NON TRADITIONAL MACHINING AND THERMAL CUTTING PROCESS CHAPTER 26

NON TRADITIONAL MACHINING AND THERMAL CUTTING PROCESS

CHAPTER 26

Advanced Machining Processes

©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

NON TRADITIONAL MACHINING1. Mechanical energy process

1. Ultrasonic Machining

2. Water & Abrasive Jet

2. Electrochemical Machining( Deburring, Grinding)

3. Thermal Energy Process1. Electric Discharge

2. Electron Beam

3. Laser Beam

4. Arc Cutting

5. Oxyfuel cutting

4. Chemical Machining1. Mechanics and Chemistry of Chemical Machining

2. CHM Processes

5. Application Considerations


REQUIREMENT for NON TRADITIONAL MACHINING

1. The need to machine newly developed metals and non-metals. These materials often have special properties (e.g.,

high strength, high hardness, high toughness) that make them difficult or impossible to machine by conventional methods.

2. The need for unusual and/or complex part geometries that cannot easily be accomplished and in some cases are impossible to achieve by conventional machining.

3. The need to avoid surface damage that often accompanies the stresses created by conventional machining.

Many of these requirements are associated with the aerospace and electronics industries.


Classification of non traditional Machining

The classification is based on principal form of energy used Mechanical Electrical Thermal Chemical


MECHANICAL ENERGY PROCESSES

Several of the nontraditional processes that use mechanical energy other than a sharp cutting tool:

1. ultrasonic machining,

2. water jet processes,and

3. other abrasive processes.


Ultrasonic Machining (USM) Used for Hard and Brittle Materials like Ceramics

and glass Abrasive particles impacts on workpiece to achieve

metal removal The tool drives the abrasives contained in a slurry The tool oscillates perpendicular to work surface at

high frequency(20,000 HZ) and low amplitude(0.075mm) and fed slowly in the work surface

Tool Material is normally soft steel & Stainless Steel

Abrasive slurry includes boron nitride, boron carbide, Aluminum Oxide etc mixed with water(20% ~60%) and the grit size is propotional to amplitude

Tool and work both undergo abrasion with a ratio of 100:1 to 1:1.


Ultrasonic Machining (USM)

Abrasives contained in a slurry are driven at high velocity against the work by a tool vibrating at low amplitude and high frequency. The amplitudes are around 0.075 mm , and the

frequencies are approximately 20,000 Hz. The tool oscillates perpendicular to the work

surface, and is fed slowly into the work, the shape of the tool is formed in the part.

It is the action of the abrasives, impinging against the work surface, that performs the cutting.

Common tool materials are soft steel and stainless steel.



FIGURE 26.1Ultrasonic machining.




Abrasive materials in USM include boron nitride, boron carbide, aluminum oxide, silicon carbide, and diamond.

Grit size ranges between 100 and 2000. The vibration amplitude should be set

approximately equal to the grit size, the gap size maintained at about twice grit size. To a significant degree, grit size determines the

surface finish on the new work surface. the material removal rate increases with increasing

frequency and amplitude of vibration.


Ultrasonic Machining (USM) The slurry consists of a mixture of water and abrasive

particles. Concentration of abrasives in water ranges from

20% to 60%. The slurry must be continuously circulated to bring

fresh grains into action. It also washes away chips and worn grits. The cutting action operates on the tool as well as the

work. As the abrasive erode the work surface, they also

erode the tool. It is therefore important to know the relative volumes of

work material and tool material removed. This ratio varies for different work materials,

100:1 for cutting glass 1:1 for cutting tool steel.



Used for hard, brittle work materials, such as ceramics,

glass, and carbides. stainless steel and titanium. Shapes obtained by USM

Shapes obtained include non-round holes, holes along a curved axis, and coining operations,

in which an image pattern on the tool is imparted to a flat work surface.


Water Jet Cutting (WJC) Hydrodynamic or Water Jet cutting (WJC) High Pressure (400 MPA) High Velocity stream(900 m/s) from a nozzle

opening of 0.1 to 0.4 mm diameter The nozzle unit consists of

– a holder made of stainless steel, and – a jewel nozzle made of sapphire, ruby, or

diamond Filtration systems is used to separate the swarf

produced during cutting. Preferred cutting fluids are polymer solutions,

– because of their tendency to produce a coherent stream.

Water Jet Cutting (WJC)

FIGURE 26.2Water jet cutting.



Standoff distance: the separation between the nozzle opening and the work surface.

– generally desirable to be small to minimize dispersion of the fluid stream ( typically 3.2 mm).

size of nozzle orifice affects the precision of cut; – smaller openings are used for finer cuts on

thinner materials. – thicker jet streams and higher pressures are

required to cut thicker stock. Typical feed rates range from 5 to 500 mm/s,

depending on work material and its thickness Used for plastics, leather, textiles, composites,

tile, carpet, cardboard etc



Applications / advantages include: 1. no crushing or burning of the work surface

typical in other mechanical or thermal processes,

2. Minimum material loss because of the narrow cut slit,

3. no environmental pollution, and 4. ease of automating the process.

A limitation of the process -- not suitable for cutting brittle materials (e.g., glass)

because of their tendency to crack during cutting.

Water-Jet Cutting Process

Figure 27.16 (a) Schematic illustration of the water-jet machining process. (b) A computer-controlled water-jet cutting machine cutting a granite plate. (c) Examples of various nonmetallic parts produced by the water-jet cutting process. (Enlarged on next slide). Source: Courtesy of Possis Corporation

Nonmetallic Parts Made by Water-Jet Cutting

Enlargement of Fig. 27.16c. Examples of various nonmetallic parts produced by the water-jet cutting process. Source: Courtesy of Possis Corporation


Abrasive Water Jet Cutting (AWJC)

When WJC is used on metallic workparts, abrasive particles must be added to facilitate cutting.

This complicates the process by adding a number of parameters that must be controlled.

Additional parameters are – abrasive type, – grit size, and – flow rate.

Typical abrasive are– Aluminum oxide, – silicon dioxide, and – garnet (a silicate mineral)

grit sizes range between 60 and 120.


Abrasive Water Jet Cutting (AWJC)The abrasive particles are added to the water stream at about 0.25 kg/min as it exits the nozzle.Nozzle orifice diameters are 0.25 to 0.63 mm somewhat larger than in WJC to permit

higher flow rates and more energy contained in the stream.

Typical standoff distances are between 1/4 and 1/2 of those in WJC.

Abrasive Water Jet Cutting (AWJC)

Abrasive Water JetCutting (AWJC)


Abrasive Jet Machining (AJM) Abrasive particles with Gas is used The gas is dry and pressure is about 0.2 ~ 1.4

MPA Gas Velocity is about 2.5 to 5 m/s nozzle orifice diameter range from 0.075 to 1.0

mm. Gases include dry air, nitrogen, carbon dioxide,

and helium. Typical distances between nozzle tip and work

surface range between 3 and 75 mm. The workstation must be set up to provide proper

ventilation for the operator. It is generally a finishing process rather than a

production cutting process


Abrasive Jet Machining (AJM)

FIGURE 26.3 Abrasive jet machining (AJM).


Abrasive Jet Machining (AJM) Applications include

– deburring, – trimming and deflashing, – cleaning, and – polishing.

Cutting is accomplished on thin flat stock of hard, brittle materials (e.g., glass, silicon, mica, and ceramics)

Typical abrasives used include – aluminum oxide (for aluminum and brass),– silicon carbide (for stainless steel and

ceramics), and – glass beads (for polishing).


Abrasive Jet Machining (AJM)

Grit sizes are small, 15 to 40 µm in diameter, – must be uniform in size.

It is important not to recycle the abrasives because used grains become fractured (and therefore smaller in size), worn, and contaminated.

Abrasive-Jet Machining

Figure 27.17 (a) Schematic illustration of the abrasive-jet machining process. (b) Examples of parts produced through abrasive-jet machining, produced in 50-mm (2-in.) thick 304 stainless steel. Source: Courtesy of OMAX Corporation.

(b)


Abrasive Flow Machining (AFM) This process is used to deburr and polish difficult-

to-reach areas abrasive particles mixed in a viscoelastic polymer

(called the media) is forced to flow through or around the part surfaces and edges.

The polymer has the consistency of putty. Silicon carbide is a typical abrasive. particularly well-suited for internal passageways

that are often inaccessible by conventional methods.

The media flows past the target regions of the part under pressures ranging between 0.7 and 20 MPa.


Abrasive Flow Machining (AFM) Other applications include

– forming radii on sharp edges, – removing rough surfaces on castings, and– other finishing operations.

The process can be automated economically for mass production.

A common setup is to position the workpart between two opposing cylinders, one containing media and the other empty.

The media is forced to flow to and fro through the part between the two cylinders, as many times as necessary to achieve the desired material removal and finish.


Electrochemical Machining (ECM)

Remove material by anodic dissolution– the shape of the workpiece is obtained by a

formed electrode tool in close proximity to the work by a rapidly flowing electrolyte.

The machining process is similar to de-plating Tool is Cathode(-) & Workpiece is Anode(+) The principle underlying the process is

– material is deplated from the anode and deposited onto the cathode in an electrolyte bath.

– In ECM the electrolyte bath flows rapidly between the two poles to carry off the deplated material, so that it does not become plated onto the tool.



FIGURE 26.4Electrochemical machining (ECM).

Electrochemical Machining

Figure 27.6 Schematic illustration of the electrochemical machining process.


Electrochemical Machining (ECM) The electrode tool,

– usually made of copper, brass, or stainless steel,

– possess the inverse of the desired final shape of the part.

– an allowance is provided for the gap between the tool and the work.

the electrode is fed into the work at a rate equal to the rate of metal removal.

MRR depends upon Faradays First Law – the amount of chemical change produced by

an electric current (i.e., the amount of metal dissolved) is proportional to the quantity of electricity passed (current x time).

V= CIt, where is C is constant i.e: specific removal rate


Electrochemical Machining (ECM) Feed rate:

We should note that this equation assumes 100% efficiency of metal removal.

The actual efficiency is 90% to 100% It depends on

• tool shape, • voltage and current density, and • other factors.

Gap distance needs to be controlled closely.– If g becomes too large, the electrochemical process

slows down. – if the electrode touches the work, a short circuit

occurs, & process stops altogether. – Typical gap range between 0.075 to 0.75 mm.



ECM requires large amounts of electrical power. rate of metal removal is determined by electrical

power, specifically the current density. The voltage is kept relatively low to minimize

arcing across the gap.


Electrochemical Machining (ECM) Water is used as the base for the electrolyte. To reduce electrolyte resistivity, salts such as NaCl

or NaNO3 are added.

the flowing electrolyte removes– the material from the workpiece, – heat and– hydrogen bubbles created in the chemical

reactions. The removed microscopic material particles must

be separated from the electrolyte through – centrifuge, – sedimentation, or – other means.

The disposal of separated particles (thick sludge) is an environmental problem.


Electrochemical Machining (ECM) Generally used in applications in which

– the work metal is very hard or difficult to machine, or

– the workpart geometry is difficult (or impossible) to accomplish by conventional machining.

Work hardness makes no difference in ECM. Typical ECM applications include:

1. die sinking, which involves the machining of irregular shapes and contours into forging dies, plastic molds, and other shaping tools;

2. multiple hole drilling, in which many holes can be drilled simultaneously with ECM;

3. holes that are not round; and4. deburring.



Advantages include: 1. little surface damage to the workpart, 2. no burrs as in conventional machining, – low tool wear (the only tool wear results from the

flowing electrolyte), and1. relatively high metal removal rates for hard and

difficult-to-machine metals. Disadvantages are: 1. significant cost of electrical power to drive the

operation and 2. problems of disposing of the electrolyte sludge.

Parts Made by Electrochemical Machining

Figure 27.7 Typical parts made by electrochemical machining. (a) Turbine blade made of nickel alloy of 360 HB. Note the shape of the electrode on the right. (b) Thin slots on a 4340-steel roller-bearing cage. (c) Integral airfoils on a compressor disk.

Knee Implants

Figure 27.8 (a) Two total knee replacement systems showing metal implants (top pieces) with an ultra-high molecular-weight polyethylene insert (bottom pieces). (b) Cross-section of the ECM process as applies to the metal implant. Source: Courtesy of Biomet, Inc.


Electrochemical Deburing (ECD) It is an adaptation of ECM designed to

– remove burrs or to – round sharp corners by anodic dissolution.

The hole in the workpart has a sharp burr. The tool is designed to focus on the burr. Portions of the tool not being used for machining

are insulated. The electrolyte flows through the hole to carry

away the burr. The same operation principles of ECM also apply

to ECD. However, cycle times are much shorter. A typical cycle time is less than a minute. The time can be increased if it is desired to round

the corner.


Electrochemical Deburing (ECD)

FIGURE 26.5Electrochemical deburring (ECD).


Electrochemical Grinding (ECG) A special form of ECM a rotating grinding wheel with a conductive bond

material is used for the anodic dissolution of the workpart surface.

Abrasives used in ECG include aluminum oxide and diamond.

The bond material is – metallic (for diamond abrasives) or – resin bond impregnated with metal particles

(for aluminum oxide). The abrasive grits protruding from the grinding

wheel at the contact with the workpart establish the gap distance in ECG.

The electrolyte flows through the gap between the grains to play its role in electrolysis.


Electrochemical Grinding (ECG)

FIGURE 26.6Electrochemical grinding (ECG).


Electrochemical Grinding (ECG) Metal removal in ECG

– deplating is responsible for 95% or more, and– the abrasive action of grinding wheel removes

the remaining 5%or less, • mostly in the form of salt films.

Therefore the grinding wheel lasts much longer than in conventional grinding.– a much higher grinding ratio. – dressing of the grinding wheel is required much

less frequently. Applications include:

– sharpening of cemented carbide tools and– grinding of surgical needles, other thin wall

tubes, and fragile parts.

Electrochemical-Grinding Process

Figure 27.9 (a) Schematic illustration of the electrochemical-grinding process. (b) Thin slot produced on a round nickel-alloy tube by this process.


Thermal energy Process characterized by very high local temperatures

– hot enough to remove material by fusion or vaporization.

the high temperatures, cause physical and metallurgical damage to the new work surface.

1. Electric discharge ProcessesDie sink

Wirecut

2. Electron Beam Machining

3. Laser Machining & cutting

4. Plasma Arc Machining

5. Oxyfuel cutting


ELECTRIC DISCHARGE PROCESSES remove metal by a series of discrete electrical

discharges (sparks) – these cause localized temperatures high

enough to melt or vaporize the metal in the immediate vicinity.

used only on electrically conducting work materials.


Electric Discharge Machining (EDM) The shape of the finished work surface is

produced by a formed electrode tool. The sparks occur across a small gap between

tool and work surface. The process takes place in a dielectric fluid. The discharges are generated by a pulsating

direct current power supply. The region in which discharge occurs is heated

to extremely high temperatures.– a small portion of the work surface is

suddenly melted and removed. The flowing dielectric flushes away the small

particle.

Electric Discharge Machining (EDM)

FIGURE 26.7 Electric discharge machining (EDM): (a) overall setup, and (b) close-up view of gap, showing discharge and metal removal.

Electrical-Discharge Machining Process

Figure 27.10 (a) Schematic illustration of the electrical-discharge machining process. This is one of the most widely used machining processes, particularly for die-sinking applications. (b) Examples of cavities produced by the electrical-discharge machining process, using shaped electrodes. Two round parts (rear) are the set of dies for extruding the aluminum piece shown in front (see also Fig. 19.9b). (c) A spiral cavity produced by EDM using a slowly rotating electrode similar to a screw thread. (d) Holes in a fuel-injection nozzle made by EDM; the material is heat-treated steel. Source: (b) Courtesy of AGIE USA Ltd.

Stepped Cavities Produced by EDM Process

Figure 27.11 Stepped cavities produced with a square electrode by the EDM process. The workpiece moves in the two principle horizontal directions (x – y), and its motion is synchronized with the downward movement of the electrode to produce these cavities. Also shown is a round electrode capable of producing round or elliptical cavities. Source: Courtesy of AGIE USA Ltd.


Electric Discharge Machining (EDM) The removal of material increases the gap at

that location,– thus it is less likely to be the site of another

spark. the individual discharges remove metal at very

localized points, – they occur hundreds or thousands of times

per second – a gradual erosion of the entire surface

occurs in the area of the gap.


Electric Discharge Machining (EDM) Two important process parameters are

– discharge current and – frequency of discharges.

The best surface finish is obtained at high frequencies and low discharge currents.

As the tool penetrates into the work, overcutting occurs. – the distance by which the machined cavity

exceeds the size of the tool on each side. Overcut is a function of current and frequency

(amount to several hundredths of a millimeter). The high spark temperatures that melt the work

also melt the tool.


Electric Discharge Machining (EDM)

FIGURE 26.8(a) Surface finish in EDM as a function of discharge current and frequency ofdischarges. (b) Overcut in EDM as a function of discharge current and frequency of discharges.


Electric Discharge Machining (EDM) Tool wear is measured as the ratio of work

material removed to tool material removed (similar to the grinding ratio). – ranges between 1.0 and 100.

Electrodes are made of graphite, copper, brass, copper tungsten, silver tungsten, and other materials.

The selection depends on – the type of power supply circuit,– the type of work material, and – whether roughing or finishing is to be done.


Electric Discharge Machining (EDM) Graphite is preferred for many applications

– because it does not melt. – It vaporizes at very high temperatures, and– the cavity created by the spark is generally

smaller than for most other materials.– a high ratio of work material removed to tool

wear.The hardness and strength of the work material are not factors in EDM.The melting point of the work material is an important property.


Electric Discharge Machining (EDM) metal removal rate can be related to melting

point approximately by the following empirical formula,

where RMR = metal removal rate, mm3/s; K = constant of proportionality whose value= 664

in SI units; I = discharge current,amps; and T

m = melting temperature of work metal, C.


Electric Discharge Machining (EDM) Dielectric fluids used include

– hydrocarbon oils, – kerosene, and – distilled or deionized water.

It serves as – an insulator in the gap (except when ionization

occurs in the presence of a spark). – flush debris out of the gap and – remove heat from tool and workpart.


Electric Discharge Machining (EDM) Applications include

– tool fabrication and – parts production.

The tooling for many of the mechanical processes are often made by EDM, including – molds for plastic injection molding, – extrusion dies, – wire drawing dies, – forging and heading dies, and – sheet metal stamping dies.

Electric Discharge Machining (EDM) As in ECM, the term die sinking used for

producing a mold cavity, is sometimes referred to as ram EDM.

the materials used to fabricate the tooling are difficult (or impossible) to machine by conventional methods.

Certain production parts also call for application of EDM. – delicate parts that are not rigid enough to

withstand conventional cutting forces, – hole drilling where the axis of the hole is at an

acute angle to the surface, and– production machining of hard and exotic

metals.


Electric Discharge Wire Cutting (EDWC) Commonly called wire EDM, It uses a small diameter wire as the electrode to

cut a narrow kerf in the work. The cutting action is achieved by thermal energy

from electric discharges between the wire and the workpiece.

The workpiece is fed past the wire along the desired path, similar to a bandsaw operation.

the wire is slowly and continuously advanced between a supply and take-up spool to present a fresh electrode of constant diameter to the work.

This maintains a constant kerf width during cutting.


Electric Discharge Wire Cutting (EDWC)

FIGURE 26.9 Electric discharge wire cutting (EDWC), also called wire EDM.

The Wire EDM Process

Figure 27.12 Schematic illustration of the wire EDM process. As many as 50 hours of machining can be performed with one reel of wire, which is then discarded.

Metal removal rate:MRR=4x104 IT w

−1 .23

whereI= current in amperes

T w=melting temperature of workpiece, °C

Wire EDM

(a) (b)

Figure 27.13 (a) Cutting a thick plate with wire EDM. (b) A computer-controlled wire EDM machine. Source: Courtesy of AGIE USA Ltd.


Electric Discharge Wire Cutting (EDWC) A dielectric is applied by nozzles directed at the

tool–work interface, or the workpart is submerged in a its bath.

Wire diameters range from 0.076 to 0.30 mm,– depending on kerf width.

Materials used for the wire include brass, copper, tungsten, and molybdenum.

Dielectric fluids include deionized water or oil. As in EDM, an overcut exists that makes the kerf

larger than the wire diameter.– overcut is in the range 0.020 to 0.050 mm.

Once cutting conditions are established, overcut remains fairly constant and predictable.



FIGURE 26.10Definition of kerf and overcut in electric discharge wire cutting.


Electric Discharge Wire Cutting (EDWC) Although EDWC seems similar to a bandsaw

operation, its precision far exceeds that of a bandsaw. – The kerf is much narrower, – corners can be made much sharper, and – the cutting forces against the work are nil. – hardness and toughness of the work material

do not affect cutting performance. The only requirement is that the work material

must be electrically conductive.


Electric Discharge Wire Cutting (EDWC) The special features make it ideal for making

stamping dies. Because of narrow kerf, it is often possible to

fabricate punch and die in a single cut. Other applications include, tools and parts with

intricate outline shapes– lathe form tools, – extrusion dies, and – flat templates.


FIGURE 26.11Irregular outline cut from a solid metal slab by wire EDM. (Photo courtesy of LeBlond Makino Machine Tool Company, Amelia, Ohio.)



ELECTRON BEAM MACHINING (EBM) One of several industrial processes that use

electron beams. – other applications include heat treating,

welding and free form fabrication. It uses a high velocity stream of electrons

focused on the workpiece to remove material by melting and vaporization.

An electron beam gun generates a continuous stream of electrons– accelerated to approximately 75% of the

speed of light– focused through an electromagnetic lens on

the work surface.


ELECTRON BEAM MACHINING (EBM)

FIGURE 26.12 Electron beam machining (EBM).

Electron-Beam Machining Process

Figure 27.15 Schematic illustration of the electron-beam machining process. Unlike LBM, this process requires a vacuum, so workpiece size is limited to the

size of the vacuum chamber.


ELECTRON BEAM MACHINING (EBM) The lens is capable of reducing the area of the

beam to a diameter as small as 0.025 mm. On impinging, the kinetic energy of the electrons

is converted into thermal energy of extremely high density that melts or vaporizes the material in a very localized area.

Applications include– high-precision cutting of any known material.– drilling extremely small diameter holes; down

to 0.05 mm,– drilling of holes with very high depth-to-

diameter ratios—more than 100:1, and – cutting slots that are about 0.025 mm wide


ELECTRON BEAM MACHINING (EBM) cuts can be made to very close tolerances with

no cutting forces or tool wear. The process is ideal for micromachining. limitations:

– carried out in a vacuum chamber to eliminate collision of the electrons with gas molecules.

– high energy requirement and – expensive equipment.


The term laser stands for light amplification by stimulated emission of radiation.

It is an optical transducer that converts electrical energy into a highly coherent light beam.

A laser light beam – is monochromatic (single wave length) and– highly collimated (the light rays in the beam

are almost perfectly parallel). using conventional optical lenses, a laser can be

focused onto a very small spot with high power densities.

Laser Beam Machining (LBM)



FIGURE 26.13 Laser beam machining(LBM).

Laser-Beam Machining

(LBM)

Figure 27.14 (a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in nonmetallic parts by LBM. (d) Cutting sheet metal with a laser beam. Source: (d) Courtesy of Rofin-Sinar, Inc.


LBM uses the light energy from a laser to remove material by vaporization and ablation.

The types of lasers used are – carbon dioxide gas lasers and – solid-state lasers.

the energy of the coherent light beam is concentrated not only optically but also in terms of time.

The light beam is pulsed so that the released energy results in an impulse against the work surface – the melted material evacuating the surface at

high velocity.



It is used to perform various types of drilling, slitting, slotting, scribing, and marking operations.

Drilling small diameter holes is possible—down to 0.025 mm.

For larger holes (above 0.50-mm) the laser beam cut the outline of the hole.

not considered a mass production process. generally used on thin stock.



The range of work materials that can be machined is virtually unlimited.

Ideal properties of a material for LBM include – high light energy absorption, – poor reflectivity, – good thermal conductivity, – low specific heat, low heat of fusion, and – low heat of vaporization.

Of course, no material has this ideal combination of properties.

The actual list of work materials includes metals with high hardness and strength, soft metals, ceramics, glass and glass epoxy, plastics, rubber, cloth, and wood.



Most arc-cutting processes use the heat generated by an arc between an electrode and a metallic workpart (usually a flat plate or sheet) to melt a kerf that separates the part.

The most common arc-cutting processes are

1.plasma arc cutting and

2.air carbon arc cutting.

3.other Arc-cutting processes

ARC-CUTTING PROCESSES


A plasma is a superheated, electrically ionized gas.

PAC uses a plasma stream operating at temperatures in the range 10,000oC to 14,000oC to cut metal by melting.– operates by directing the high-velocity plasma

stream at the work, – thus melting it and – blowing the molten metal through the kerf.

The arc is generated between an electrode and the anode workpiece.

The plasma flows through a water-cooled nozzle.

Plasma Arc Cutting (PAC)

FIGURE 26.14 Plasma arc cutting (PAC).



The plasma jet is a high-velocity, collimated stream with extremely high temperatures at its center, – hot enough to cut through metal in some

cases 150 mm (6 in) thick. Gases used in PAC include nitrogen, argon,

hydrogen, or mixtures of these gases. These are referred to as the primary gases. Secondary gases or water are often directed to

surround the plasma jet to confine the arc and clean the kerf of molten metal as it forms.



Application: cutting of flat metal sheets and plates.

Operations:– manually by hand-held torch, or – by the torch under numerical control (NC).

can be used to cut nearly any electrically conductive metal. – Metals frequently cut include plain carbon

steel, stainless steel,and aluminum. For NC applications feed rates for 6-mm thick

plate can be as high as – 200 mm/s aluminum and – 85 mm/s steel plate.



Feed rates must be reduced for thicker stock. – e.g. the maximum feed rate for cutting 100-

mm thick aluminum stock is around 8 mm/s. Disadvantages:

1.the cut surface is rough, and

2.metallurgical damage at the surface is the most severe among the nontraditional metalworking processes.



In this process, – the arc is generated between a carbon

electrode and the metallic work, and – a high-velocity air jet is used to blow away the

melted portion of the metal. It can be used

– to form a kerf for cut the piece, or – to gouge a cavity in the part. – Gouging is used to prepare the edges of

plates for welding.

Air Carbon Arc Cutting


used on a variety of metals, including– cast iron, – carbon steel,– low alloy, – stainless steels, and – various nonferrous alloys.

Spattering of the molten metal is a hazard and a disadvantage of the process.

Air Carbon Arc Cutting


1. gas metal arc cutting,

2. shielded metal arc cutting,

3. gas tungsten arc cutting, and

4. carbon arc cutting.

Other Arc-Cutting Processes


A widely used family of thermal cutting processes, popularly known as flame cutting.– use the heat of combustion of certain fuel

gases combined with – the exothermic reaction of the metal with

oxygen (primary mechanism) The cutting torch deliver a mixture of fuel gas and

oxygen in the proper amounts, and direct a stream of oxygen to the cutting region.

The purpose of the oxyfuel combustion is to raise the temperature in the region of cutting to support the reaction.

It is performed either manually or by machine.

OXYFUEL-CUTTING PROCESSES (OFC)


commonly used to cut ferrous metal plates, with the following reactions:

The second reaction is the most significant in terms of heat generation.



cutting of nonferrous metals is different. – generally characterized by lower melting

temperatures than the ferrous metals, and– more oxidation resistant. – the heat of combustion plays a more important

role in creating the kerf. to promote the metal oxidation reaction, chemical

fluxes or metallic powders are added to the oxygen stream.



Fuels used in OFC include

– acetylene (C2H

2),

– MAPP (methylacetylene-propadiene— C3H

4),

– propylene (C3H

6), and

– propane (C3H

8).

Acetylene burns at the highest flame temperature and is the most widely used fuel for welding and cutting. – However, there are storage and handling

hazards



material is removed by means of a strong chemical etchant.

include – chemical milling, – chemical blanking, – chemical engraving, and – photochemical machining (PCM).

CHEMICAL MACHINING (CHM)


1. Cleaning.

first step

ensures that material will be removed uniformly.

2. Masking.

A protective coating called a maskant, made of a material chemically resistant to the etchant.

applied to those portions of the work surface that are not to be etched.

Maskant materials include neoprene, polyvinylchloride, polyethylene, and other polymers.

Mechanics and Chemistry of CHM


1. Etching.

material removal step.

part is immersed in an etchant that chemically attacks portions that are not masked.

When the desired amount of material has been removed, the part is withdrawn from the etchant and washed.

2. Demasking.

The maskant is removed from the part.



Masking can be accomplished by any of three methods:

1. cut and peel,

2. photographic resist, and

3. screen resist.

1- Cut and peel maskant is applied over the entire part by

– dipping, – painting, or – spraying.

The resulting thickness of the maskant is 0.025 to 0.125 mm.

Masking Methods


After it has hardened, the maskant peeled by a knife in the areas to be etched. – performed by hand, usually guiding the knife

with a template. generally used for

– large workparts, – low production quantities, and – where accuracy is not critical.

cannot hold tolerances tighter than ±0.125 mm.

1- Cut and peel


It uses photographic techniques to perform masking.

contain photosensitive chemicals. – applied to the work surface and exposed to

light through a negative. – These areas can then be removed from the

surface using developing techniques. – This leaves desired surfaces of the part

protected by the maskant and– the remaining areas unprotected, vulnerable to

chemical etching.

2- Photographic Resist (Photoresist)


normally applied where small parts are produced in high quantities, and

Close tolerances are required.– Tolerances closer than ±0.0125 mm can be

held

2- Photographic Resist (Photoresist)


The maskant is applied by means of silk screening methods.– the maskant is painted onto the workpart

surface through a silk or stainless steel mesh.– Embedded in the mesh is a stencil. – The maskant is thus painted onto the work

areas that are not to be etched. generally used in applications in between the

other two methods in terms of accuracy, part size, and production quantities.

Tolerances of ±0.075 mm can be achieved.

3- Screen Resist


Material removal rates are generally indicated as penetration rates, mm/min, – because rate of chemical attack of the work

material by the etchant is directed into the surface.

The penetration rate is unaffected by surface area.

Depths of cut are as much as 12.5 mm.– However, many applications require depths

only several hundredths of a millimeter.



Along with penetration into the work, etching also occurs sideways under the maskant. – The effect is referred to as the undercut, and – it must be accounted for in the design of the

mask.

the undercut is directly related to the depth of cut.

etch factor:


Chemical-Machining

Figure 27.3 (a) Schematic illustration of the chemical-machining process. Note that no forces or machine tools are involved in this process. (b) Stages in producing a profiled cavity by chemical machining; note the

undercut.


It is used largely to remove material from aircraft wing and fuselage panels for weight reduction.

applicable to large parts where substantial metal is removed.

surface finish varies with different work materials. Surface finish depends on depth of penetration.

– As depth increases, finish becomes worse, Metallurgical damage from chemical milling is

very small, perhaps around 0.005 mm into the work surface.

The cut and peel maskant method is employed

Chemical Milling


Chemical Milling

FIGURE 26.16Sequence of processing steps in chemical milling: (1) clean raw part, (2) apply maskant, (3) scribe, cut, and peel the maskant from areas to be etched, (4) etch, and (5) remove maskant and clean to yield finished part.


Chemical Milling

Chemical Milling

Figure 27.2 (a) Missile skin-panel section contoured by chemical milling to improve the stiffness-to-weight ratio of the part. (b) Weight reduction of space-launch vehicles by the chemical milling of aluminum-alloy plates.

These panels are chemically milled after the plates first have been formed into shape by a process such as roll forming or stretch forming. The design of the chemically machined rib patterns can be modified readily at minimal

cost.


It uses chemical erosion to cut – very thin sheetmetal parts—down to 0.025 mm

thick and/or – for intricate cutting patterns.

In both instances, conventional punch-and-die methods do not work because – the stamping forces damage the sheet metal, – the tooling cost would be prohibitive, or – both.

It produces burr free parts. Maximum stock thickness is around 0.75 mm. Also, hardened and brittle materials can be

processed.

Chemical Blanking


Chemical Blanking

FIGURE 26.17Sequence of processing steps in chemical milling: (1) clean raw part, (2) apply maskant, (3) scribe, cut, and peel the maskant from areas to be etched, (4) etch, and (5) remove maskant and clean to yield finished part.


Maskant is applied either by the photoresist method or the screen resist method.

Tolerances as close as 0.0025 mm can be held on 0.025mm thick stock

As stock thickness increases, more generous tolerances must be allowed.

Because chemical etching takes place on both sides, it is important that the masking procedure provides accurate registration between the two sides. – Otherwise, the erosion into the part from

opposite directions will not line up. This is especially critical with small part sizes and

intricate patterns.

Chemical Blanking


FIGURE 26.18 Parts made by chemical blanking. (Courtesy of Buckbee-Mears, St.Paul.)


It is a chemical machining process for making name plates and other flat panels that have lettering and/or artwork on one side.

It can be used to make either recessed lettering or raised lettering.

Masking is done by either the photoresist or screen resist methods.

Chemical Engraving


Chemical machining in which the photoresist method is used.

The term can be applied correctly to chemical blanking and chemical engraving when these photographic resist method.

employed in metalworking when close tolerances and/or intricate patterns are required.

These processes are also used extensively in the electronics industry to produce intricate circuit designs on semi-conductor wafers.

the term etch factor here correspond to anisotropy,– defined as the depth of cut d divided by the

undercut

Photochemical Machining (PCM)



FIGURE 26.19Sequence of processing steps in photochemical machining: (1) clean raw part; (2) apply resist (maskant) by dipping, spraying, or painting; (3) place negative on resist; (4) expose to ultraviolet light; (5) develop to remove resist from areas to be etched; (6) etch (shown partially etched); (7) etch (completed); (8) remove resist and clean to yield finished part.


There are various ways to photographic exposure The figure shows the negative in contact with the

surface of the resist during exposure. – This is contact printing,

but other methods expose the negative through a lens system to enlarge or reduce the size of the pattern printed on the resist surface.

Photoresist materials in current use are sensitive to ultraviolet light but not to light of other wavelengths. – Therefore, no need to carry out the processing

steps in dark



APPLICATION CONSIDERATIONS



A, Good application; B, fair application, C, poor application; D, not applicable; and blank entries indicate no data available during compilation.



A, Excellent; B, good,C, fair, D, poor.

Documents

NON TRADITIONAL MACHINING AND THERMAL CUTTING PROCESS CHAPTER 26