Chapter Ⅰ USMChapter Ⅰ USM

USM is able to ef­f ectively machine all hard materials whether they are electrically conductive or not. USM is able to ef­f ectively machine all hard materials whether they are electrically conductive or not.

The process and cutting toolThe process and cutting tool

• The process is performed by a cutting tool, which oscillates at high frequency, typically 20-40 kHz, in abrasive slurry.

• The shape of the tool corresponds to the shape to be produced in the workpiece.

• The high-speed reciprocations of the tool drive the abrasive grains across a small gap against the workpiece .

• The tool is gradually fed with a uniform force. • The impact of the abrasive is the energy principally

responsible for material removal in the form of small wear particles that are carried away by the abrasive slurry.

• The tool material, being tough and ductile, wears out at a much slower rate.

Ultrasonic MachiningUltrasonic Machining

Elements of ultrasonic machiningElements of ultrasonic machining

• The tool is oscillated by a longitudinal magnetostriction

• A magnetic field variation at ultrasonic frequencies

• The length of a ferromagnetic object changes

Material removalMaterial removal

• Occurs when the abrasive particles, suspended in the slurry between the tool and workpiece, are struck by the downstroke of the vibration tool.

• The impact propels the particles across the cutting gap, hammering them into the surface of both tool and workpiece. Collapse of the cavitation bubbles in the abrasive suspension results in very high local pressures.

• Under the action of the associated shock waves on the abrasive particles, microcracks are generated at the interface of the workpiece.

• The effects of successive shock waves lead to chipping of particles from the workpiece.

Material removalMaterial removal

The basic components to the cutting The basic components to the cutting action are believed to beaction are believed to be

• Small, tabletop-sized units to large-capacity machine tools,

• Bench units, and as self-contained machine tools. • Power range from about 40 W to 2.5 kW. • The power rating strongly influences the material

removal rate.

Subsystems of USM SystemSubsystems of USM System

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• The power supply is a sine-wave generator • The user can control over both the frequency and power of the

generated signal. • It converts low-frequency (50/60 Hz) power to high-frequency

(10-15 kHz) power• Supply to the transducer for conversion into mechanical

motion.

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• Two types of transducers are used in USM to convert the supplied energy to mechanical motion.

• They are based on two different principles of operation

- Magnetostriction

- Piezoelectricity

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• Magnetostrictive transducers are usually constructed from a laminated stack of nickel or nickel alloy sheets.

• Magnetostriction is explained in terms of domain theory .

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• Domains are very small regions, of the order of l0-8 ~ l0-9 cm3,

• In which there are forces that cause the magnetic moments of the atoms to be oriented in a single direction.

• In each domain the atomic magnetic moments are oriented in one of the directions of easy magnetization

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• In the cubic-lattice crystals of iron and nickel there are six directions of easy magnetization.

• In unmagnetized material all these directions are present in equal numbers, the magnetic moments of the orderless, unorientated domains compensate one another

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• When the material is placed in a sufficiently strong magnetic field, the magnetic moments of the domains rotate into the direction of the applied magnetic field and become parallel to it.

• During this process the material expands or contracts, until all the domains have become parallel to one another.

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• As the temperature is raised, the amount of magnetostrictive strain diminishes .

• Magnetostrictive transducers require cooling by fans or water.

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• Such as quartz or lead,zirconate,titanate, generate a small electric current when compressed.

• Conversely, when an electric current is applied, the material increases minutely in size.

• When the current is removed, the material instantly returns to its original shape.

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• Piezoelectric materials are composed of small particles bound together by sintering.

• The material undergoes polarization by heating it above the Curie point.

• Such transducers exhibit a high electromechanical conversion efficiency that eliminates the need for cooling.

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• The magnitude of the length change is limited by the strength of the particular transducer material.

• The limit is approximately 0.025 mm.

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• Its function is to increase the tool vibration amplitude and to match the vibrator to the acoustic load.

• It must be constructed of a material with good acoustic properties and be highly resistant to fatigue cracking.

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• Monel and titanium have good acoustic properties and are often used together with stainless steel, which is cheaper.

• However, stainless steel has acoustical and fatigue properties that are inferior to those of Monel and titanium, limiting it to low amplitude applications.

• Nonamplifying holders are cylindrical and result in the same stroke amplitude at the output end as at the input end.

• Amplifying toolholders have a cross section that diminishes toward the tool, often following an exponential function.

• An amplifying toolholder is also called a concentrator.

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• Amplifying holders remove material up to 10 times faster than the nonamplifying type.

• The disadvantages of amplifying toolholders include increased cost to fabricate, a reduction in surface finish quality, and the requirement of much more frequent running to maintain resonance.

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• Tools should be constructed from relatively ductile materials.

• The harder the tool material, the faster its wear rate will be.

• It is important to realize that finishing or polishing operations on the tools are sometimes necessary because their surface finish will be reproduced in the workpiece.

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• The geometry of the tool generally corresponds to the geometry of the cut to be made,

• Because of the overcut, tools are slightly smaller than the desired hole or cavity

• Tool and toolholder are often attached by silver brazing.

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• The criteria for selection of an abrasive for a particular application include hardness, usable life, cost, and particle size.

• Diamond is the fastest abrasive, but is not practical because of its cost.

• Boron carbide is economical and yields good machining rates.

• Silicon carbide and aluminum oxide are also widely used.

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• Coarse grits exhibit the highest removal rates,when the grain size becomes comparable with the tool amplitude, cut more slowly.

• The larger the grit size, the rougher the machined surface.

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• With an abrasive concentration of about 50% by weight in water， but thinner mixtures are used to promote efficient flow when drilling deep holes or when forming complex cavities.

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ExampleExample

• Find the machining time for a hole 5mm in diameter in a tungsten carbide plate 1cm thick.

• The grains are 0.01mm in diameter, the feed force is 3N, and the amplitude of oscillation is 20 micro m at a frequency of 25KHz.

• The fracture hardness is approximately 6900N/mm2.

• The slurry is mixed in equal parts water and abrasive.

ExampleExample

The acoustic head is the most complicated part of the machine.

It must provide a static force, as well as the high frequency vibration

 

- Basic machine layout- Basic machine layoutExampleExample

Magnetostrictive materials should have a good coupling of magnetic and mechanical energy

Basic machine layoutBasic machine layoutExampleExample

• If a tool is designed to increase flow, better cutting speeds will occur.

• Tools

- hard but ductile metal

- stainless steel and low carbon

- aluminum and brass tools wear near 5 to 10 times faster

• Abrasive Slurry - common types of abrasive

- boron carbide (B4C) good in general, but expensive - silicon carbide (SiC) glass, germanium, ceramics

- corundum (Al2O3) - diamond (used for rubies , etc)

- boron silicon-carbide (10% more abrasive than B4C)

• liquid

- water most common

- benzene

- glycerol

- oils

• high viscosity decreases mrr

• typical grit size is 100 to 800

Little production of heat and stress, but may chip at exit side of hole.

Sometimes glass is used on the back side for brittle materials.

• Mechanics of material removal - brittle fracture caused by impact of abrasive grains due to vibrating at high frequency

• Medium - slurry • Abrasives: B4C; SiC; Al2O3; diamond; 100-800 grit size • Vibration freq. 15-30 KHz, amplitude 25-100 micro m

• Tool material soft steel • Material/tool wear = 1.5 for WC workpiece, 100 for glass • Gap 25-40 micro m • Critical parameters - frequency, amplitude, tool material, grit size,

abrasive material, feed force, slurry concentration, slurry viscosity

• Material application - metals and alloys (particularly hard and brittle), semiconductors, nonmetals, e.g., glass and ceramics

• Shape application - round and irregular holes, impressions • Limitations - very low mrr, tool wear, depth of holes, and cavities small.

1. A cylindrical impression with a diameter of 10mm and a depth of 1mm has to be made on a tungsten carbide surface. The feed force is constant and equal to 5N. The average diameter of the grains in the abrasive slurry is 0.01mm. The tool oscillates with an amplitude of 30 micro m at 20 KHz. The slurry contains 1 part of abrasive to about 1 part of water. The fracture hardness of tungsten carbide workpiece may be taken as 7000 N/mm2. Estimate the machining time.

2. A square through hole of 5mm by 5mm has to be drilled in a 5mm thick tungsten carbide sheet. The slurry is made of 1 part of 10 micro m radius boron carbide grains mixed with 1.5 parts of water. The feed force is 4N. The tool oscillates with an amplitude of 0.015mm at 25KHz. Assuming that only 20% of the pulses are effective, calculate the time required to complete the job.

3. In an ECM operation, a pure copper block is being machined. If a current of 5000A is used, determine the volume rate of material removal from the copper block.

4. The composition of a Nimonic alloy turbine blade is 18% cobalt, 62% Ni, and 20% chromium. It is being machined electrochemically with a current of 1500A. Find out the volume removal rate if the density of the alloy is 8.3g/cm3. The dissolution valency of chromium is 6, whereas that for both nickel and cobalt is 2.

5. The composition of a monel alloy workpiece undergoing electrochemical machining is as given here:

63% Ni, 31.7% Cu, 2.5% Fe, 2% Mn, 0.5% Si, 0.3% C

if the machining current is 1000A, estimate the volume removal rate.

6. The equilibrium gap when machining (electrochemically) iron, using NaCl solution in water as the electrolyte, is found to be 0.2mm. The current density is 200A/cm2, the operating voltage being 12V. Iron dissolves at a valency 2, the density of iron is 7.8 g/cm3, and the specific resistance of the electrolyte is 2.8 ohm cm. Calculate the metal removal rate/unit work surface area. The overvoltage may be taken as 1.5V.

7. In an electrochemical trepanning operation on a flat iron surface, an electrode in the form of a tube (with an outer diameter of 1cm). A laser beam with a power intensity of

2 * 105 W/mm2 is used to drill a 0.2mm diameter hole in a tungsten sheet of 0.4mm thickness. If the efficiency of the operation is only 10%, estimate the time required.

8. TRUE / FALSE - Water is the main cutting tool in Ultra Sonic machining.

9. Why are the vibrations in USM so small?

10. USM will be used to add the following pattern to an object, If the tool is Tungsten carbide, and the work is Cu, with an amplitude of oscillation of 10 μm, at 30KHz, how long will the operation take? (Note: the grain diameter is 20μm, and the head has a static force of 6N)

11. When is the abrasive added into the flow for the various abrasive jet machining processes?

12. Why is the depth of material removed by abrasive jet

machining so variable?

13. Describe the ability of the abrasive processes to

produce sharp corners.