electro chemical machinig

8/13/2019 Electro chemical machinig

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ChapterINTRODUCTION

As the world is advancing forth technically in the field of space research,

missile and nuclear industry; very complicated and precise components having some

special requirements are demanded by these industries. The challenge is taken by the

new developments taking place in the manufacturing field. The wordunconventional is used in the sense that the materials like Hastalloy, itralloy,

imonics etc. are such that they can!t be machined by conventional methods but

require some special techniques. The conventional methods, in spite of recent

advancements are inadequate to machine such materials from stand point of

economic production.

The unconventional methods of machining have several specific advantages

over conventional methods of machining and these promise formidable tasks to be

under taken and set a new record in the manufacturing technology. These methods

are not limited by hardness, toughness, and brittleness of materials and can produce

any intricate shape on any work piece material by suitable control over the various

physical parameters of the process. However unconventional methods are not

substitutes for conventional machining methods, but are only complementing them.

on conventional energies are classified according to the nature of the

energy employed in machining

"i# Thermal and $lectro thermal

"ii# %hemical and $lectro %hemical

"iii# &echanical

'n thermal and electro thermal methods, heat energy is concentrated on small

areas of the work piece, to melt and vapori(e the tiny bits of work material. The

)


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required shape is machined by repetition of this process. "$.*.&, +..&., -.A.&,

$..&., '..&.#. 'n chemical and electrochemical machining the work piece material

in contact with a chemical solution is etched "anodic dissolution# in a controlled

manner "$.%.&., $.%.., $.%.H., and $.%.*.#. 'n mechanical methods the material is

removed by mechanical erosion of the work piece material "/.0.&., A..&., and

2..&.#

There are various unconventional machining processes like

$lectro discharge machining

$lectro chemical milling

$lectro chemical grinding -lasma arc machining

+aser beam machining

Abrasive 3et machining

2ater 3et machining

/ltrasonic machining

Abrasive 3et machining is one of the unconventional machining process by

which the material is removed by the erosion of high velocity abrasive particles

mi4ed with gas. And it is a process which is suited for machining super alloys and

refractory type materials.

'n this pro3ect work an attempt has been made to study the abrasive 3et

machining process and fabrication the e4perimental setup used for the abrasive 3et

machining process. And in this study it is observed that the various process

parameters like velocity of 3et, no((le diameter, and abrasive particle si(e. 0tand off

distance will affect the metal removal rate and surface finish of the work piece

material.

5


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LITERATURE SURVEY

6


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Chapter 2LITERATURE SURVEY

/nconventional methods of machining have several specific advantages over

conventional methods of machining. These methods are not limited by hardness,

toughness, and brittleness of materials and can produce any intricate shape on any

work piece material by suitable control over various physical parameters of the

processes. However unconventional methods are not substitutes for conventional

machining methods, but are only complimenting them.

).1 *'77$8$T T9-$0 :7 /%:$T':A+ &$TH:*0

).1.1$+$%T8: %H$&'%A+ &A%H'' "$.%.

This process is developed on the principle of faraday and ohm. 'n this

process, an electrolytic cell is formed by the anode "work piece# and the cathode

"tool# in the midst of a following electrolyte. The metal is removed by the controlled

dissolution of the anode according to the well


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2hen the electrodes are connected to about )>v electric supply source. 7low of

current in the electrolyte is established due to positively changed ions being attracted

towards the cathode and ice versa. %urrent density depends on the rate at which

ions arrive at respective electrodes which is proportional to the applied voltage,

concentration of electrolyte, and gap between the electrodes. *ue to electrolysis

process at the cathode, hydro4yl "


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).1.5 %H$&'%A+ &'++'

This is a form of etching process. 'n this process hob is first cleaned properly

and then some sort of preventive coating is applied "masking# on that particular

portion which is not to be machined. The preventive coating is of vinyl plastic. This

is applied with the help of a template then the 3ob is e4posed to the etching material.

%hemical etching reagent depends upon work material. 7or aluminum it is caustic

soda, and for steel it is acid "itric#. The metal is removed by the chemical

conversions of the metal in to metallic salt

).1.6 +A0$8 $A& &A%H''

+aser is as electro magnetic radiation. 't produces monochromatic light

which is in the form of collimated beam. Though has been introduced recently but is

making much heal way in industry. 't finds user for micro drilling, micro welding

etc. however it has low efficiency and high energy input requirements.

+ight Amplification by 0timulated $mission of 8adiation is called

+.A.0.$.8. beam. 'n this process use is made of laser beam.

$lectrons are arranged in different cells in as atom and each cell has a set

number of electrons. 'f any atom so e4cited, i.e. we pump some amount of e4ternal

energy then the atomic cell will be in e4cited condition and some electrons will

3ump up to ne4t energy level i.e. electrons 3ump from one orbit to ne4t one.

).1.= /+T8A0:'% &A%H'' "/.0.&.#

/ltrasonic waves are sound waves which propagate at frequency of 1B to 5>

kcCsec and are there beyond the human ear to respond. o heat is generated in case

of ultrasonic machining and it provides better working performance. These

processes is best suited for machining circular and non circular holes in hard to

machine and brittle metals whether conducting or non< conducting. /ltrasonic

waves can be generated by pie(oelectric or magneto stricture effects.

).1.@ $+$%T8: $A& &A%H'' "$..&.#

D


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'n this process, the material is removed with the help of a high velocity

"traveling at half the speed of light i.e. 1@>,>>> kmCsec#. 7ocused stream of electrons

which are focused magnetically upon a very small area. These heat and raise the

temperature locally above the boiling point and this melt and vapori(e the work

material at the point of bombardment. This process is best suited for micro cutting of

material because the evaporated area is a function of the bean power and the method

of focusing which can be easily controlled.

The electrons are obtained in 7ree 0tate by heating the cathode. &etal is

vacuum to the temperature at which they attain sufficient speed for escaping to the

space around the cathode. These can then be made to move under the effect of

electric or magnetic field and can be accelerated greatly. The acceleration is carried

out by electric field and focusing and concentration is done by controllable magnetic

fields.

).1.D A8A0'$ $T &A%H''

'n this process a focused stream of abrasive particles carried by high pressure

gas or air at a velocity of about )>> to 6>> mCsec is made to impinge on the work

surface through a o((le, and the work material is removed by erosion by the high

velocity of abrasive material removal rate which depends on flow rate and si(e of

abrasive particles. This process is best suited for machining super alloys and

refractory type of materials, and also machining thin sections of hard materials and

making intricate hard holes.

).1.B A8A0'$ 2AT$8 $T &A%H''

A narrow focused, water 3et is mi4ed with abrasive particles. This 3et is

sprayed with very high pressure resulting in high velocities that cut through all

materials. The water 3et reduces cutting forces and virtually eliminates heating. The

basic cutting mechanism is erosion.

on contact and no tool wear. ood for materials that canEt stand high

B


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temperatures of other methods for stress distortion or metallurgical reasons.

*eveloped in 1F@>Es. Typical pressures are 1>> k -si. +ower pressures are good

for soft materials and metals need higher pressures. The required 3et pressure

decreases with the use of harder abrasives. 0teel plates over 5G thick can be cut.

An abrasive water 3et is a 3et of water, which contains abrasive material.

/sually the water e4its a no((le at a high speed and the abrasive material is in3ected

into the 3et stream. This process is some times known as entrainment in that the

abrasive particles become part of the moving water much as passengers become part

of a moving train. Hence as with a train the water 3et becomes the moving

mechanism.

Abrasive 3et users need in


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7ig ).5? et formation geometry

).).) 2AT$8 -8:-$8T'$0

2ater is a liquid usually regarded as being incompressible, and this is a

sound statement at most ordinary pressures. However, at the high pressures used for

driving an abrasive 3et, water compresses up to 11 percent. The compression

percentage at pressures up to 1>>,>>> pounds per square inch "-0'#.

7or the mathematically inclined, the empirical equation is given by

ecause of the compressibility factor, a pump plunger pumping at =>,>>>

-0' moves at least 1> percent of its stroke length before any water begins to e4it the

check valve. 'f the pump cylinder contains any dead volume, an even greater force is

required.

At the end of the stroke, the dead volume is filled with energy containing

compressed water. -ump mechanical efficiency depends on how well this stored

energy is recovered.

1>

#1.)"........#>>>,=5C1" 1@).>

> eqp+=


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The two main pump types on the marketcrank drive and intensifierdiffer

in the ability to recover this energy. %rank drive pumps recover the e4panding

waterEs energy 3ust like an engine recovers e4panding hot gas energy. 'n the

hydraulically driven plunger of an intensifier pump, the energy is dumped as heat

into the hydraulic oil.

%ompressibility is useful in smoothing out pulsation from the pumping

equipment. 'ntensifier pumps typically pulse at about once per second with quite

large pulses and use an accumulator comprising about )>> cubic inches of fi4ed

water volume. The waterEs compressibility keeps the 3et running during the

intensifierEs plunger reversal cycle.

Three


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from >.@ to >.D. The uncertainty and variability in this value more than masks the

compressibility effects mentioned previously.

0ome literature refers to the orifice coefficient as orifice efficiency, but this

gives the false impression that it is in some way related to energy efficiency. 'n fact,

the orifice almost perfectly converts pressure energy into the waterEs kinetic energy,

with the orifice coefficient affecting only the amount of water flowing through its

effect on the stream diameter.

The amount of water flowing, J, simply is the stream area times the stream

speed. /nfortunately, we usually know the hole si(e rather than the stream si(e, so

the orifice coefficient is used to ratio the hole area down to the stream area. The

equation for the flow through an orifice is?

Assuming inches for the hole diameter, -0' for pressure, -& for flow, and

the liquid is water, the equation is?

This equation predicts the flow to within a few percent, with the ma3or uncertainty

coming from the orifice coefficient. The power required to force the water through

the no((le is calculated as the product of the upstream pressure and the flow rate.

2hen we use -0', -&, and horsepower as units of measure, the equation for

no((le power is?

1)

#).)"...........C)6C ) eqpdpCQ d =

#5.)"..............B6).)F ) eqpdCQ d=

#6.)".............5.1D16C. eqQppowerhorse =


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7ig ).6? ariation of flow rate with pressure and flow rate

The relationships among pressure, flow, no((le si(e, and no((le power are

such that if any two of the quantities are known, the other two can be calculated.

These relationships are shown graphically in 7igure ).6. The stream horsepower

entering the mi4ing tube top affects the cutting rate.

).).6 &'K' T/$ A* %/TT' -8:%$00

As discussed, the power required to drive the 3et is independent of what

occurs with the 3et in the mi4ing tube and below. The mi4ing tube and cutting

process usually are treated as a single phenomenon to be investigated by e4periment.

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This important second half of the flow process is described by empirical results

rather than results from engineering calculations. 'n a publicly reported study by

Ieng, 1 result was gathered from a large series of e4periments and equations were

fitted to the data, resulting in the equation?

2here

- Lstagnation pressure of water 3et in -0', typically =>,>>> -0'

d L orifice diameter in inches, typically >.>16 in.

&a L abrasive flow rate in lb.Cmin., typically >.B lb.Cmin.

fa L abrasive factor "1.> for garnet#

J L quality desired. 0et to 1.> for calculating separation speed

H L material thickness in inches

*m L mi4ing tube diameter in inches, typically >.>5> to >.>6> in.

L traverse speed in '-&

& L material machinability

Table 2.1

&achinability, &, of arious

&aterials

Hardened Tool 0teel

&ild 0teel

%opper

Titanium

Aluminum

B>

BD

11>

11=

)1

/nlike the previous equations, this one is valid only for parameters near to

those tested e4perimentally, as listed above. This relatively simple equation

summari(es the results from many test cutting hours, but says nothing about the

detailed behavior of the 3et, other than its ability to cut various materials, nor does it

provide an e4planation of why things are as they are.

16

#=.)"...........1@5

1=.1

@1B.>

565.>5D6.1=F6.1

eqDmHQ

MadMpfaV

=


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-rocesses within the mi4ing tube are very comple4. The flow contains high percent

water, F percent abrasive, and 1 percent air.

At the tubeEs top, the water is at the high speed calculated above. 'n fact, it is

highly supersonic with respect to the air. The air and abrasives are moving very

slowly in comparison. 2ithin the 6> ft.Csec. This acceleration is about 6@,>>>

times the acceleration of gravity, and it causes many abrasive grains to break. The

grains rub on and erode the mi4ing tubeEs sides. 7igure ).= shows the comple4,

sausage like wear pattern in a used mi4ing tube, indicating a very comple4 flow

pattern.

Fig 2.5: comple !lo" pa##e$%

+ikewise, the cutting process is comple4. The 3et e4it point lags the entrance

point by an amount that depends on the cutting speed. 't flops from side to side,

causing striations on the cut surface. 't produces a tapered kerf that ranges from

being widest at the top at high speed to widest at the bottom at low speed. &oreover,

the total kerf width depends on the cutting speed.

The H- and flow rate required to drive the 3et cutting process are determined

completely by the water no((le. o((le flow physics are well < known, and

quantities can be predicted accurately from scientific principles.

The flow in the mi4ing tube below the water no((le and in the material being

cut is comple4. *etermining it requires empirical study. 8esults of these empirical

studies often are proprietary, because machine manufacturers use this information to

compete on the ability to make good parts easily. However, this information is

embedded within the machinesE controllers and provides the user with the ability to

make e4cellent parts without detailed process knowledge.

1=


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).5 A8A0'$ -A8T'%+$0

).5.1%+A00'7'%AT': A* -8:-$8T'$0 :7 A8A0'$ &AT$8'A+0

A large number of different typos of abrasive materials are used in the

abrasive water< 3et technique. The evaluation of an abrasive material for abrasive

water.>>@# 1=)F.@)

0pessartine < 11.@15 ">.>>=# 1=@@.1=

-yrope < 11.6=D ">.>>=# 1=>5.BB

rossulare 5> 11.B@D ">.>>=# [email protected]

Andradite B> < F> 1).>F1 ">.>>F# 1D@D.@1

0ince the abrasive particles erode the material and this is a mechanical

operation, which is a cross between the shearing and compressing the material by

the particle, it can be seen that the above characteri(ation of the particles is crucial.

The particles must be hard so that they are the erodes as opposed to being the

1@


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eroded. The shape is important. -articles with sharp edges can be envisioned to be

good cutters and upon impacting the material at one of their sharp edges can cause

high stress concentrations. 7ig. ).@ gives a few e4amples of shapes that would be

good for such purposes

7ig ).@? Typical shapes of garnet abrasive used for abrasive water


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turbulence of the 3et. Therefore much effort has gone into the Abrasive -article

*elivery 0ystem

Here are several other potential designs, two of which try to give theparticles speed either due to gravitational force or air pressure, and the other being a

design using a premi4 of the water and particles before 3et formation.

Fig 2./: 'a$#icle -a#e$ '$emi

Fig 2.0 Ab$a+i,e 'a$#icle $a,i# Fee3

1B


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Fig 2.14: Fo$ce3 Ai$ 'a$#icle Fee3

7or illustrative purposes 7ig. will be used since it is a basic model that

combines the 3et former, the mi4ing chamber and the collimator "which is also

known as the focus tube#. The top section is the e4iting straight portion of the 3et

former. Here the 3et e4its the no((le and enters into the mi4ing chamber. 7rom

ernoulli!s equation one can appro4imate the 3et speed into the chamber as?

2here - is the pressure developed in the straight portion of the 3et former

and r 2 is the density of the water. Here it is assumed that the pressure in the 3et

former is significantly greater than in the mi4ing chamber and the velocity of the

e4iting 3et into the mi4ing chamber is much greater than the fluid velocity in the 3et

former. 2ith these assumptions then the formula above is a good appro4imation to

the 3et speed as it enters the mi4ing chamber.

The abrasive particles are supplied from the right inlet tube and are pulled in

by a pressure differential created by the moving fluid past the inlet port "similar to

the lift developed on an aircraft wing as the air above the wing moves faster than the

air under the wing bottom thus creating an upward pressure potential or lift.# 7rom

1F

#@.)"...................

)> eqw

p

V th

=


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A simple conservation of momentum, the speed of the resulting abrasive

particle loaded water 3et can be estimated as

2here ma and mw are the mass flow rates of the abrasive particles and 3et water

respectively and Ais the speed of the abrasive water 3et as it enters the bottom

portion of the mi4ing chamber. 'n applying the conservation of momentum it has

been assumed that the impact of the 3et stream water and the abrasive particles is a

perfectly inelastic collision.

The purpose of the final reduced cross


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ABRASIVE WATER JETMACHINING VS OTHER METHODS

)1


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Chapter3A8A0'$ 2AT$8 $T &A%H'' 0 :TH$8&$TH:*0

7ig 5.1? Abrasive water 3et machining

Abrasive water 3et machining is a relatively new machining technique in thatit makes use of the impact of abrasive material to erode the work piece material. 't

relies on the water to accelerate the abrasive material and deliver the abrasive to the

work piece. 'n addition the water afterwards carries both the spent abrasive and the

eroded material away from the working area. %onventional machining practices

such as milling use a solid tool to cut the material usually by a shearing process.

%onventional machining may also use a liquid medium in con3unction with the

cutting tool but its purpose is not to deliver but to carry away the material. 'n

addition for both conventional and abrasive water 3et machining the liquid medium

will also act as a heat sink, taking heat away from the machining area.

5.1 %:&-A8' 2'TH +A0$80

Abrasive water 3ets can machine many materials that lasers cannot.

"8eflective materials in particular, such as Aluminum and %opper.

/niformity of material is not very important to an Abrasive 3et.

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Abrasive 3ets do not heat your part. Thus there is no thermal distortion or

hardening of the material.

-recision abrasive 3et machines can obtain about the same or higher

tolerances than lasers "especially as thickness increases#.

9our capital equipment costs for water 3et are generally much lower than that

for a laser. '.e. for the price of a laser, you can purchase several abrasive 3et1G ">.>)=mm# work,

but thatEs the e4ception, not the norm, and only on certain shapes and

materials.#

o heat affected Ione with Abrasive 3ets.

Abrasive 3ets require less setup.

&ake bigger parts.

&any $*& shops are also buying water 3ets. 2ater 3ets can be considered

to be like super


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same fi4turing on the $*& to do secondary operations and final cutting to

e4tra tolerance.

5.5 %:&-A8' 2'TH -+A0&A C 7'$ -+A0&A

Abrasive 3ets provide a nicer edge finish

Abrasive 3ets donEt heat the part

Abrasive 3ets can cut virtually any material

Abrasive 3ets are more precise

-lasma is typically faster

2ater 3ets would make a great compliment to a plasma shop where more

precision or higher quality is required or for parts where heating is not good,

or where there is a need to cut a wider range of materials.

Fig &.8: )o3e$% mac7i%e+ a$e $ela#i,el clea% a%3 9ie#.

5.6 %:&-A8' 2'TH 7+A&$ %/TT'

Abrasive 3ets provide a much nicer edge finish

Abrasive 3ets donEt heat the part

Abrasive 3ets can cut virtually any material

Abrasive 3ets are more precise

)@


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7lame cutting is typically faster

7lame cutting is typically cheaper, if you can use it.

2ater 3ets would make a great compliment to a flame cutting where more

precision or higher quality is required or for parts where heating is not good,

or where there is a need to cut a wider range of materials.

5.= %:&-A8' 2'TH &'++'

There is only one tool to qualify on an abrasive 3et

0etup and 7i4turing typically involves placing the material on the table with

an abrasive 3et

%leanup is much faster with an abrasive 3et

-rogramming is easier and faster

&achine virtually any material, including?

brittle materials

pre hardened materials

:therwise difficult materials such as Titanium, Hastalloy, 'nconel, 00 5>6,

hardened tool steel....

2ater 3ets are used a lot for complimenting or replacing milling operations.

They are used for roughing out parts prior to milling, for replacing milling

entirely, or for providing secondary machining on parts that 3ust came off the

mill. 7or this reason, many traditional machine shops are adding water 3et

capability to provide a competitive edge.

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Fig &.: -7e% compa$i%g "i#7 milli%g

5.@ %:&-A8' 2'TH -/%H -8$00

+ower cost per piece for short runs

-lace holes closer to the materials edge

7ast turn


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Fig &.*: -7e% compa$e3 "i#7 p%c7 p$e++

Abrasive 3ets also play a big part as 3ust one part in a larger manufacturing process.

7or e4ample, abrasive 3ets are often used to machine features into an e4isting part, or

to do pre


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Chapter 4

A(RASIVE ET )AC;ININ

'n this process a focused stream of abrasive particles carried by high pressure

gas or air at a velocity of about )>> to 6>> mCsec is made to impinge on the work

surface through a o((le, and the work material is removed by erosion by the high

velocity of abrasive material removal rate which depends on flow rate and si(e of

abrasive particles. This process is best suited for machining super alloys and

refractory type of materials, and also machining thin sections of hard materials and

making intricate hard holes.

6.1 $lements of A..&.

The criteria used for evaluating the A..& process, vi(., material removal

rate, geometry of cut, are influenced by the following elements.

"i# o((le

"ii# Abrasives

"iii# elocity of fluid

5>


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"iv# 0tand off distance

"v# %arrier gas

6.1.1 o((le?

The abrasive particles are directed in to the work surface at high velocity

through no((le therefore, the material of the no((le is sub3ected to a great degree of

abrasion wear and hence these are made of hard materials.

Tungsten carbide no((les are used for circular cross section in the range of

>.1).B> mm diameter, and for rectangular section of si(e >.>B >.=> to >.1B

5.B mm

The rate of material removal and the si(e of the machined area are influenced

by the distance of the no((le from the work piece. The abrasive particles from the

no((le follow a parallel path only for a short distance and then the 3et floares

resulting in the aver si(ing of the hole. The material removal rate initially increases

with increase in the distance of the no((le from the work piece because of the

acceleration of particle after leaving the no((le. This increase is ma4imum up to a

distance of about Bmm and then it steadily drops off because of increase in

machining area of the some amount of abrasive and decrease in velocity of abrasive

particle stream due to drag.

7or precision machining, the no((les are provided with e4tended taper at the

tip in order to minimi(e secondary erosion by abrasive particles that rebound from

the work surface.

6.1.) 0tand off distance

0tand off distance plays a very important role in material removal rate in

A..&. as is evident from the e4perimental curve shown in fig.

51


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m.r.p

stand off distance

't is evident from fig. That the material removal rate increases with the

increase of tip distance up to a certain limit after which it remains unchanged for a

certain tip distance and then flows gradually. The stand off distance has also a

direct effect on the width of cut owing to the out ward flowing of the A. after a

short distance.

6.1.5 elocity of fluids

The velocity of the carrier gas converging the abrasive particles changes

considerably with the change of abrasive particle density as indicated in fig.

$4it velocity of gas

$4it velocity of gas %ritical velocity

Abrasive particle density

The e4it velocity of gas can be increased to critical velocity when the

internal gas pressure is nearly twice the pressure at the e4it of the no((le for an

abrasive particle density of (ero .'f the density of abrasive particles is gradually

increased, the e4it velocity will go on decreasing for the same pressure condition. 't

is due to fact that the kinetic energy of the fluid is utili(ed for transporting the

abrasive particles. However proper analysis is yet to be done in this regard.

5)


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7rom the above analysis, it is evident that the increased mass flow rate of

abrasive will result in a decreased velocity of fluid and will thereby cause a decrease

in the available energy for erosion and ultimately the material removal rate

.However, it is convenient to e4plain this fact with the help of a well defined term

known as mi4ing ratio.

&i4ing ratio =

6.1.6 %arrier gas

The carrier gas or propellant which is used in A..&. process is usually air,

carbon dio4ide "co2#, and nitrogen ")#, but never use o4ygen. %ommercial gas

cylinders can be employed satisfactorily.

The no((le pressure which generally affects the cutting process is generally

maintained between ) to B.= kgfCunit. Higher no((le pressure results in greater metal

removal rate, but it decreases the no((le life. The most suitable no((le pressure has

been found to be = kgfCunit.

6.) TH$ -H90'%0

1. 7ine particles ">.>)=mm# are accelerated in a gas stream "commonly air at a

few times atmospheric pressure#.

). The particles are directed towards the focus of machining "less than 1mm

from the tip#.

5. As the particles impact the surface, they fracture off other particles.

55

&ass flow rate of Abrasive

&ass flow rate of fluid or gas


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Fig 8.1: Ab$a+i,e 6e# mac7i%i%g

As the particle impacts the surface, it causes a small fracture, and the gas

stream carries both the abrasive particles and the fractured "wear# particles away.

rittle and fragile work pieces work better.

6.).1 The factors are in turn effected by

1. < The abrasive? composition; strength; si(e; mass flow rate

). < The gas composition, pressure and velocity

5. < The no((le? geometry; material; distance to work; inclination to work

6.).) The abrasive

1.


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1. < -ressure is typically >.) Cmm) to 1Cmm)

). < as composition effects pressure flow relationship

6.).6 o((le

1. < &ust be hard material to reduce wear by abrasives? 2% "lasts 1) to 5> hr#;

sapphire "lasts 5>> hr#

). < %ross sectional area of orifice is >.>=.) mm)

5. < :rifice can be round or rectangular

6. < Head can be straight, or at a right angle

Fig 8.2: Tpe+ o! %o


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6.5 TH$ 8$+AT':0H'- $T2$$ H$A*, A* :II+$ T'-

*'0TA%$.

Fig 8.&: $ela#io%+7ip be#"ee% 7ea3= a%3 %o


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6.= 0/&&A89 :7 A& %HA8A%T$8'0T'%0

1. &echanics of material removal < brittle fracture by impinging abrasive grains

at high speed

). &edia < Air, %:)

5. Abrasives? Al):5, 0ic, >.>)=mm diameter, )gCmin, non> mCsec

=. -ressure L ) to 1> atm.

@. o((le < 2%, sapphire, orifice area >.>=.) mm), life 1)> hr., no((le tip

distance >.)=. +imitations < low metal removal rate "6> mgCmin, 1= mm5Cmin#, embedding

of abrasive in work piece, tapering of drilled holes, possibility of stray

abrasive action.

6.@ -8:%$00 %HA8A%T$8'0T'%0

[email protected] &$TA+ 8$&:A+ 8AT$

A typical material removal rate for abrasive 3et machining is 1@ mm5Cmin in

cutting glass. A minimum practical width of cut of about >.1 mm cab be obtained by

using a rectangular no((le with an orifice of >.>D=>.= mm placed at a distance of

>.>B mm.

Abrasive 3et machining is a finishing process that removes material from work

pieces by focusing a high speed stream of abrasive particles carried in an air 3et.

/ses a high velocity stream of abrasive particles carried in an air or gas 3et

's used for machining delicate or very hard materials

5D


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o((les are usually made of tungsten or sapphire to resist abrasion

-roduces a taper in deep cuts

*istance of no((le from work piece affects the si(e of the machined area.

Fig 8.: Ac#io% o! ab$a+i,e pa$#icle+ o% "o$> piece

Fig 8.*: +7o"+ #7e "o$> piece mac7i%e3 b A)

6.@.) Accuracy and surface finish

2ith close control of the various parameters a tolerance in the region of

>.>= mm can be obtained. :n normal production work an accuracy of >.1 mm is

5B


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easily held. &asking to define the machining area is sometimes used go prevent

stray cutting. %opper, glass and rubber are used as masking materials. 8ubber has a

good life but has gives poor definition. The corner radius obtained can be limited to

>.1 mm. taper is around >.>= mm per 1>mm penetration.

The surface finish ranges from >.6mm# thick part may take hours.

A 1C1@G "1.=mm# part will cut in seconds

6.D $T 2H$ 'T $K'T0 TH$ :II+$

+ike a fire hose. The 3et starts out coherent, then fans out. 2hen cutting

thick metal, the 3et will be held coherent by the metal you are cutting.

5F


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Fig 8./: e# "7e% i# ei#+ #7e %o


41/53

eneral purpose machine shops? Here is where ' see the biggest growth in abrasive

3et use. This is because abrasive 3ets are such good all


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&odel shops C rapid prototyping? 7ast turn around of single piece production

in nearly any material makes water 3ets great for these kinds of applications.

2ealthy hobbyists? ' have seen a few people buy them for their homes. '

want one in my garage too.

0chools? &any of the larger si(e universities that offer engineering classes

also have water 3ets. They are great tools for the classroom environment

because they are easy to learn, program, and operate, and because they can

make one


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FABRICATION OF MODEL FORABRASIVE JET MACHINING

65


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Chapter 57A8'%AT': :7 &:*$+ 7:8 A8A0'$ $T

&A%H''

The e4perimental set up of abrasive 3et cutting machine is consists of air

compressor of ma4 pressure allowed is D kgCcm). The abrasive powder silicon

carbide is supplied from a hopper. The hopper is made of galvani(ed iron sheet. The

outlet of hopper is connected to a >.=.=


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5

= 6 ) 1

@

6=


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1. %ompressor

). all valve

5. Hopper

6. 'nclined T 3oint

=. -ipe line

@. o((le

6@


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PROCESS VARIABLES INABRASIVE JET MACHINING

6D


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Chapter 6-8:%$00 A8'A+$0 ' A8A0'$ $T &A%H''

@.1 -$87:8&A%$ :7 A8A0'$ $T &A%H''

The performance of A..&. is 3udged by the following

&etal removal rate

7inish of the work piece

8ate of wear of the no((le

@.1.1 &etal removal rate in A..&

Taking into consideration the fact that metal removal is due to the chipping

of the work surface brought about by the impacting abrasive particles. &athematical

model for the computation of the metal removal rate in A&. They have shown that

the rate of metal removal J in A& can be obtained by $q.

#1.@".........1)

6C5

)C55

6 eqavNdKQ

y

=

7rom eq it can be seen that

#).@"....)C5 eqNvQAs a first degree appro4imation, i can be assumed that

#5.@"......eqpN

#6.@".......)C1 eqpv

Therefore#=.@".........1>D= eqpQ

@.1.1.1 TH$ 7A%T:80 A77$%T' &$TA+ 8$&:A+ 8AT$

The variables that influence the rate of metal removal and accuracy of

machining in this process ate?

"i# %arrier gas.

6B


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should be so designed that the pressure loss due to bends, friction, etc. is as little as

possible.

CONCLUSION

=1


52/53


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The abrasives used for cutting are aluminum o4ide and silicon carbide where

as sodium bi

electro chemical machinig

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