Electric Discharge Machining[1]

8/12/2019 Electric Discharge Machining[1]

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ELECTRIC DISCHARGE MACHINING

Electric discharge machining provides an effective manfactring techni!e that ena"les

the prodction of parts made of special materials #ith complicated geometr$ #hich isdifficlt to prodce "$ conventional machining processes% Controlling the process

parameters to achieve the re!ired dimensional accrac$ and finish placed this machiningoperation in a prominent position% &rom that reason' electric discharge machining has

fond "road applications in indstr$% The a"sor"ing interest for electric dischargemachines has reslted great improvements in EDM technolog$% No#ada$s' sophisticated

electric discharge machines are availa"le for most of machine shop applications%

&igre (%(% )asic Elements of an EDM s$stem

)asicall$ Electric Discharge Machining *EDM+ is a process for eroding and removingmaterial "$ transient action of electric spar,s on electricall$ condctive materials% This

process is achieved "$ appl$ing consective spar, discharges "et#een charged #or,piece

and electrode immersed in a dielectric li!id and separated "$ a small gap% -sall$'locali.ed "rea,do#n of the dielectric li!id occrs #here the local electrical field is

highest% Each spar, melts and even evaporates a small amont of material from "oth

electrode and #or,piece% /art of this material is removed "$ the dielectric flid and the

remaining part resolidifies rapidl$ on the srfaces of the electrodes% The net reslt is thateach discharge leaves a small crater on "oth #or,piece and electrode% Application of

consective plses #ith high fre!encies together #ith the for#ard movement of the toolelectrode to#ards the #or,piece' reslts #ith a form of a complementar$ shape of the

electrode on the #or,piece%

The material removal rate' electrode #ear' srface finish' dimensional accrac$' srface

hardness and te0tre and crac,ing depend on the si.e and morpholog$ of the craters


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formed% The applied crrent' voltage and plse dration' thermal condctivit$' electrical

resistivit$' specific heat' melting temperatre of the electrode and #or,piece' si.e and

composition of the de"ris in dielectric li!id can "e considered as the main ph$sicalparameters effecting to the process% Among them' applied crrent' voltage and plse

dration are the parameters #hich can "e controlled easil$%

Ever$ EDM machine has the follo#ing "asic elements as sho#n in &igre (%(%

*i+ Spar, generator

*ii+ Servo s$stem

*iii+ Dielectric li!id

iv% Mechanical strctre

1.1.1 Spark Generator

The re!ired energ$ is in the form of plses sall$ in rectanglar form% Recent stdies

has "een sho#n that application of plses in the form of trape.oids reslted #ith amar,ed improvement in ctting efficienc$% The optimm plse form is not e0actl$ a

trape.oid' "t similar% 123 4

Electrical energ$ in the form of shot dration implses #ith a desired shape shold "e

spplied to the machining gap% &or this prpose' spar, generators are sed as the sorceof electrical plses in EDM% The generators can "e distingished according to the #a$ in

#hich the voltage is transformed and the plse is controlled% The discharge ma$ "e

prodced in a controlled manner "$ natral ignition and rela0ation' or "$ means of acontrolla"le semicondctor s#itching elements% No#ada$s' sophisticated compter aided

spar, generators are in se as a reslt of fast development in electronics indstr$% These

t$pe of generators give s a "etter manner in controlling ph$sical parameters%

1.1.2 Servo System

)oth electrode and #or,piece are eroded dring the process' after a certain timedimensions of the electrodes #ill "e changed considera"l$% The reslt is increase in

interelectrode gap% This #ill increase the voltage re!ired for spar,ing% This pro"lem can

"e solved "$ increasing the plse voltage or decreasing the gap distance% The former is

not feasi"le since most of the electrical energ$ is sed for overcoming "rea,ing strengthand prodcing plasma in dielectric li!id rather than machining' in addition to that' the

re!ired voltage can increase to the levels that spar, generator can not sppl$' therefore5the interelectrode gap shold "e maintained constant dring the process% This can "e

achieved "$ a servo s$stem #hich maintains a movement of the electrode to#ards the

#or,piece at sch a speed that the #or,ing gap' and hence' the spar,ing voltage remains

naltered%


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1.1.3 Dielectric Circuit

High cooling rates dring resolidification process changes the chemical composition of

the "oth electrodes and dielectric li!id machining particles called de"ris are formed%&ormation of sch particles effects on machining performance' therefore' dielectric li!id

shold "e circlated to prevent contamination in #or,ing gap% This circlation is done "$a dielectric circit #hich is composed of a pmp' filter' tan, and gages%

1.1.4 Mechanical Structure

EDM machines has similar constrction #ith conventional drilling and milling machineframes #ith vertical tool feeding and hori.ontal #or,ta"le movements% Since there is not

a real contact "et#een electrodes' that678s #h$' it is considered that' the frame elements

not ta,ing mch force as in conventional machining so simpler design is possi"le% Thisconsideration needs a little "it attention' "ecase gas """les collapses at the end of

discharge and case high frontal shoc, #aves' therefore5 the frame shold "e strong

enogh to ,eep its dimensional sta"ilit$%

1.2 Operation Types of lectric Dischar!e Machines

Electric discharge machining ena"les the machining operation in several #a$s% Some of

these operations are similar to conventional operations sch as milling and die sin,ing

others have its o#n characteristic% Different classifications are possi"le and also it shold

"e ,eep in mind that' crrent developments in its technolog$ adds different t$pes ofoperations% )t a simple and general classification can done "$ considering famos

applications sch as '

i% Die Sin,ing EDMii% 9ire EDMiii% EDM Milling

iv% 9ire Electric Discharge Grinding

1.2.1 Die Sinkin! DM

The tool electrode has the complementar$ form of finished #or,piece and literall$ sin,sinto the rogh material% Comple0 shapes are possi"le' "t needs more machining time "t

dimensional accrac$ is high #hen compared #ith #ire EDM%

1.2.2 "ire DM

The electrode is a #ire that cts throgh the #or,piece and rene#ed constantl$ to avoidraptre% The #ire is cheaper than the comple0 electrodes sed in die sin,ing electric

discharge machining% Less material shold "e removed' #hich leads short machining time

and electrode #ear% )t' the operation is possi"le onl$ for rled srfaces and the #ire

ma$ "end dring machining' case s"stantial shape errors%


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1.2.3 DM Millin!

-sall$ a rotating c$lindrical electrode follo# a path throgh the #or,piece' $ielding the

desired final geometr$% It is advantageos #hen large holes or comple0 geometries arere!ired%

1.2.4 "ire lectric Dischar!e Grin#in!

In the case #here small holes are needed' a relativel$ large electrode ma$ "e reversi"l$

eroded against a sacrificial #or,piece% In this case the polarit$ "et#een the electrode and

the #or,piece is reversed' so that the material removal predominantl$ ta,es place on theelectrode%

1.3 Material $emoval Mechanism

A perfect general theor$ for EDM can not constrcted since each machining condition

has its o#n particlar aspects and involves nmeros phenomena' i%e%' heat condctionand radiation' phase changes' electrical forces' """le formation and collapse' rapid

solidification% In addition' theories of ho# spar,s eroded the #or,piece and electrode

have never "een completel$ spported "$ the e0perimental evidence since its ver$difficlt to o"serve the process scientificall$% Ths' most of the p"lished stdies are

mostl$ concerned #ith simplified models of different events of EDM% Development of

high:speed compters and comprehensive nmerical techni!es ena"led scientist to

involve more parameters in their models than "efore' "t still man$ aspects of the processcan not "e e0plained in detail%

Melting' vapori.ation and even ioni.ation of the electrode materials occrs at the point

#here the discharge ta,es place% &lshing action of the dielectric li!id plls a#a$ #holevapori.ed and some of melted material% The reslt is formation of a crater on "othelectrode srfaces% Theoretical models "ased on one spar, can "e e0tended to the

machining #ith same side effects%13;4Generall$ the ph$sics of the spar,s can "e

investigated in three phases%

*i+ )rea,do#n *Ignition+ phase

*ii+ Discharge phase

iii% Erosion*Crater &ormation+ phase

)rea,do#n phase ta,es a relativel$ small percent of the total spar, time% It varies from

fe# microseconds to several hndreds depending on discharge conditions% Erosion is onl$o"served onl$ in later stages of spar,' partl$ after the discharge has eased%12


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)rea,do#n in li!ids is the initial condition for plasma formation% There are several

proposed theories #hich tries to e0plain the "rea,do#n phase' "t consistent reslts #ith

e0periments can not "e o"tained% A simplified e0pression can "e given as "elo#%

The charge indced on the t#o electrodes "$ the po#er sppl$ creates a strong electric

field% This field is strongest #here the electrodes are closest to each other% This is thepoint #here discharge ta,es place% Molecles and ions of the dielectric flid are polari.ed

and oriented "et#een these t#o pea,s and forming a narro#' lo#:resistance channel%9hen the dielectric strength of the li!id in the gap e0ceeded de to the electric field'

"rea,do#n occrs% Electrons emitted from "oth electrodes and the stra$ electrons and

ions in the dielectric li!id are accelerated% 9hen accelerated electrons and ions reachedto the electrons a crrent flo# starts #hich is the "eginning of the discharge phase% 1=


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Appl$ing consective spar, discharges and driving one electrode to#ards the other #ill

erode the #or,pice gradall$ in a form complementar$ to that of the tool electrode

%

4. Characteristic of lectrical Dischar!e Machine# Surfaces

The general appearance of the crater formed after spar,ing is almost the sane for the

different materials e0cept for their si.es and depth% Spar,ing leaves a #ell de fined ridge

#hich cold have come onl$ throgh the deposition of the molten material from the

crater% This is de to machined srface #hich pro?ects ot from the original srfaceindicating redeposition of the material after erosion% Ths #ith the formation of the

crater' material is splashed to the sides forming the ridge% 13@4 Thermal properties of

machined material and dielectric flid and energ$ condition of the spar, can "econsidered as the main parameters #hich determines the si.e of the crater%

A spar, eroded srface is matt srface #ith random distri"tion of overlapping craters%The violent natre of the process leads a ni!e strctre on the srfaces of the machined

parts% This natre compelled interest from several researchers%

1.4.1 Thermally (nfluence# ,ayers

E0perimental o"servations revealed mainl$ t#o thermall$ effected' distinct la$ers of

materials ,no#n as the #hite la$er and the heat effected .one%

The #hite la$er incldes some particles resolidified dring the process and can not "e

flshed a#a$ "$ dielectric flid% The la$er is densel$ infiltrated #ith car"on that has a

totall$ different strctre from that of the original #or,piece% Car"on enrichment occrs#hen the h$drocar"on dielectric "rea,s do#n and penetrates in to the hot #or,piece

srface%

Heat effected .one lies "elo# the #hite la$er strctre% It is partl$ effected "$ car"ondra#n "$ the dielectric% It retains some strctral properties of the original material%

-naffected material is lies ?st "elo# to this la$er%

1.4.2 Stress Distri-ution

The violent natre of the process leads a ni!e strctre on the srfaces of the machined

parts% High cooling rates can "e attained on sperheated material de to !enching effectof the dielectric% The high thermal contraction rates prodces high stresses and case

severe slip' t#ining and cleavage on or near the crater% Another sorce of stresses can "econsidered as the shoc, #aves prodced "$ the e0plosion of """le at the end of the

discharge% Ths' the resolidified la$er is highl$ stressed% These stresses changes the

material properties drasticall$ and prodce a net#or, of micro crac,s arond the crater

even in dctile materials%


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Electric Discharge Machining[1]

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