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1、Unit 9 The Special-Purpose Machining Unit 9 The Special-Purpose Machining EquipmentEquipment Lesson 21 Electro-discharge Machines 9.1Lesson 22 Types of Electro-discharge Machines 9.2Reading Materials 21 Traveling-wire EDM Machine 9.3Reading Materials 22 Electrochemical Machines 9.49.1 9.1 Lesson 21
2、Electro-discharge MachinesLesson 21 Electro-discharge Machines Electricaldischargemachining(EDM),orelectro-dischargemachining,orsparkmachining,asitisso-called,isbasedontheerodingeffectofanelectricsparkonboththeelectrodesusedtoproduceit.Whenapplyingtheelectro-dischargemachiningprocess,thematerialiser
3、odedasaresultofanelectricaldischargebetweentoolandwork.AsinthecaseofECM,EDMisacopyingprocesswherethereisnocontactbetweentoolandwork.ContrarytoECM,thereissomeerosionofthetoolinEDM,whichmustbeallowedforinthetooldesigntoensureaccuracyofmachining.AfurtherdifferencearisesfromthefactthatinEDMthereisnofixe
4、dtoolfeed,butthegapsizemustbemaintainedinaccordancewiththerateofmetalremovalandtheconditionsexistingwithinthegap.1EDMisaformofmetalremovalinwhichpulsatingdirectcurrentisappliedtoashapedtool(electrode)andaworkpiece,bothofwhicharecapableofconductingelectricity.Thetwoareheldclosetoeachotherwithanon-con
5、ductingfluidservingasaninsulatorbetweenthemWhenavoltagehighenoughtobreakdowntheinsulatorisreached,asparkjumpsthegapbetweenthetoolandworkpiece.Asaresult,thissparkremovesasmallportionofmaterial.Themainadvantageisthatelectricaldischargemachiningprocesseswithgoodsurfacefinishandcanmachinemetalofanyhardn
6、ess.9.2 9.2 Lesson 22 Types of Lesson 22 Types of Electro-discharge MachinesElectro-discharge Machines Therearetwobasictypesofelectro-dischargemachinesdescribedasbelow:Electro-dischargedie-sinkingmachinesTheconstructionprinciplesofanEDMdie-sinkingmachineareshowninFigure9-1.Sparkerosiontakesplaceinac
7、ontainerfilledwiththedielectric,inwhichtheworkpieceisclamped.Thecontrolledfeedoftheelectrodeisthroughanelectro-hydraulicorelectro-mechanicalservosystem.Theelectricalenergyforerosionisprovidedbytheerosiongenerator.ThefilteringunitseparatedtheerodedmaterialfromthedielectricTheappliedvoltageionizestheg
8、apatthebeginningofthedischarge.Atthepointofthehighestfieldstrength,achannelisformedthroughwhichthedischargecurrentflows.Ateachendofthechannel,thematerialmeltsandthechannelanditssurroundinggasbubbleexpand.Whenthevoltageisfullydischarged,thechannelcollapsesandthemoltenmaterialvaporizes,simulatingamini
9、atureexplosion.Theresultantcraterisahallmarkoftheirregularandscarredsurfacefinishofspark-erodedwork.Electro-dischargecuttingmachinesAnimportantapplicationofthespark-erosionprocessisthecuttingofmetalbywireelectrodesTheprocessisusedfortheproductionofaperturesincuttingtoolsandthemanufactureoftoolelectr
10、odesforEDM.Figure9-2illustratestheprinciple.Thecuttingtoolisacopperorbrasswire,whichenterstheworkduringcuttingwithoutphysicalcontactThissufferswearasaresultoftheactionofsparkerosion,andforthisreasonfreshwireisconstantlysupplied,soitisso-calledwireelectricaldischargemachines(WEDM).Theworkingmediumfor
11、electro-dischargecuttingisusuallyde-ionizedwater,whichisfedtotheworkareawiththeuseofflushingjets.9.3 9.3 Reading Materials 21 Reading Materials 21 Traveling-wire EDM MachineTraveling-wire EDM Machine Aswithallelectricaldischargemachining,theactualmetalremovalistheresultofasparkdischargejumpingthegap
12、fromatoolthroughadielectrictotheworkpiece.Uponstrikingtheworkpiece,thesparkgeneratesanintenselocalizedheatthatvaporizesamicroscopicportionoftheworkpiecesurface.Intimetheworkpieceissimplyerodedaway.Thetraveling-wiremethodemploysareelofcopperwirethatisslowlyfedpasttheworkpieceandfunctionsmuchthesameas
13、abandsawexceptthewireisusedonlyonce.Injumpingfromthewiretotheworkpiece,thesparkserodeawayaclearlydefinedpath,whichcanbeverycloselycontrolledwithNC.Hardenedtoolsteelsandcarbidesareeffectivelymachinedwiththismethod.Thereisonlyonerequirementtheworkpiecemustbeelectricallyconductive.Traveling-wireelectri
14、caldischargemachiningisshowninFigure9-3.9.4 9.4 9.4 9.4 Reading Materials 22 Electrochemical MachinesReading Materials 22 Electrochemical Machines Theprincipleunderlyingelectrochemicalmachining（ECM）restsonanexchangeofchargeandmaterialbetweenapositivelychargedanodicworkpieceandanegativelychargedcatho
15、detoolinanelectrolyteInsuchconditions,theanodedissolveswhilstthecathode（tool）isnotaffectedThevolumeofmetalremovalmaybecalculatedaccordingtoFaradaysLaw：V=CItWhereCaconstantdependentonworkmaterial,Ithecurrentflowingbetweenthetoolandthework,tthetimeoferosion.Thecurrentisdependentonthegapbetweenthetoola
16、ndthework,theareaoferosionandtheconductivityoftheelectrolyte,aswellasthesupplyvoltage.Theworkinggapmaintainedbetweentheelectrodeandworkallowsmachiningtotakeplacewithoutphysicalcontact.TheelectrolyteispumpedintotheworkinggapandalsoservesasacoolantwhichisnecessaryduetothehighenergydensityThematerialwh
17、ichhasbeenerodedfromtheworkformsasludge,andmustbeseparatedfromtheelectrolytebyfilterorcentrifuges.Electrochemicaldie-sinkingmachinesThemaincomponentsofanelectrochemicaldie-sinkingmachine(Figure9-4).Afeedingdeviceadvancesthetooltowardstheworkinaccordancewiththerateofmetalremoval.Whenproducinginternal
18、forms,adesignproblemarisesinrelationtotheshapeandsizeofthetool.Thegapisnotconstant,butisafunctionofthestateofthesurfacetobeerodedandtherateoftooladvanceIf,forexample,acylindricalboreistobesunk,asimplecylindricaltoolisnotsuitable(Figure9-5).Thiswouldresultinaconstantlyincreasinggapsizeandthecurrentde
19、nsitywoulddecreaseinproportion(Figure9-5left).Withatoolsuitablyinsulatedonitssides(Figure9-5right),theoffendingexcessiveerosionofcylindricalsideswillbesuppressed.Electrochemicaldie-sinkingmachinesareconstructedinanopen“C”structureforsmall-tomedium-sizedwork,andformachineshavinglargerworkareastheclosed“O”constructionformisused.