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UNIT - II

MECHANICAL ENERGY BASED PROCESSES

Introduction:

In mechanical energy methods, the material is removed by mechanical erosion of the work piece

material.

Some of the processes are:

1) Abrasive Jet Machining (AJM)

2) Abrasive Water Jet Machining (AWJM)

3) Water Jet Machining (WJM)

4) Ultrasonic Machining (USM)

Abrasive Jet Machining:

Abrasive jet machining is a material removal process in which the material is removed due to the

action of a high velocity stream of gas with small abrasive particles.

The mechanism of material removal is erosion or chipping action.

The jet of inert gas and abrasive particles strike the work piece at high velocity (150 - 300 m/s)

which lead to removal of material from the work piece. It is shown in fig 2.1 & 2.2

The process can be employed for cutting, cleaning, etching, polishing, deburring and trimming.

fig 2.1 Principle Of AJM



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fig 2.2 Schematic Diagram of AJM

Construction:

The important elements of a Abrasive jet machining are shown in fig 2.3

i) Gas Propulsion Systemii) Abrasive feeder / Hopper

iii) Nozzle

iv) Abrasivesv) Mixing Chamber

vi) Machining Chamber

fig 2.3 CONSTRUCTION OF ABRASIVE JET MACHINING

Air Compressor

Abrasive

Control

Valves &

Variable

MixerWork PieceNozzle



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i) Gas Propulsion System:

It supplies clean and dry air to propel the abrasive particles.

It has compressor or cylinder filled with gas to carry the abrasive.

Dry air, Nitrogen, Carbon dioxide are the gases usually used.

ii) Abrasive feeder / Hopper:

Abrasive feeder is the container in which the abrasive particles are stored.

The main function of the abrasive feeder is to control the quantity of abrasive particles which are

propelled by carrier gas to the mixing chamber.

iii) Nozzle:

The nozzle is made up of either circular or rectangular cross section.

The nozzle material is tungsten carbide (WC) or synthetic sapphire or aluminum oxide.

Tungsten carbide nozzle offers a life of 12 - 20 hours.

Synthetic sapphire has high wear resistance so it offers the average life of 300 hours.

Nozzle is designed in such a way that loss of pressure due to bends, friction should be minimum.

iv) Abrasives:

An abrasive is small, hard particle having sharp edges and an irregular shape used to clean, grind

or polish and to remove the material from the work surface.

Different types of abrasive are used in abrasive jet machining like aluminum oxide, silicon

carbide, Dolomite, Glass beads, Crushed glass, Sodium bicarbonate.

Abrasive particles must be hard, high toughness, irregular in shape & edges should be sharp.

v) Mixing Chamber:

In the mixing chamber the hard abrasive particles and compressed air get mixed to be supplied

to the machining chamber.

To ensure the proper mixing of the abrasive particles with air, vibrator is provided at the bottom

of the mixing chamber.

vi) Machining Chamber:

In the machining chamber the standoff distance between the nozzle and work piece is maintained

approximately 0.8mm.



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Chamber is well closed region in which the abrasive particles content should be well below the

harmful limit.

If toxic material (say beryllium) is machined, special attention should be given to the dust

collection system.

Working Principle:

Gases like dry air, Nitrogen, Carbon dioxide are compressed to the pressure range of 0.2 to 1.4

MPa, before entering into the mixing chamber.

Abrasive particles are sent to the mixing chamber from abrasive feeder.

The proper mixing of gas with abrasives are ensured by inducing vibration to the mixing

chamber through the vibrator.

The gas abrasive mixture propels through the nozzle orifice of diameter 0.075 to 1.0 mm at

velocities of 200 - 300 m/s.

Standoff distance of 0.25 - 0.75 mm is maintained between the nozzle tip and work surface.

The jet of gas-abrasive which is coming out from the nozzle strikes the work surface at high

velocity, erodes the material from the work piece.

Process Parameters:

i) Standoff distance or Nozzle tip distance

ii) Abrasive flow rate

iii) Gas Pressureiv) Mixing Ratio

v) Abrasive velocity

vi) Abrasive grain size

i) Standoff distance (SOD) or Nozzle tip distance:

The standoff distance has great influence on diameter of cut, its shape and size, material removal

rate. It is shown in fig 2.4

For maximum MRR, recommended range SOD is 0.75 mm to 1 mm.

Normally lower the SOD, higher the accuracy produced, reduces the taper in machined surface.

Higher range of SOD is recommended (12.5 mm to 75 mm) for cleaning and frosting purpose.



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fig 2.4

ii) Abrasive flow rate / Mass flow rate:

Increase in abrasive flow rate increases the MRR. It is shown in fig 2.5

MRR decreases as abrasive flow velocity goes down for further increase in abrasive flow rate.

fig 2.5

iii) Gas pressure / Nozzle pressure:

Increase in gas pressure increase the MRR, therefore kinetic energy of abrasive particles is

responsible for removal of material by erosion. It is shown in fig 2.6

fig 2.6



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iv) Mixing ratio:

Mixing ratio is defined as ratio between volume flow rate of abrasive to volume flow rate of gas.

Increasing the value of mixing ratio increases the MRR but larger value may reduce the jet

velocity and sometimes block the nozzle, which consequently reduces the MRR. It is shown in

fig 2.7 & 2.8

fig 2.7 fig 2.8

vi) Velocity of Abrasive particles:

Increase in velocity of abrasive particles increases the MRR. It is shown in fig 2.9

fig 2.9

vii) Abrasive grain size:

Coarse abrasives are more irregular in shape, hence their cutting ability is high.

Fine abrasives are less irregular in shape, as a result their cutting ability is poor.

Larger sized abrasive are used for rapid MRR. It is shown in fig 2.10

Smaller sized abrasive are used for good surface finish and precision.



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fig 2.10

Process Capabilities:

Hard materials can be easily produced.

Possible to cut narrow slots of range 0.12 to 0.25 mm.

Produces good surface finish of range 0.25 to 1.25 microns.

Surface damage is insignificant because heat generation is low.

Possible to produce sharp radius of 0.2 mm on machined edge.

The process can maintain the tolerance of 0.12 mm.

Steel of 1.5 mm thick, glass of 6.3 mm thick can be easily cut by AJM with low MRR.

Advantages:

There is no direct contact between tool and work piece.

Complex shapes can be easily cut.

During machining, no heat is generated.

Cheaper than other processes.

Environmentally clean and safe process.

Harder materials like titanium, Aluminum, Steel can be effectively cut by this process.

Brittle materials like glass, ceramic, quartz, stone can be also cut by this process.

Disadvantages:

Metal removal rate is very low.

Not suitable for mass production because of the high maintenance required.

Soft material cannot be machined.

Machining accuracy is poor

Nozzle wear rate is high.

The abrasive powder used in this process cannot be reused.



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Applications:

Fine drilling and micro welding

Machining of semi-conductor

Machining of intricate profile on hard and brittle materials

Cleaning and polishing of plastics, nylon and Teflon component.

Frosting of the interior surface of the glass tubes.

Abrasive Water Jet Machining (AWJM)

Abrasive water jet machining is a mechanical energy based material removal process in which

abrasives are mixed with water to form the abrasive slurry.

The process is similar to abrasive jet machining except that the water is used as carrier medium

instead of dry air.

In this process, a jet of water and small abrasive particles in the form of stream mix up in the

mixing chamber and pass through the nozzle.

This high velocity stream of abrasive with water impinges on work piece and removes the work

material by erosion or chipping action.

The materials which are electrically non conductive and difficult to machines can be easily cut

more rapidly and effectively.

The process can be employed for cutting, drilling and cleaning of hard materials.

Construction:

The important elements of a Abrasive water jet machining are shown in fig 2.11

i) Pumping unit

ii) Abrasive feed system

iii) Abrasive water jet nozzle

iv) Catcher

i) Pumping unit:

The main purpose of the pumping unit is to produce and supply ultra high pressure water to the

mixing chamber.



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ii) Abrasive feed system:

It controls the supply of the abrasive particles to nozzle.

There are two types of abrasive feed system

In the first type, stream of dry abrasive deliver to the nozzle where it mixes with water jet.

This type has a limitation that it cannot supply the abrasive to the longer distance.

To overcome this difficulty, we can directly supply the slurry(Mixture of abrasive and water),

which is prepared before entry to the nozzle.

fig 2.11 CONSTRUCTION OF ABRASIVE WATER JET MACHINING

This forms the second type of abrasive feed system

The desired velocity of a jet of abrasive water mixture is about 700 m/sec

iii) Abrasive water jet nozzle:

Nozzle is made up of sapphire, tungsten carbide(WC) or boron carbide.

Nozzle may serve two functions. They are,

Mixing of abrasive with water.

Forming the high velocity abrasive water jet.



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Based on their internal design, nozzle is of two types:

a) Single side feed jet nozzle

b) Multi jet central feed nozzle.

(a) Single side feed jet nozzle:

Abrasive are fed from the side and mix with water in mixing chamber. It is shown in fig 2.12

This type does not provide an optimum mixing efficiency.

Exit port of nozzle is subjected to rapid wear.

This type of nozzle is least expensive.

(b) Multi jet central feed nozzle:

Abrasives are centrally fed surrounded by multiple water jet. It is shown in fig 2.13

This type provides better mixing of abrasive with the water jet.

Fabrication of nozzle is difficult and expensive.

It has longer life.

fig 2.12 & 2.13

4. Catcher:

It uses the hard and replaceable inserts to break the jet quickly and completely.

Catcher is used to absorb the residual energy of the water jet and dissipate the same.



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Working principle:

The water jet and a stream of abrasives coming from two different directions mix up and flow

through the nozzle.

Velocity of abrasives rises rapidly by gaining the momentum of water jet.

The pressure of the water is maintained about 400 MPa which produces a jet speed of about 900

m/sec.

This high velocity jet strikes the work piece and has the ability to machine ceramics, composites,

rocks, etc.,

Process Parameters:

The performance of AWJM is influenced by the following parameters

i) Pressure of the water

ii) Mass flow rate of water

iii) Size of the abrasive particles

iv) Abrasive flow rate

v) Standoff distance

vi) Type of abrasive material

i) Pressure of the water:

The effect of pressure on depth of cut is given. It is shown in fig 2.14

The depth of cut increases with increase in water pressure.

On the other hand this increase in pressure increases the nozzle wear rate and cost of pump

maintenance and decrease the volumetric efficiency.

Critical pressure or threshold pressure is the pressure below which no machining would take

place.

Different materials have different critical pressure.

fig 2.14



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2. Mass flow rate of water:

It is directly proportional to square root of pressure and square of diameter of the nozzle.

Generally the percentage increase in the depth of cut is always lower than percentage increase in

flow rate of water.

3. Size of the abrasive particles:

Commonly used particle size ranges from 100 to 150 grit. It is shown in fig 2.15

Increase in the particle size will increase the irregularities in the abrasive particle which

consequently increases the depth of cut.

fig 2.15

4. Abrasive flow rate:

As abrasive flow rate increases the number of abrasive particles cutting the work piece also

increase consequently increasing the MRR. It is shown in fig 2.16

MRR increases only up to a certain value, beyond which it starts decreasing the abrasive flow

rate and velocity goes down for further increase in abrasive flow rate.

fig 2.16



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5. Standoff distance:

An increase in SOD rapidly decreases the machined depth. It is shown in fig 2.17

There is an upper value of SOD beyond which the process will no longer cut the material.

Smaller the SOD, deeper is the cut.

fig 2.17

6. Type of Abrasive material:

Commonly used abrasives are garnet, silica sand, silicon carbide, aluminum oxide and glass

beads.

Selection of abrasive material will be based on type of work piece material.

Reuse of abrasive particles is not recommended.


The process can cut even thick material (upto 200 mm)

High quality of machined edges can be achieved.

Automation of the process can be easily done.

Thermal stress are not developed in the work surface.

Applications:

Both metals and non-metals can be machined.

Metals like copper, lead, aluminum, tungsten carbide can be cut.

Non-metals like Silica, glass, acrylic, concrete, graphite can be cut.

Sandwiched honeycomb structural material can be machined which finds its application in

aerospace industries.



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Advantages:

It is economical and environmentally safe.

Reduced need for secondary finishing operations.

Low cutting forces on work piece.

No need of direct physical tool.

No heat affected zone.

Disadvantages:

High equipment cost.

Hazard from rebounding abrasives.

High level of noise.

Low nozzle life.

Water Jet Machining (WJM)

When the high velocity of water jet comes out of the nozzle and strikes the material, its kinetic

energy is converted into pressure energy including high stresses in the work material.

When this induced stress exceeds the ultimate shear stress of the material, small chips of the

material get loosened and fresh surface is exposed.

Construction:

The basic elements of water jet machining system are shown in fig 2.18

i) Pumping unit

ii) Intensifier

iii) Accumulator

iv) Nozzle

v) Drainer / Catcher

i) Pumping unit:

Pump unit has the oil pump.

Electric motor drives the oil pump.

Oil from the reservoir is pumped to an intensifier.



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ii) Intensifier:

It is used to produce the high pressure water.

The pressure of water is as high as 40 times that of the oil pressure.

iii) Accumulator:

Accumulator is a container which is used to store high pressure water to ensure the smooth out

flow.

Rigid high pressure tubing and connectors are used to transfer the water to the nozzle from the

accumulator.

fig 2.18 CONSTRUCTION OF WATER JET MACHINING

iv) Nozzle:

Nozzle is made of Synthetic sapphire and it offers a good life (250 - 500 hrs)

Nozzle internal diameter ranges from 0.07 mm to 0.5 mm.

Presence of foreign particles (dirt) in water results in failure of nozzle by chipping.



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v) Catcher / Drainer:

Catcher is used to absorb the residual energy of the water jet and dissipate the same.

Catcher uses hard and replaceable inserts to break the jet quickly and completely.

Working Principle:

Water from the reservoir is pumped to the intensifier using a hydraulic pump.

The intensifier increases the pressure of the water to the required level. Usually, the water is

pressurized to 200 to 400 MPa.

Pressurized water is then sent to the accumulator. The accumulator temporarily stores the

pressurized water.

Pressurized water then enters the nozzle by passing through the control valve and flow regulator.

Control valve controls the direction of water and limits the pressure of water under permissible

limits.

Flow regulator regulates and controls the flow rate of water.

Pressurized water finally enters the nozzle. Here, it expands with a tremendous increase in its

kinetic energy. High velocity water jet is produced by the nozzle.

When this water jet strikes the work piece, stresses are induced. These stresses are used to

remove material from the work piece.

The water used in water jet machining may or may not be used with stabilizers. Stabilizers are

substances that improve the quality of water jet by preventing its fragmentation.

Process Parameters:

The following process parameters are needed to utilize the WJM process successfully.

(i) Material Removal Rate (MRR).

(ii) Geometry and surface finish of work material.

(iii) Wear rate of the nozzle.



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(i) Material Removal Rate:

Material removal rate is directly proportional to the reactive force (F) of the jet.

MRR F

Mass flow rate depends on nozzle diameter (d) and fluid pressure, jet velocity depends on fluid

pressure.

When MRR increases, the SOD also increases up to a certain limit after that is remains

unchanged for a certain tip distances and the falls gradually.

(ii) Geometry and surface finish of work material:

Geometry and surface finish of work material mainly depends upon the following parameters

such as Nozzle design, Jet velocity, Cutting speed, Depth of cut, Properties of the material to be

machined.

(iii) Wear rate of the nozzle:

Nozzle wear rate depends upon the following factors such as Hardness of the nozzle material,

Pressure of the jet, Velocity of the jet, Nozzle design.


Non conductive material can be machined effectively.

The process does not require any predrilled hole to start cutting.

Machined surfaces does not have burrs, thermal damage.

Very thick materials can be cut by more than one pass.

The process can be used to cut the materials which are porous, fibrous, granular or soft.

Insulation of the cables can be removed with low pressure water jet,

Advantages:

Water jet machining is a relatively fast process.

It prevents the formation of heat affected zones on the work piece.

Low operating cost.

Low maintenance cost.

It automatically cleans the surface of the work piece.



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WJM has excellent precision. Tolerances of the order of ±0.005″ can be obtained.

It does not produce any hazardous gas.

It is eco-friendly.

Disadvantages:

Only soft materials can be machined. Very thick materials cannot be easily machined.

Initial investment is high.

Applications:

Water jet machining is used to cut thin non-metallic sheets.

It is used to cut rubber, wood, ceramics and many other soft materials.

It is used for machining circuit boards. It is used in food industry.

Ultrasonic Machining (USM)

In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz)

with an amplitude of around 15 – 50 μm over the work piece.

Generally the tool is pressed downward with a feed force, F. Between the tool and work piece,the machining zone is flooded with hard abrasive particles generally in the form of a water based

slurry.

As the tool vibrates over the work piece, the abrasive particles act as the indenters and indent

both the work material and the tool.

Abrasive particles , as they indent , the work material would remove the material from both tool

and work piece.

It is employed to machine hard and brittle materials (both electrically conductive and non -

conductive material) having hardness usually greater than 40 HRC.

Construction:

The basic elements of Ultrasonic machining system are shown in fig 2.19



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i) Power Supply

ii) Transducer

iii) Tool feed mechanism

iv) Tool holder

v) Tool

vi) Abrasive slurry

i) Power Supply:

Power supply unit of the ultrasonic machining has sine wave generator.

Generator converts low frequency power (50/60 Hz) to high frequency (10 - 15 KHz) electrical

power.

ii) Transducer:

Essentially transducer converts electrical energy to mechanical vibration.

The high frequency electrical signal is transmitted to transducer which converts it into

high frequency low amplitude vibration.

There are two types of transducer used in USM

a) Piezo electric transducer

b) Magnetostricitve transducer

a) Piezo electric transducer:

Piezoelectric crystals like quartz, zirconium, lead and titanate generate a small electric current

when they are compressed.

When an electric current is passed through the piezoelectric crystal (quartz) it expands, when the

current is removed the crystal attains its original size. This effect is known as piezoelectric

effect.

b) Magneto restrictive transducer:

When an object made of ferromagnetic materials (Nickel & Nickel alloy sheets) is placed in the

continuously changing magnetic field, a change in its length takes place.

These are available up to 2.4KW power supply & 20% - 30% efficiency.



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fig 2.19 CONSTRUCTION OF ULTRASONIC MACHINING

iii) Tool feed mechanism:

A constant gap is maintained between the tool and work piece in order to ensure the proper

standoff distance.

It must supply sufficient cutting force.

It must reduce the force at a specified depth

For obtaining high accuracy, feed mechanism should be very precise and sensitive.

iv) Tool holder:

Tool holder is used to connect the tool to the transducer.

It transmits energy and sometimes amplifies the amplitude of vibration.

Tool holder is made of monel, titanium and stainless steel.

Tool holder may be of two types:

a) Amplifying type

b) Non-amplifying type



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a) Amplifying type:

The tool motion at the one end will be increased six times than the other end.

MRR is 10 times greater than non amplifying type.

More expensive and demands higher operating cost.

This type of tool holder gives poor surface quality.

b) Non amplifying type:

It has circular cross section

It gives same amplitude at both the ends.

The tool holder can be of different shape such as tapered, exponential, stepped.

Machining of tapered or stepped horn is much easier as compared to the exponential.

v) Tool:

Relatively ductile material like brass, stainless steel, mild steel are used as tool material.

Tool must be properly surface finished.

Tool should not have scratches or machining marks to avoid early fatigue failure.

vi) Abrasive slurry:

Commonly used abrasives are Al2O3, Sic & B4C (Boron Carbide).

Abrasive Slurry act as a medium to carry abrasives to the cutting zone.

Vibrating Abrasives attain kinetic energy and strike the work piece surface with a force much

higher than their own weight.

Each down stroke of the tool accelerates numerous abrasive particles resulting in the formation

of thousands of tiny chips per second.

Working Principle:

This process is performed by a cutting tool, which oscillates at high frequency, typically 20-

40kHz.

The shape of the tool is just mirror part of the shape to be produced in the work piece.

As the tool vibrates with a specific frequency, an abrasive slurry (usually a mixture of abrasive

grains and water of definite proportion) is made to flow through the tool work interface.

The impact force arising out of vibration of the tool end and the flow of slurry through the work

tool interface actually causes thousands of microscopic abrasive grains to remove the work

material by abrasion which is carried away by the abrasive slurry.



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

Process Parameters:

i) Amplitude of tool oscillation (a0)

ii) Frequency of tool oscillation (f)

iii) Tool materialiv) Type of abrasive

v) Grain size of the abrasive - (d 0)

vi) Feed force - F

vii) Volume concentration of abrasive in water slurry - C

viii) Ratio of work piece hardness to tool hardness, λ = σ ω / σt

ix) Material Removal rate

i) Amplitude of tool oscillation (a0):

Normally increasing the amplitude of tool oscillation will increase the MRR. It is shown in fig

2.20

fig 2.20

ii) Frequency of tool oscillation(F):

MRR has a directly proportional relation with frequency. So increase in frequency will increasethe MRR. It is shown in fig 2.21

For same amplitude increasing the frequency will increase the MRR.



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fig 2.21

iii) Tool material:

Relatively ductile material like brass, stainless steel, mild steel are used as tool material.

Tool must be properly surface finished.

iv) Type of abrasive:

Commonly used abrasives are Al2O3, Sic & B4C (Boron Carbide).

For same volume concentration, boron carbide (B4C) particles will offer more MRR than Al2O3.

v) Grain size (or) Grit size (d0):

Finer grain size will produce higher surface finish with lesser material removal rate. It is shown

in fig 2.22

Coarse grains will produce poor surface finish with material removal rate.

fig 2.22



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vi) Feed force (F):

Increasing the feed force will increase the material removal rate. It is shown in fig 2.23

fig 2.23

vii) Volume concentration of abrasives 'C':

Generally increasing the volume concentration will make more abrasive particles to suspend in

the slurry which makes more number of particles to contact with the work piece.

So increasing the volume concentration will increase the MRR. It is shown in fig 2.24

fig 2.24

viii) Ratio of work piece hardness to tool hardness, λ

Increasing the ratio of work piece hardness to tool hardness will reduce material removal rate,

because the tool has to remove the harder material closer to its hardness value.

ix) Material Removal Rate:

Material removal rate depends on both fracture and plastic deformation. It is shown in fig 2.25

MMR is inversely proportional to the cutting area of the tool.

Tool vibrations also affect the material removal rate.

Type of abrasive, its size and concentration also directly affect the MRR.



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Materials like ceramic and glass can be easily machined.

fig 2.25


Materials such as carbides, ceramics, tungsten, glass etc can be easily machined by this

technique.

It can machine work piece harder than 40 HRC to 60 HRC. Tolerance range 7 micron to 25 microns can be achieved.

Aspect ratio 40:1 has been achieved.

Holes up to 76 micron have been drilled hole depth up to 51mm have been achieved easily.

Linear material removal rate 0.025 to 25mm/min

Advantages:

It can be used machine hard, brittle, fragile and non conductive material.

Any size of holes can be generated.

It is burr less and distortion less processes.

Power consumption is very low.

Cost of metal removal is low

Noiseless operation

High accuracy and surface finish

Equipment is safe to operate

Disadvantages:

Metal removal rate is very low.

Depth of hole is limited to the size of tool.



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Tool wear is induced which affects the performance.

Sharp corners cannot be generated.

Power consumption is high when compared with other mechanical processes.

Depth of cylindrical holes is presently limited to 2.5 times the diameter of the tool.

Applications:

Holes as small as 0.1mm can be drilled

Precise and intricate shaped articles can be machined

It has been efficiently applied to machine glass, ceramics, tungsten, precision mineral stones,

etc..

Several machining operations like drilling, grinding, turning, threading, profiling etc., on all

materials both conducting and non-conducting.

TOOL FEED MECHANISM

Feed system is used to apply the static load between the tool and work piece during ultrasonic

machining operation.

The basic requirements of tool mechanism are as follows:

i) The tool should be moved slowly to prevent breaking.

ii) The tool has to come back to its initial position after finishing its machining operation.

There are three types of feed mechanism which are used in ultrasonic machining processes.

a) Gravity feed mechanism

b) Spring loaded feed mechanism

c) Pneumatic (or) Hydraulic feed mechanism

a) Gravity Feed Mechanism:

In this mechanism, counter weights are used to apply the load to the head through a pulley.

In order to reduce the friction, ball bearing are used. It is shown in fig 2.26

Gravity feed mechanism is generally preferred because of its simple construction.



fig 2.26

b) Spring Loaded Feed Mechanism:

In this mechanism, spring pressure is used to feed the tool during machining operation.

This type of mechanism is also preferred because of its sensitive and compact ability. It is shown

in fig 2.27

fig 2.27

c) Pneumatic or Hydraulic Feed Mechanism:

In order to get high feed rate, pneumatic feed mechanism is used.

fig 2.28

energy based process

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