuracy and repeatability.

(iii) Hardened work-pieces can also be machined since the deformation caused by it does

not affect the final dimensions.

(iv) EDG is a burr free process.

(v) Hard die materials with complicated shapes can be easily finished with good surface

finish and accuracy through EDG process.

(vi) Due to the presence of dielectric fluid, there is very little heating of the bulk material.

(vii) The ED milling is the unique development in EDG process for machining insulating

materials such as Al2O3.

Applications:

(i) Steel and carbide at the same time without wheel loading.

(ii) Thin sections on which abrasive wheel pressures might cause distortion.

(iii) Brittle materials or fragile parts on which abrasive materials might cause fracturing.

Five Marks Questions with Answers

1. Explain the general principle, working and applications of Electric Discharge

Machining process with neat sketch.

Ans.:

Principle:

Electric discharge machining is a process of metal removal based on the principle of erosion

of metals by an interrupted electric spark discharge between the electrode tool and the

workpiece. Fundamentally, the electric erosion effect is understood by the breakdown of

electrode material accompanying any form of electric discharge. The discharge is usually

through a gas, liquid or any cases through solids. A necessary condition for producing a

discharge is ionization of the dielectric i.e. splitting up of its molecules into ions and

electrons.

Electric Discharge Machining

Working:

Fig shows the schematic layout of the EDM system. The main components are (i) power

supply (ii) dielectric medium (iii) work- piece and the tool (iv) servo control.

The work-piece and the tool are electrically connected to a D.C. electric power supply. The

work-piece is connected to the positive terminal of the electric source, so that it becomes the

anode and the tool is the cathode. A gap known as the spark gap ranges of 0.005 to 0.05 mm

is maintained between the work-piece and the tool and suitable dielectric slurry is forced

through this gap at pressure of 2kgf/cm2 or less. When a suitable voltage range of 50-450v is

applied, the dielectric breaks down and electrons are emitted from cathode and the gap is

ionized. In fact a small ionized fluid column is formed owing to formation of electrons in the

spark gap where the process of ionization collision takes place. When more electrons collect

in the gap the resistance drops causing electric spark to jump between work surface and tool.

Each electric discharge or spark causes a focused stream of electrons to move with a very

high velocity and acceleration from the cathode towards the anode and creates compression

shock waves on both the electrode surface, closest to the tool. The generation of compression

shock waves develops a local rise in temperature. The whole sequence of operation occurs

within a few-microseconds. However the temperature of spot hit by electrons is of order of

10,0000C. This temperature is sufficient to melt a part of the metals. The forces of electric

and magnetic fields caused by the spark produce a tensile force and tear-off molten particles

from this spot in the work piece. A part of the metal may vaporize and fill up the gap. The

metal is thus removed in this way from the work piece. The electric and magnetic fields on

the heated metal cause a compressive force to act on the cathode tool so that metal removal

from the tool is at a slower rate than that from the work-piece.

The current density in the discharge of channel is of the order of 10000A/cm2, power density

of the order of 500 MW/cm2.

Electro-hydraulic servo control is usually preferred. The servo gets its input signal from the

difference between a selected reference voltage and the actual voltage across the gap. The

signal is amplified and the tool as it wears a little is advanced by hydraulic control. A short

circuit across the gap causes the servo to reverse the motion of the tool until the correct gap is

stabilized.

Applications:

(i) Hardened steel dies, stamping tools, wire drawing and extrusion dies, header dies,

forging dies, intricate mould cavities and such parts are made by the EDM process.

(ii) The process is widely used for machining of exotic materials that are used in aerospace

and automotive industries.

(iii) EDM being a non-contact type of machining process, it is very well suited for making

fragile parts that cannot take the stress of machining.

(iv) Ex: washing machine agitators, electronic components, printer parts and difficult to

machine features such as the honeycomb shapes.

(v) Deep cavities, slots and ribs can be easily made by EDM.

(vi) Micro-EDM process can successfully produce micro-pins, micro-nozzles and micro-

cavities.

2. Describe the process of Wire cut EDM and list its advantages, applications, and

limitations.

Ans.:

EDM, primarily, exists commercially in the form of die-sinking machines and wire process, a

slowly moving wire travels along a prescribed path and removes material from the

workpiece. Wire EDM uses electro-thermal mechanisms to cut electrically conductive

materials. The material is removed by a series of discrete discharges between the wire

electrode and the work-piece in the presence of dielectric fluid, which creates a path for each

discharge as the fluid becomes ionized in the gap. The area where discharge takes place is

heated to extremely high

temperature, so that the surface is melted and removed. The removed particles are flushed

away by the flowing dielectric fluids.

Schematic process of Wire cut EDM

The wire EDM process can cut intricate components for the electric and aerospace industries.

This non-traditional machining process is widely used to pattern tool steel for die

manufacturing cutting machines (Wire EDM). The concept of wire EDM is shown in Figure.

In this the wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or

brass coated wires are also used extensively in this process. The wire used in this process

. Wire EDM can also

employ to cut cylindrical objects with high precision.

This process is usually used in conjunction with CNC and will only work when a part is to be

cut completely through. The melting temperature of the parts to be machined is an important

parameter for this process rather than strength or hardness. The surface quality and MRR of

the machined surface by wire EDM will depend on different machining parameters such as

applied peak current, and wire materials.

The wires for wire EDM is made of brass, copper, tungsten, molybdenum. Zinc or brass

coated wires are also used extensively in this process. The wire used in this process should

cut cylindrical objects with high precision.

Advantages:

(v) Wire EDM has more effective metal-cutting capabilities than laser, flame-cut, plasma,

or die cutting.

(vi) The most intricate parts and delicate shapes, including small or odd angles, sharp

corners, contours, cavities, and external or internal tapers can be cut.

(vii) Wire EDM machines cut to very tight tolerances +/- .0001" (.0025mm).

(viii) Since the entire process is computer- and robotics-controlled, we can create duplicate

parts that are virtually identical

Applications:

(i) Wire EDM has been employed for making various types of dies. It is possible to control

tolerances effectively.

(ii) The process is also used for fabrication of press tools and electrodes for use in other

areas of EDM.

Limitations:

(i) Only able to machine conductive materials.

(ii) More expensive process than conventional process

3. Discuss the basic requirements and considerations of tool materials in EDM

process. Name any four tool materials.

Ans.:

The spark erosion process is basically a copying process and the shape and accuracy of the

machined part will therefore depend primarily on the shape and accuracy of the tool or

cutting electrode. The spark erosion machine should ensure that under identical conditions,

the maximum accuracy in size and shape is obtained.

It has been observed that in the EDM erosion process, both the work piece as well as the

electrode get eroded. Hence, the accuracy of the machined part obtained depends on the

electrode wear.

The basic requirements of tool (electrode) materials in EDM process are:

EDM Performance

of the EDM cut and remove metal. In generating a spark, peak current is discharged only

after the gap between the electrode and work-piece has been ionized. At this point, the

electrode emits electrons that collide with the molecules of the dielectric fluid. As a result,

the fluid is vaporized and an energy channel is formed that allows the spark to take place. For

this to happen, the electrode material must be hot enough for electrons to absorb enough

energy to escape and work to create the energy channel.

For copper electrodes to release electrons in the gap, the temperature must be high enough for

these electrons to absorb sufficient energy. As a result, these high temperatures tend to burn

some of the copper electrode away. In order to generate this heat, the on times for copper

electrodes are generally much higher than for graphite electrodes. However, due to its carbon

base, a graphite electrode is able to emit these electrons at much lower temperatures and does

not require the extended on times for electrons to release and create the energy channel.

Therefore, the time required to form the energy channel is considerably less. Since the

graphite initializes the spark faster, significantly higher metal removal rates are the result.

Wear

Electrode wear is a constant concern because excessive wear results in adding electrodes or

redressing electrodes more often. Graphite is able to achieve electrode wear of less than 1

percent in relation to the depth of cut at machine parameters much more aggressive than

particles from the work-piece may penetrate into the structure of the graphite electrode and

reduce the amount of wear caused by the EDM process. The melting temperature of most

standard work metals is around 1,500°C. This exceeds that of copper, which is around

1,100°C. Any molten particles being ejected out of the EDM cut are likely to carry a

thermoelectric charge and have temperatures that do not permit them to affix to the copper

electrode. In this case, the molten particles may actually create secondary discharging and

erode the copper electrode away.

This means that the high amperage and long on times of a roughing condition (and the

process of replating) actually preserve the graphite electrode but is detrimental to the copper

electrode which erodes away at these settings. On the contrary, in the finishing stages, with

low amperage and short on-times, the graphite electrode has a tendency to wear at a faster

rate than copper. However, since electrode wear is a ratio of the amount of material removed

in the EDM cut, the actual wear on either a graphite or copper electrode is minimal in the

finishing stage and sometimes immeasurable.

Surface finish

Because it is cast as a solid with no porosity, it goes without saying that copper electrodes

provide very fine surface finishes. However, with the sophistication of

technology, the surface finish gap between graphite and copper has narrowed significantly.

Fine-grain graphite electrodes are now able to deliver surface finishes similar to copper with

comparable electrode wear. With the proper electrode material selection and machine

parameters, graphite is able to achieve near mirror finishes without the use of a powder

additive and mirror-like finishes with the additive.

Electrode material considerations:

Graphite is produced with a wide range of material characteristics, enabling to match the

electrode material properties to the EDM application. Less-critical applications with electrode

features such as a large radius, an open tolerance or minimal EDM requirements would use

an economically-priced graphite material with large particles and lower strengths. However, a

highly detailed electrode with critical features, extreme tolerances and stringent EDM

requirements would entail a more premium graphite to fit the needs of this application.

On the other hand, due to the high purity value required for efficient machining, the types of

copper available on the market and used in EDM applications are limited. This minimizes the

ability to match material characteristics to the EDM application. The most commonly used

types are electrolytic copper and tellurium copper, which vary slightly in elemental

composition and are both cast as a solid.

Copper is often considered a commodity material and can be less costly than graphite in the

bulk or blank stage. Many graphite materials are considered specialty materials and, as such,

can be more costly. In addition, copper is recyclable whereas graphite is limited in its ability

to be reclaimed. However, with the wide range of graphite materials available on the market

today, it is possible to find some low-quality EDM grades that are more economical than

copper. To establish the true cost of copper versus graphite, the value of machining the

electrode must also be considered. Even with the more expensive graphite materials, the

machining costs often offset any savings that are realized with the copper.

Various electrode materials used are graphite, copper, copper graphite, brass, zice alloys,

steel, copper tungsten, silver-tungsten, tungsten etc.

4. Explain about R-C relaxation circuit used in EDM process.

Ans.:

In the EDM process, Electrical energy in the form of short duration impulses are required to

be supplied to the machining gap. For this purpose, especially designed generators are

employed. The generators for spark erosion are distinguished according to the way in which

the voltage is transformed and the pulse is controlled and also on the basis of the

characteristics of discharge.

The discharge may be produced in a controlled manner by natural ignition and relaxation or

by means of a controllable switching element. For example, electronic value, thyristor,

transistor etc. The discharge may take place with constant or changing polarity.

On the basis of these facts generators for EDM can be classified into:

(i) Relaxation generators

(ii) Rotary pulse generators

(iii) Static pulse generators

(iv) Relaxation Generators:

Relaxation generators:

The relaxation or the R-C circuit shown in figure was the first to be used in EDM. The circuit

consists of a DC power source, which charges the capacit c.

Initially when the capacitor is in the unchanged condition, when the power supply is on with

a voltage V0, a heavy current ic will flow in the circuit as shown to charge the capacitor.

Elementary relaxation circuit for EDM

c = V0 [1-

]

From the above equation the condenser voltage (Vc), with a time constant equal to (R C) and

after tc = RC, the condenser voltage (Vc) will be 63 percent of the supply voltage (V0). A

discharge, across the working gap will occur if Vc equals the breakdown voltage (Vb) of the

dielectric within the gap. After the discharge, the dielectric deionizes, the capacitor is

recharged and the cycle repeats itself. The time taken to recharge the capacitor to the

breakdown voltage must be sufficient to allow the dielectric to deionize.

In practice the spark gap is adjusted so that the discharge takes place corresponding to the gap

voltage. Although a higher gap voltage would be liberate much more energy, the time

required to recharge the condenser increases. Thus the benefit from higher energy content per

spark is more than offset by a reduction in the number of condenser discharge per unit time. It

has been found in an R-C circuit, that for a given condenser and breakdown voltage, there

In a relaxation generator, the spark repetition rate, for a given supply voltage and capacitance

, cannot be increased beyond a critical value and is determined by the sped at which the

spark gap is deionized and cleared of the debris after each discharge. Forced circulation of

dielectric through the gap is necessary if high metal removal rate is desired. As the working

gap is is of the order of 0.025-0.05 mm, forced circulation is difficult, especially when large

electrode is involved. In such cases, a lower erosion rate must be accepted than is possible

with small size electrodes.

The fundamental advantages of relaxation circuits are their

(i) Comparative cheapness

(ii) Simplicity of design

(iii) Robustness and

(iv) Relatively extensive range of discharge.

They remain the only practical means of generating low energy ranges and high frequencies

required for fine finishing and delicate operations.

In spite of many modifications of relaxation circuits, they are liable to result in

(i) High tool wear

(ii) Slow metal removal rates, compared with other types of generators.

5. Explain the principle, working and process of metal removal in Electric Discharge

Grinding (EDG) with neat sketch.

Ans.:

Detail of EDG process

Principle:

Electrical discharge grinding (EDG) is a non-traditional thermal process for machining

difficult to machine hard and brittle electrically conductive materials. EDG has been

developed by replacing the stationary electrode used in electrical discharge machining

(EDM) with rotating electrode. In EDG process, material is removed melting and

vaporization as same as EDM process. But there are ample differences with EDM instead of

mechanism of material. In EDG process, an electrically conductive wheel is used as a tool

electrode instead of stationary tool electrode used in EDM. There is no contact with work-

piece and tool electrode (rotating wheel) except during electric discharge. Due to the

rotational motion of wheel electrode, the peripheral speed of wheel transmitted to the

stationary dielectric into gap between work-piece and wheel resulting flushing efficiency of

process is enhanced. Therefore, the molten material is effectively ejected from gap and no

debris accumulation take place into gap while in EDM debris accumulation is major problem

which adverse effect on performances of process. Due to the enhanced in flushing, higher

material removal and better surface finish is obtained as compare to the conventional EDM

process. At the same machining condition, EDG gives better performances than EDM and it

is machined extremely hard materials faster (2-3 times) as compare to the conventional

grinding. The high speed of wheel is not always beneficial and after a certain value of speed,

the spark becomes instable and produces adverse effect on performance. There is no physical

contact between work-piece and wheel, so that the process becomes more advantageous for

machining thin and fragile electrically conductive materials.

Working and process of metal removal:

The detail of EDG process has been illustrated in figure and wheel-work-piece interaction is

shown in. In this process, a rotating eclectically conductive metallic wheel is used which is

known as grinding wheel. The grinding wheel used in this process, having no any abrasive

particles and rotates its horizontal axis. Due to the similarities of process with conventional

grinding and material is removed due to the electrical discharge, it is known as electrical

discharge grinding (EDG). In this process, the spark is generated between rotating wheel and

work-piece. The rotating wheel and work-piece both are separated by dielectric fluid and

during machining both (work-piece and wheel) are continuously dipped into dielectric fluid.

The dielectric fluids are mainly Kerosene oil, Paraffin oil, Transformer oil or de-ionized

water. The main purpose of dielectric is to make a conductive channel during ionization when

suitable breakdown voltage is applied. The servo control mechanism utilized to maintain the

constant gap between work-piece and wheel in range of 0.013-0.075 mm. A pulse generator

is used for maintaining the DC pulse power supply in ranges of voltage, current and

frequency are 30-400V, 30-100A and 2-500 kHz respectively. When pulse power supply is

applied, the spark takes place into gap due to the ionization and striking of ions and electrons

at their respective electrodes. Due to spark, high temperature generated between ranges of

8000°C to 12000°C or as so high up to 200000c by each spark resulting material is meted

from both the electrodes. Simultaneously DC pulse power supply switch is deactivated

resulting the breakdown of spark occurs and fresh dielectric fluid entering into gap. Due to

the high flushing efficiency, the molten materials flush away in form of micro debris from

gap and formed the crater on work surface.

In EDG without abrasive particle, the wheel is made of graphite which rotates on is

horizontal axis but instead of graphite wheel, some other materials are used for making wheel

for EDG process such as copper, brass and mild steel. Due to the high wear resistance, the

mild steel wheel gives low wheel wear as compare to the copper and brass wheel. The main

developments in EDG without abrasives are: electro-discharge grinding and electro-discharge

milling (ED milling). In EDG process with abrasive particle, the rotating wheel replaced with

metal bonded abrasive wheel or and such types of wheel is known as composite wheel. In

composite wheel, the main purposes of abrasive particles are: to enhanced the material

removal, to achieve better surface finish and requirement of low grinding forces. Electro-

discharge abrasive grinding (EDAG) is the main development of EDG process with abrasive

wheel.

Multiple Choice Questions with Answers

1. What is the value of order of frequency applied between the two electrodes in EDM?

a) 1 kHz

b) 3 kHz

c) 5 kHz

d) 7 kHz

2. What are the magnitudes of voltages used in Electro discharge machining?

a) 1 to 20 V

b) 20 to 120 V

c) 120 to 220 V

d) 220 to 320 V

3. How is material removed in Electro discharge machining?

a) Melt and evaporate

b) Corrode and break

c) Mechanical erosion takes place

d) None of the mentioned

4. What are the values of temperature that are obtained while machining using EDM?

a) 2000 to 3000 C

b) 4000 to 6000 C

c) 8000 to 12000 C

d) 15000 to 20000 C

5. Which of the following are main components of EDM?

a) Dielectric system

b) Servomechanism

c) Power supply

d) All of the mentioned

6. Which of the following parameters determines the size of cavities?

a) Size of electrode

b )Radius of orbit

c) All of the mentioned

d) None of the mentioned

7. Which one among the following, is the most important factor in determining the tool

wear?

a) Melting point

b) Boiling point

c) Power supplied

d) Feed rate

8. Which of the following are the main requirements of dielectric fluids?

a) Viscosity

b) High flash point

c) Minimum odor

d) All of the mentioned

9. What happens to the material removal rate if the sparks are very less in EDM?

a) Decreases

b) Increases

c) Increase and then decrease

d) All of the mentioned

10. What happens to the surface roughness values if the MRR increases in EDM?

a) Increases

b) Decreases

c) Decrease and increase

d) None of the mentioned

11. Which of the following materials can be machined using Electro discharge

machining?

a) Heat resistant alloys

b) Super alloys

c) Carbides

d) All of the mentioned

12. Which of the following are the applications of Electro discharge machining?

a) Holes

b) Slots

c) Texturing

d) All of the mentioned

13. Which motion of tool is used for machining spherical surfaces in Electro discharge

machining?

a) Oscillatory

b) Vibratory

c) Rotary

d) All of the mentioned

14. Which path of the components in wire EDM determines the path to be machined?

a) Horizontal worktable movement

b) Vertical worktable movement

c) All of the mentioned

d) None of the mentioned

15. Which of the following materials are machined using Wire Electro discharge

machining?

a) Polycrystalline diamond

b) Cubic Boronitride

c) Matrix composites

d) All of the mentioned

Answer:1.(c), 2.(b), 3.(a), 4.(c), 5.(d), 6.(c), 7.(a), 8.(d), 9.(a), 10.(a), 11.(d), 12.(d), 13.(c),

14.( a), 15.)(d)

Fill in the Blanks with Answers

1. Copper has _____ Electro discharge machining wear and _____ conductivity.

2. Quality of hole produced by orbiting motion is _____ to that obtained by using

stationary electrode.

3. Few large holes are _____ than many small flushing holes in Electro discharge

machining.

4. _____amount of burr is produce when we use Wire Electro discharge machining for

machining of work pieces?

5. _____ is the value of gap maintained between the electrodes when we use servo

mechanism.

6. Electrode wear ratios can be expressed as_____.

7. In Electro discharge machining, _____ type of dielectric flow mentioned below is

desirable.

8. In Electro discharge machining, materials with low melting point have _____ type of

material removal rate.

9. If the sparks are very less in EDM material removal rate _____.

10. Wire EDM is a special form of Electro discharge machining which contains _____

electrode.

Answer: 1.Good, better, 2.Superior, 3.Worse, 4.No burr, 5.200 500 µm, 6. End wear, Side

wear, 7. Steady flow, 8. High 9. Decreases 10. Continuously moving

UNIT-IV

Two Marks Questions with Answers

1. Explain the mechanism of metal removal of EBM.

Electron Beam Machining (EBM) is a thermal process. Here a steam of high speed electrons

impinges on the work surface so that the kinetic energy of electrons is transferred to work

producing intense heating. Depending upon the intensity of heating the work-piece can melt

and vaporize. The process of heating by electron beam is used for annealing, welding or

metal removal. During EBM process very high velocities can be obtained by using enough

voltage of 1, 50,000 V can produce velocity of 228,478 km/sec and it is focused on 10 200

µm diameter. Power density can go up to 6500 billion W/sq.mm. Such a power density can

vaporize any substance immediately. Complex contours can be easily machined by

maneuvering the electron beam using magnetic deflection coils.

2. Differentiate the thermal and non-thermal processes of EBM

Thermal type: In this type of EBM process the surface of the thermo electronic cathode is

heated to high temperature that the electrons acquire speed to escape out to the shape around

the cathode. As a result the work piece is heated by the bombardment of these electrons in a

localized area to melt and vaporize at the point of bombardment.

Non thermal type: In this type of EBM process the electron beam is used to cause a

chemical reaction

3. Quote the advantages and limitations of EBM

Ans.:

Advantages:

(v) This process is not dependent on the work piece material properties.

(vi) This process can be applied to hard and soft materials

(vii) This process suitable for cutting delicate shapes.

(viii) No mechanical distortion

Limitations:

(vi) High capital cost of the equipment and necessary regular maintenance applicable for

any equipment using vacuum system.

(vii) Need of auxiliary backing material.

(viii) Low MRR

(ix) Very high specific energy consumption.

(x) Heat affected zone is rather less in EBM but recast layer formation cannot be avoided.

4. What is the full form of LASER in advanced machining processes? Explain the

different types of lasing medium.

Ans.:

The full form of LASER is Light Amplification by Simulated Emission of Radiation.

Many materials can be used as the heart of the laser. Depending on the lasing medium lasers

are classified as solid state and gas laser. Solid-state lasers are commonly of the following

type

Ruby which is a chromium alumina alloy having a wavelength of

Nd-

Nd-

These solid-state lasers are generally used in material processing.

The generally used gas lasers are

Helium Neon

Argon

CO2 etc.

Lasers can be operated in continuous mode or pulsed mode. Typically CO2 gas laser is

operated in continuous mode and Nd YAG laser is operated in pulsed mode.

5. List some EBM parameters.

The process parameters, which directly affect the machining characteristics in EBM, are:

(i) The accelerating voltage

(ii) The beam current

(iii) Pulse duration

(iv) Energy per pulse

(v) Power per pulse

(vi) Lens current

(vii) Spot size

(viii) Power density

Three Marks Questions with Answers

1. Explain the process parameters of EBM

The process parameters, which directly affect the machining characteristics in Electron Beam

Machining, are:

The accelerating voltage

The beam current

Pulse duration

Energy per pulse

Power per pulse

Lens current

Spot size

Power density

In EBM the gun is operated in pulse mode. This is achieved by appropriately biasing the

biased grid located just after the cathode. Switching pulses are given to the bias grid so as to

Beam current is directly related to the number of electrons emitted by the cathode or

-amp to 1 amp.

Increasing the beam current directly increases the energy per pulse. Similarly increase in

pulse duration also enhances energy per pulse. High-energy pulses (in excess of 100 J/pulse)

can machine larger holes on thicker plates.

The energy density and power density is governed by energy per pulse duration and spot size.

Spot size, on the other hand is controlled by the degree of focusing achieved by the

electromagnetic lenses. A higher energy density, i.e., for a lower spot size, the material

removal would be faster though the size of the hole would be smaller.

The plane of focusing would be on the surface of the work-piece or just below the surface of

the work-piece.

2. Explain the advantages, limitations and applications of LBM.

Ans.:

Advantages:

i. In laser machining there is no physical tool. Thus no machining force or wear of the

tool takes place.

ii. Large aspect ratio in laser drilling can be achieved along with acceptable accuracy or

dimension, form or location

iii. Micro-holes can be drilled in difficult to machine materials

iv. Though laser processing is a thermal processing but heat affected zone specially in

pulse laser processing is not very significant due to shorter pulse duration.

Limitations:

i. High initial capital cost

ii. High maintenance cost

iii. Not very efficient process

iv. Presence of Heat Affected Zone specially in gas assist CO2 laser cutting

v. Thermal process not suitable for heat sensitive materials like aluminium glass fibre

laminate

Applications:

Laser can be used in wide range of manufacturing applications

Material removal drilling, cutting and tre-panning

Welding

Cladding

Alloying

Drilling micro-sized holes using laser in difficult to machine materials is the most

dominant application in industry. In laser drilling the laser beam is focused over the desired

spot size. For thin sheets pulse laser can be used. For thicker ones continuous laser may be

used.

Five Marks Questions with Answers

1. Summarize Electron Beam Machining with neat sketch and the process

parameters.

Ans.:

Electron Beam Machining (EBM) is a thermal process. Here a steam of high speed

electrons impinges on the work surface so that the kinetic energy of electrons is

transferred to work producing intense heating. Depending upon the intensity of heating

the work-piece can melt and vaporize. The process of heating by electron beam is used

for annealing, welding or metal removal. During EBM process electrons with very high

velocities can be obtained by using enough voltage of 1, 50,000 V can produce velocity

of 228,478 km / sec and it is focused on 10 200 µm diameter. Power density can go up

to 6500 billion W / sq.mm. Such a power density can vaporize any substance

immediately. Complex contours can be easily machined by maneuvering the electron

beam using magnetic deflection coils. To avoid a collision of the accelerating electrons

with the air molecules, the process has to be conducted in vacuum. So EBM is not

suitable for large work pieces. Process is accomplished with vacuum so no possibility of

contamination. No effects on work piece because about 25-50 µm away from machining

spot remains at room temperature and so no effects of high temperature on work.

Schematic diagram of Electron Beam Machining process

Process parameters:

The process parameters, which directly affect the machining characteristics in Electron Beam

Machining, are:

The accelerating voltage

The beam current

Pulse duration

Energy per pulse

Power per pulse

Lens current

Spot size

Power density

In EBM the gun is operated in pulse mode. This is achieved by appropriately biasing the

biased grid located just after the cathode. Switching pulses are given to the bias grid so as to

Beam current is directly related to the number of electrons emitted by the cathode or

-amp to 1 amp.

Increasing the beam current directly increases the energy per pulse. Similarly increase in

pulse duration also enhances energy per pulse. High-energy pulses (in excess of 100 J/pulse)

can machine larger holes on thicker plates.

The energy density and power density is governed by energy per pulse duration and spot size.

Spot size, on the other hand is controlled by the degree of focusing achieved by the

electromagnetic lenses. A higher energy density, i.e., for a lower spot size, the material

removal would be faster though the size of the hole would be smaller.

The plane of focusing would be on the surface of the work-piece or just below the surface of

the work-piece.

2. Discuss the Process capability of EBM and explain how it influences on machining

quality?

Ans.:

EBM can pr

mm, i.e., with a l/d ratio of around 10. Fig. shown schematically represents a typical hole

drilled by electron beam. The hole can be tapered along the depth or barrel shaped. By

focusing the beam below the surface a reverse taper can also be obtained. Typically as shown

in Fig. there would be an edge rounding at the entry point along with presence of recast layer.

Generally burr formation does not occur in EBM.

A wide range of materials such as steel, stainless steel, Ti and Ni super-alloys, aluminium as

well as plastics, ceramics, leathers can be machined successfully using electron beam. As the

mechanism of material removal is thermal in nature as for example in electro-discharge

machining, there would be thermal damages associated with EBM. However, the heat-

affected zone is rather narrow due to shorter pulse duration in EBM. Typically the heat-

Variations in drilling speed with volume of material removal for steels and aluminium

Some of the materials like Al and Ti alloys are more readily machined compared to steel.

Number of holes drilled per second depends on the hole diameter, power density and depth of

the hole as well as material type as mentioned earlier. Fig. depicts the variation in drilling

speed against volume of material removed for steel and aluminium alloy.

EBM does not apply any cutting force on the work-pieces. Thus very simple work holding is

required. This enables machining of fragile and brittle materials by EBM. Holes can also be

drilled at a very shallow angle of as less as 20 to 300.

3. Compare of thermal and non-thermal processes of EBM and list the advantages,

limitations and applications.

Ans.:

(i) Thermal type: In this type of EBM process the surface of the thermo electronic

cathode is heated to high temperature that the electrons acquire speed to escape out to the

Aluminium alloy

Steel

shape around the cathode. As a result the work piece is heated by the bombardment of these

electrons in a localized area to melt and vaporize at the point of bombardment.

(ii) Non thermal type: In this type of EBM process the electron beam is used to cause a

chemical reaction.

Advantages:

(ix) This process is not dependent on the work piece material properties.

(x) This process can be applied to hard and soft materials

(xi) This process suitable for cutting delicate shapes.

(xii) No mechanical distortion

Limitations:

(xi) High capital cost of the equipment and necessary regular maintenance applicable for

any equipment using vacuum system.

(xii) Need of auxiliary backing material.

(xiii) Low MRR

(xiv) Very high specific energy consumption.

(xv) Heat affected zone is rather less in EBM but recast layer formation cannot be avoided.

Applications:

(iv) Suitable for drilling fine holes.

(v) Cutting integrated circuit board

(vi) Apply to soft as well as hard materials

4. With the help of a neat diagram explain the working of a Laser Beam Machining.

Ans.:

As the name implies in Laser Beam Machining the source of energy is the LASER

(Light Amplification by Simulated Emission of Radiation). The laser beam focuses optical

energy on the surface of the work-piece. A laser beam can be so powerful when used with

lens system that it can melt and vaporize diamond as the energy density can be of the order of

105 kW/cm2. This huge amount of energy is released due to some specific atoms having

higher energy levels and particular frequency.

Different types of lasers are used in Laser beam machining (LBM). For example solid state,

gas and semiconductor. At times high power lasers are required for machining and welding

and in those cases only solid state lasers can provide such power levels.

Ruby-laser or crystalline aluminium oxide or saphire is the most commonly used solid state

laser. Generally these lasers are fabricated in into rods having length about 150 mm. Their

ends are well furnished to close optical tolerances. Figure below shows a schematic view of

laser beam machining process.

(a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of

holes produced in nonmetallic parts by LBM.

A small amount of chromium oxide is added to dope the ruby crystal. A flash of high

intensity light, generally Xenon-filled flash lamp is used to pump the laser. To fire the xenon

lamp a large capacitor is required to be discharged through it and 250 to 1000 watts of

electric power is needed to do this. The intense radiation discharged from the lamp excites the

fluorescent impurity atoms (chromium atoms) and these atoms reach a higher energy level.

After passing through a series of energy levels when the atoms fall back to original energy

level, an intense beam of visible light emission is observed. This beam is reflected back from

the coated rod ends and makes more and more atoms excited and stimulated and return to

ground level. A stimulated avalanche of light is obtained which is transmitted through the

coated part (~80% reflective). This light which is highly coherent in time and space has a

very narrow frequency band, is highly in phase and quite parallel. If this light is focused in

association with ordinary lenses on the desired spot of the w/p, high energy density is gained

which helps to melt and vaporize the metal.

5. State the mechanism of metal removal, advantages and limitations of laser beam

machining process with neat sketch.

Ans.:

Mechanism of metal removal:

Laser beam machining (LBM) is a non-traditional subtractive manufacturing process, a form

of machining, in which a laser is directed towards the work piece for machining. This process

uses thermal energy to remove material from metallic or nonmetallic surfaces. The laser is

focused onto the surface to be worked and the thermal energy of the laser is transferred to the

surface, heating and melting or vaporizing the material. Laser beam machining is best suited

for brittle materials with low conductivity, but can be used on most materials.

Mechanism of metal removal of LBM

Advantages:

(i) No tool wear as there is no direct contact between tool and workpiece.

(ii) Metal and non-metals (e.g plastics and rubbers) irrespective of their brittleness and

hardness can be machined.

(iii) Laser beam can go through a long distance as a result LBM can be used to weld, drill or

cut areas which are difficult to reach.

(iv) Laser beam welding gives the opportunities to weld/cut magnetic as well as heat treated

materials without losing their properties. (some change in the properties is observed in

the heat affected zone).

(v) Any environment is suitable for laser beam machining through transparent medium

and magnetic fields.

(vi) Very little distortion is observed and tow materials can be easily joined together.

(vii) Difficult-to-machine or refractory materials can be drilled.

(viii) Micro sized holes can created in all types of materials.

(ix) Energy obtained is of high density as a result high heat is obtained.

(x) Beam configuration and size of exposed area is easily controllable.

(xi) Precise location of the spot is ensured.

(xii) By applying unidirectional multiple pulses deep holes of very short diameter can be

drilled.

Limitations:

(xv) The initial cost is very high and lifespan of the flash lamp is short.

(xvi) The safety procedures are needed to be followed very strictly.

(xvii) Material removal rate is not up to the mark.

(xviii) While machining some plastics bum or char is noticed.

(xix) Too deep holes are not possible to drill.

(xx) Machined holes are not round shaped or straight.

(xxi) Overall efficiency is very low (0.3 ~0.5 %).

Multiple Choice Questions with Answers

1. Which of the following are the applications of EBM process?

a) Drilling

b) Cutting

c) Engraving

d) All of the mentioned

2. Which of the following are the advantages of EBM process?

a) Drilling rates

b) No distortion

c) High accuracy

d) All of the mentioned

3. How are the production times for a material in electron beam machining?

a) Very small

b) Small

c) Moderate

d) Long

4. Which type of thin cast layer is produced when we use EBM?

a) Thin layer

b) Thick layer

c) No layer

d) All of the mentioned

5. The tungsten filament cathode is heated to what temperatures in EBM?

a) 1000 to 1500 C

b) 1500 to 2000 C

c) 2500 to 3000 C

d) 3500 to 5000 C

6. What happens to the cutting speed with an increase in the work piece?

a) Increases

b) Decreases

c) Enhances

d) Remains same

7. State whether following statement is true or false regarding the drilling using LBM.

a)True

b) False

8. Which of the following has high machining speeds in Laser Beam Machining?

a) Metals

b) Non metals

c) Metal alloys

d) All of the mentioned

9. How are minimum energies required for plastics when compared to that required for

metals?

a) Lower than

b) Higher than

c) Same as

d) None of the mentioned

10. What happens to the material removal rate when reflectivity is higher?

a) Will decrease

b) Will increase

c) Will enhance

d) Remains same

Answer:1.(d), 2.(d), 3.(d), 4.(a), 5.(c), 6.(b), 7.(b), 8.(b), 9.(a), 10.(a)

Fill in the Blanks with Answers

1. The full form of LBM in advanced machining processes is _____.

2. Peak power should be increased in Laser drilling process by ______ pulse energy.

3. Laser beam machining uses ______ type of power sources for machining.

4. Machining of LBM takes place when power density is ____ than what is lost by

conduction and radiation.

5. _____ pulse energy and _____ pulse duration are suitable for drilling.

6. _____are the values of currents used in EBM process.

7. _____ is the value of largest diameter of the hole drilled on EBM.

8. The tolerance value obtained in EBM_____.

9. The capital equipment cost of equipment used in EBM_____.

10. _____type of thin cast layer is produced in EBM.

Answer: 1.Laser Beam Machining, 2. Increasing, 3.High power, 4.Greater, 5. High, short, 6.

20 and 100 mA, 7. 1.5 mm, 8.± 10 %, 9. High cost, 10. Thin layer

UNIT-V

Two Marks Questions with Answers

1. Discuss the plasma arc welding and its applications.

Ans.:

Plasma arc welding (PAW) is a process of joining of metals, produced by heating with a

constricted arc between an electrode and the work piece (transfer arc) or the electrode and the

constricting nozzle (non transfer arc). Shielding is obtained from the hot ionized gas issuing

from the orifice, which may be supplemented by an auxiliary source of shielding gas.

Transferred arc process produces plasma jet of high energy density and may be used for high

speed welding and cutting of Ceramics, steels, Aluminum alloys, Copper alloys, Titanium

alloys, Nickel alloys.

Non-transferred arc process produces plasma of relatively low energy density. It is

used for welding of various metals and for plasma spraying (coating).

Welding cryogenic, aerospace and high temperature corrosion resistant alloys.

Nuclear submarine pipe system (non-nuclear sections, sub assemblies).

Welding steel rocket motor cases.

Welding of carbon steel, stainless steel, nickel, copper, brass, monel, inconel, aluminium,

titanium, etc.

Melting, high melting point metals.

2. Define the term Plasma. Explain the basic principle of PAM.

Ans.:

Plasma is defined as a superheated, electrically ionized gas .In this process gases are heated

and charged to plasma state. Plasma state is the superheated and electrically ionized gases at

approximately 5000oC. These gases are directed on the work-piece in the form of high

velocity stream.

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt

and displace material in its path .This is a method of cutting metal with a plasma-arc, or

tungsten inert-gas-arc, torch. The torch produces a high velocity jet of high temperature

ionized gas called plasma that cuts by melting and removing material from the work piece.

Temperatures in the plasma zone range from 20,000° to 50,000° F (11,000° to 28,000° C). It

is used as an alternative to oxyfuel-gas cutting, employing an electric arc at very high

temperatures to melt and vaporize the metal.

3. Infer chemical machining (CHM).

Ans.:

Chemical machining is the subtractive manufacturing process of using baths of temperature-

regulated etching chemicals to remove material to create an object with the desired shape. It

is mostly used on metals, though other materials are increasingly important. It was developed

from armor-decorating and printing etching processes developed during the Renaissance as

alternatives to engraving on metal. The process essentially involves bathing the cutting areas

in a corrosive chemical known as an etchant, which reacts with the material in the area to be

cut and causes the solid material to be dissolved; inert substances known as maskants are

used to protect specific areas of the material as resists.

This controlled chemical dissolution will simultaneously etch all exposed surfaces even

though the penetration rates of the etch may be only 0.0005 0.0030 in./min. The basic

process takes many forms: chemical milling of pockets, contours, overall metal removal,

chemical blanking for etching through thin sheets; photochemical machining (PCM) for

etching by using of photosensitive resists in microelectronics; chemical or electrochemical

polishing where weak chemical reagents are used (sometimes with remote electric assist) for

polishing or deburring and chemical jet machining where a single chemically active jet is

used.

4. Identify the basic steps in chemical milling.

Ans.:

The basic steps in CHM for metal removal are:

(i) Residual stress relieving: If the part to be machined has residual stresses from the

previous processing, these stresses first should be relieved in order to prevent warping

after chemical milling.

(ii) Preparing: The surfaces are degreased and cleaned thoroughly to ensure both good

adhesion of the masking material and the uniform material removal.

(iii) Masking: Masking material is applied (coating or protecting areas not to be etched).

(iv) Etching: The exposed surfaces are machined chemically with etchants.

(v) Demasking: After machining, the parts should be washed thoroughly to prevent further

reactions with or exposure to any etchant residues. Then the rest of the masking

material is removed and the part is cleaned and inspected.

5. Discuss limitations of CHM.

Ans.:

The limitations of CHM are: Handling and disposal of chemicals can be troublesome; hand

masking, scribing, and stripping can be time-consuming, repetitive, and tedious and design

changes can be implemented quickly; metallurgical homogeneous surfaces are required for

best results; slower process, very low MRR so high cost of operation; deep narrow cuts are

difficult to produce; difficult to get sharp corner; hydrogen pickup and intergranular attack

are a problem with some materials; complex designs become expensive; simultaneous

material removal, from all surfaces, improves productivity and reduces wrapping; the

straightness of the walls is subjected to fillet and undercutting limitations; difficult to

chemically machine thick material (limit is depended on workpiece material, but the

thickness should be around maximum (10mm); porous materials and part designs that have

deep, narrow cavities or folded-metal seams should be avoided.

6. Write the application of CHM.

The applications of CHM are:

Chemical Milling

It is widely used in aircraft industry. It is the preparation of complicated geometry on the

workpiece using CHM process.

Chemical Blanking

In this application cutting is done on sheet metal workpieces. Metal blanks can be cut from

very thin sheet metal, this cutting may not be possible by conventional methods.

Photochemical Machining

It is used in metal working when close (tight) tolerances and intricate patterns are to be made.

This is used to produce intricate circuit designs on semiconductor wafers.

Three Marks Questions with Answers

1. Explain the process details of PAM

Ans.:

Process details:

Plasma Gun

Gases are used to create plasma like, nitrogen, argon, hydrogen or mixture of these gases.

The plasma gun consists of a tungsten electrode fitted in the chamber. The electrode is given

negative polarity and nozzle of the gun is given positive polarity. Supply of gases is

maintained into the gun. A strong arc is established between the two terminals anode and

cathode. There is a collision between molecules of gas and electrons of the established arc.

As a result of this collision gas molecules get ionized and heat is evolved. This hot and

ionized gas called plasma is directed to the work-piece with high velocity. The established

arc is controlled by the supply rate of gases.

Power Supply and Terminals

Power supply (DC) is used to develop two terminals in the plasma gun. A tungsten electrode

is inserted to the gun and made cathode and nozzle of the gun is made anode. Heavy potential

difference is applied across the electrodes to develop plasma state of gases.

Cooling Mechanism

As we know that hot gases continuously comes out of nozzle so there are chances of its over

heating. A water jacket is used to surround the nozzle to avoid its overheating.

Tooling

There is no direct visible tool used in PAM. Focused spray of ho0t, plasma state gases works

as a cutting tool.

Work-piece:

Work-piece of different materials can be processed by PAM process. These materials are

aluminium, magnesium, stainless steels and carbon and alloy steels. All those material which

can be processed by LBM can also be processed by PAM process

2. Quote useful applications of plasma arc machining process.

Ans.:

Single runs autogenously and multi-run circumferential pipe welding.

In tube mill applications.

Welding cryogenic, aerospace and high temperature corrosion resistant alloys.

Nuclear submarine pipe system (non-nuclear sections, sub assemblies).

Welding steel rocket motor cases.

Welding of stainless steel tubes (thickness 2.6 to 6.3 mm).

Welding of carbon steel, stainless steel, nickel, copper, brass, monel, inconel,

aluminium, titanium, etc.

Welding titanium plates up to 8 mm thickness.

Welding nickel and high nickel alloys.

Melting, high melting point metals.

Plasma torch can be applied to spraying, welding and cutting of difficult to cut metals

and alloys.

3. Explain the function of masking and etching in CHM.

Ans.:

Masking is similar to masking action is any machining operation. This is the action of

selecting material that is to be removed and another that is not to be removed. The material

which is not to be removed is applied with a protective coating called maskant. This is made

of a materials are neoprene, polyvinylchloride, polyethylene or any other polymer. Thinkers

of maskent are maintained up to 0.125 mm. The portion of workpiece having no application

of maskent is etched during the process of etching. After the process is completed demasking

is done. Demasking is an act of removing maskent after machining.

Etching

In this step the material is finally removed. The work-piece is immersed in the enchant where

the material of work-piece having no protective coating is removed by the chemical action of

enchant. Enchant is selected depending on the work-piece material and rate of material

removal; and surface finish required. There is a necessity to ensure that maskant and enchant

should be chemically in active. Common enchants are H2SO4, FeCL3, HNO3. Selection of

enchant also affects MRR. As in CHM process, MRR is indicated as penetration rates

(mm/min).

4. Quote the advantages, limitations and applications of PAM

Ans.:

Advantages:

(a) It gives faster production rate.

(b) Very hard and brittle metals can be machined.

(c) Small cavities can be machined with good dimensional accuracy.

Limitations:

(a) Its initial cost is very high.

(b) The process requires over safety precautions which further enhance the initial cost of the

setup.

(c) Some of the work-piece materials are very much prone to metallurgical changes on

excessive heating so this fact imposes limitations to this process.

(d) It is uneconomical for bigger cavities to be machined.

Applications:

The chief application of this process is profile cutting as controlling movement of spray

focus point is easy in case of PAM process. This is also recommended for smaller machining

of difficult to machining materials.

Five Marks Questions with Answers

1. Describe the principle of operation and process details of Plasma Arc Machining

(PAM) with neat sketch.

Ans.:

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas to melt

and displace material in its path called PAM, this is a method of cutting metal with a plasma-

arc, or tungsten inert-gas-arc, torch. The torch produces a high velocity jet of high

temperature ionized gas called plasma that cuts by melting and removing material from the

work piece. Temperatures in the plasma zone range from 20,000° to 50,000° F (11,000° to

28,000° C). It is used as an alternative to oxy-fuel-gas cutting, employing an electric arc at

very high temperatures to melt and vaporize the metal.

Principle of operation:

PAM is a thermal cutting process that uses a constricted jet of high-temperature plasma gas to

melt and separate metal. The plasma arc is formed between a negatively charged electrode

inside the torch and a positively charged work piece. Heat from the transferred arc rapidly

melts the metal, and the high-velocity gas jet expels the molten material from the cut.

Schematic diagram of Plasma Arc Machining process

Applications of PAM

The chief application of this process is profile cutting as controlling movement of spray focus

point is easy in case of PAM process. This is also recommended for smaller machining of

difficult to machining materials.

2. Discuss applications of plasma in manufacturing industries.

Ans.:

The applications of plasma in manufacturing industries are:

Industrial

Plasma cleaning and Plasma activation in manufacturing of industrial products has become a

reliable tool in the optimization of process specifications.

differentiating your product from your competition with quality leads to customer retention,

cost reduction, and growth. The ability to consistently provide a stronger bond and a cleaner

surface in manufacturing processes provides a more reliable, defect free part to your

customer.

The application of plasma surface treatment can be used in industrial applications when a

requirement for increased bonding strength is needed in the area of painting, adhesive

assembly or bonding. Plasma processing can also be used to clean just about any surface

imaginable.

Plasma science and industry

The application of Gas Plasma to the everyday manufacturing of industrial products is

growing at an enormous rate. The need in industry to optimize every aspect of a product has

become necessary to remain competitive. No industry is untouched by the application of Gas

Plasma in the area of optimized product performance. Some of the markets that have seen

the most growth in application of Gas Plasma are aerospace, automotive, electronics, food

packaging, glass, marine, medical, military, optics, packaging, paint, paper, plastics, and

textiles.

own ability to innovate, the need to drive our product market and apply the technology at

hand.

Industrial manufacturing cleaning and adhesion optimization

The optimization of materials in the area of product performance is critical to being

competitive in Unfortunately the best materials for the product are not

always the easiest to assemble, bond or keep clean. Gas Plasma provides solutions to those

difficult manufacturing problems providing clean, activated surfaces that promote and

optimize a reliable bond for product performance. Gas Plasma is a clean, environmentally

friendly alternative to solvents and complicated, heavily regulated clean lines.

Application:

Single runs autogenously and multi-run circumferential pipe welding.

In tube mill applications.

Welding cryogenic, aerospace and high temperature corrosion resistant alloys.

Nuclear submarine pipe system (non-nuclear sections, sub assemblies).

Welding steel rocket motor cases.

Plasma torch can be applied to spraying, welding and cutting of difficult to cut metals

and alloys.

3. Demonstrate the working principle of chemical machining with neat sketch.

Ans.:

Chemical machining (CHM) process is a controlled chemical dissolution (CD) of a work-

piece material by contact with strong reagent (etchant). Special coatings called maskants

protect areas from which the metal is not to be machined. It is one of the non-conventional

machining processes as shown in figure.

Schematic sketch of CHM

The advancement of technology causes to the development of many hard-to-machine

materials: stainless steel, super alloys, ceramics, refractories and fiber-reinforced composites

due to their high hardness, strength, brittleness, toughness and low machinability properties.

Sometimes, the machined components require high surface finish and dimensional accuracy,

complicated shape and special size, which cannot be achieved by the conventional machining

processes. Moreover, the rise in temperature and the residual stresses generated in the

workpiece due to traditional machining processes may not be acceptable. These requirements

have led to the development of non- traditional machining (NTM) processes. In these

processes, the conventional cutting tools are not employed. Instead, energy in its direct form

is utilized. Chemical energy is used in chemical machining. This process is the precision

contouring of metal into any size, shape or form without the use of physical force, by a

controlled chemical reaction. Material is removed by microscopic electrochemical cell action,

as occurs in corrosion or chemical dissolution of a metal.

This controlled chemical dissolution will simultaneously etch all exposed surfaces even

though the penetration rates of the etch may be only 0.0005 0.0030 in./min. The basic

process takes many forms: chemical milling of pockets, contours, overall metal removal,

chemical blanking for etching through thin sheets; photochemical machining (PCM) for

etching by using of photosensitive resists in microelectronics; chemical or electrochemical

polishing where weak chemical reagents are used (sometimes with remote electric assist) for

polishing or deburring and chemical jet machining where a single chemically active jet is

used.

Chemical machining offers virtually unlimited scope for engineering and design ingenuity, to

gain the most from its unique characteristics, chemical machining should be approached with

the idea that this industrial tool can do jobs not practical or possible with any other metal

working methods. Chemical machining will likely prove to be of considerable value in the

solution of problems that are constantly arising as the result of the introduction of new

materials.

4. Explain the process details of CHM.

Ans.:

Following steps are normally followed in the process of CHM :

Cleaning

The first step of the process is a cleaning of workpiece, this is required to ensure that material

will be removed uniformly from the surfaces to be processed.

Masking

Masking is similar to masking action is any machining operation. This is the action of

selecting material that is to be removed and another that is not to be removed. The material

which is not to be removed is applied with a protective coating called maskant. This is made

of a materials are neoprene, polyvinylchloride, polyethylene or any other polymer. Thinkers

of maskent is maintained up to 0.125 mm. The portion of work-piece having no application

of maskent is etched during the process of etching.

Etching

In this step the material is finally removed. The work-piece is immersed in the enchant where

the material of work-piece having no protective coating is removed by the chemical action of

enchant. Enchant is selected depending on the work-piece material and rate of material

removal; and surface finish required. There is a necessity to ensure that maskant and enchant

should be chemically in active. Common enchants are H2SO4, FeCL3, HNO3. Selection of

enchant also affects MRR. As in CHM process, MRR is indicated as penetration rates

(mm/min).

Demasking

After the process is completed demasking is done. Demasking is an act of removing maskent

after machining.

5. Write the applications of CHM

Ans.:

Nontraditional machining processes are widely used to manufacture geometrically complex

and precision parts for aerospace, electronics and automotive and many other industries.

There are different geometrically designed parts, such as deep internal cavities, miniaturized

microelectronics and nontraditional machining processes may only produce fine quality

components. All the common metals including aluminum, copper, zinc, steel, lead, and nickel

can be chemically machined. Many exotic metals such as titanium, molybdenum, and

zirconium, as well as nonmetallic materials including glass, ceramics, and some plastics, can

also be used with the process. CHM applications range from large aluminum alloy airplane

wing parts to minute integrated circuit chips. The practical depth of cut ranges between 2.54

to 12.27 mm. Shallow cuts in large thin sheets are of the most popular application especially

for weight reduction of aerospace components. Multiple designs can be machined from the

same sheet at the same time. CHM is used to thin out walls, webs, and ribs of parts that have

been produced by forging, casting, or sheet metal forming.

Further process applications related to improving surface characteristics include the

following:

Elimination of alpha case from titanium forgings and super plastic formed parts.

Exclusion of the decarburized layer from low alloy steel forgings.

Removal of sharp burrs from conventionally machined parts of complex shapes.

Removal of a thin surface from forgings and castings prior to penetration inspection

below the surface (required for the detection of hidden defects).

Chemical machining is an effective method for the machining of shallow holes and

depressions, for profiling of the edges of sheet-metal work-pieces, and for machining of

shallow cavities of large surface areas (particularly in light alloys).

Multiple Choice Questions with Answers

1. In PAM material removal rates depend on which of the following factors?

a) Work piece material

b) Type of cutting

c) Shielding gases

d) All of the mentioned

2. How much distortion is produced while machining using PAM?

a) 10%

b) 20%

c) 30%

d) No distortion is produced

3. What is the maximum thickness of the walls of tube machined using plasma arc?

a) 10 mm

b) 30 mm

c) 50 mm

d) 70 mm

4. Which of the following solutions cannot be used as chemical reactive solution in

CHM?

a) Acidic solution

b) Alkaline solution

c) Neutral solution

d) None of the mentioned

5. Special coatings applied on work piece materials in order to protect them from

chemical reaction are known as?

a) Maskants

b) Protective coverings

c) Protective varnishing

d) None of the mentioned

6. State whether the following statement about Chemical milling is true or false.

a) True

b) False

7. Scribing plates are used to define, which of the following parameters in Chemical

milling?

a) Areas to be exposed

b) Volumes to be exposed

c) Areas not to be exposed

d) Volumes not to be exposed

8. In CHM which of the following factors, MRR will not be depend on?

a) Chemical uniformity

b) Metallurgical uniformity

c) Frequency uniformity

d) Temperature uniformity

9. State whether following statement is true or false about etching rates.

a) True

b) False

10. Which type of etching rate, mentioned below, produces low surface roughness?

a) Very low

b) Low

c) Medium

d) High

Answer: 1.(d), 2.(d), 3.(c), 4.(c), 5.(a), 6(a), 7(a), 8.(c), 9.(a), 10.(d)

Fill in the Blanks with Answers

1. PAM is the only process which works faster in ____ steel than _____ steel.

2. The value of the voltage range used in PAM is ____ to ____.

3. In PAM low power factors indicates _____ energy required and _____ removal rates.

4. The cut edge of the material tends to be _____ than the base metal in PAM.

5. To machine high quality parts using CHM _____ factors need not be necessary?

6. For softer materials, _____type of etching rates are obtained in CHM.

7. The depth of cut tolerances increases when machining ____ depths at high machining

rates?

8. ____are the advantages when we use Chemical milling process.

9. The values of scrap rates obtained in CHM are ____.

10. Good surface quality and absence of burr eliminates ____.

Answer: 1. Stainless, mild, 2. 30 250 V, 3. Low, high, 4. Harder, 5. Frequency of

vibrations, 6. Low, 7. Larger, 8. No burrs, 9. 3%, 10. Finishing operations

17. BEYOND SYLLABUS TOPICS WITH

MATERIAL

18. RESULT ANALYSIS

Academic Year Students Appeared Students Passed Pass Percentage (%)

2014-15 41 41 100

2015-16 35 34 97

2016-17 103 93 90

2017-18 59 39 66

EXTRA QUESTION SET

1.

processes? Is it justified to use this word in the context of the utilization of these

processes on the shop floor?

2. Make a comparison between traditional and unconventional machining processes in

terms of cost, application, scope, Machining time, advantages and limitations.

3. Explain the reasons for the development of Unconventional Machining Process. Discuss

about the criteria recommended in selection of these processes.

4. Give the complete classification of the unconventional machining process and explain

the factors to be considered for the selection of a process.

5. Explain the principle of USM and its equipment. Explain the factors, which influence

the MRR in USM.

6. Compare USM with traditional Abrasive machining.

7. Explain the following in detail i) Types of transducers for USM ii) Feed mechanisms in

USM iii) USM typical applications iv) Abrasives for USM

8. Explain the functions of Transducer and horns used in USM. List the tool materials

used.

9. Briefly explain the effect of operating parameters on MRR. List the applications of

USM.

10. With neat sketch describe the principle and equipment for Abrasive Jet machining.

(OR) Write the names of various elements of AJM and explain them in brief.

11. What is the fundamental principle of abrasive jet machining? Briefly explain with a neat

diagram, the AJM process. In AJM, how is material removal rate increased? Also state

how nozzle life is improved in such a machining process.

12. Explain the process parameter which controls the AJM machining quality.(or) With a

neat sketch explain the process of AJM? Explain the process control measures to be

taken to control quality and MRR.

13. Describe the principle and equipment for Water Jet Machining. Explain the different

applications and process control features of WJM.

14. Compare USM, WJM and AJM in terms of process capabilities and limitations.

15. With suitable sketches, explain the need for the insulation in an ECM process. List the

advantages, disadvantages and applications for this process.

16. Explain the ECM process. Explain how a replica of the tool is obtained. Mention the

advantages and applications.

17. Describe the principle of ECG and ECH. Discuss about the process parameters that

influences the ECM. List their applications and advantages.

18. Explain the working principle of electrochemical discharge grinding and discuss the

process capabilities and applications.

19. Explain the working of electro chemical grinding process with a neat sketch and explain

why the life of the ECG wheel is much higher than conventional grinding. Also list

down its advantages and limitations?

20. Briefly discuss about electro chemical deburring process parameters of chemical

machining process that influence the performance of the machining?

21. With the help of a neat sketch, explain the working of a spark erosion machine. (or)

With the help of neat sketch, describe the EDM process.

22. What are the desirable properties of a dielectric fluid? Give some examples for

dielectric fluids.

23. Explain the functions of dielectric fluid.

24. What are the important process parameters that control the material removal rate in

EDM? Explain any four factors

25. Explain the process of wire cut EDM and list any two of its advantages, limitations and

applications.

26. Explain the process of Electrical discharge grinding (EDG) and list any two of its

advantages, limitations and applications.

27. Explain the process of Electrical discharge wire cutting processes and list any two of its

advantages, limitations and applications.

28. Describe the Laser Beam Machining equipment and Electron Beam Machining

equipment. (i) Explain the production of laser beam and working principle of LBM?

29. What are the applications of EBM process?

30. Explain the features of EBM unit. Explain the effect of increasing the accelerating

potential on MRR.

31. Explain the principle of LBM with neat sketch and list out the advantages and

disadvantages?

32. Explain the thermal features of Laser beam machining. Discuss the performance of

various types of Lasers.

33. Discuss about the process capabilities of EBM and the process parameters of EBM in

improving machining quality.

34. What is the principle of plasma arc machining? What are the two stages in which the

process of material removal is affected? What is the main industrial application of

plasma cutting systems?

35. Define plasma. What are the gases used in PAM? What are the advantages of plasma

arc welding? What are the metals that can't be machined by plasma arc machining?

36. Briefly explain the following with respect to chemical machining: i) Characteristics of

cut peel maskants ii) Selection of maskants iii) Advantages of photo-resist maskant iv)

Limitations of chemical machining.

37. State the principle of chemical machining process. What is the purpose of etchants in

CHM? Name the etchants used in CHM. What is the use of maskant in CHM?