参考成果第6章

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CHAPTER6

TheEDMWorkpiieceSurface

6.1Workpiece'sSurfaceQuality

Workpiece'ssurfacequalitymachinedbyEDMmainlyincludesthreeparts:

Surfaceroughness,

Alteredmetalzoneand

Surfacemechanicalproperties.

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6.1.1．SurfaceRoughness

ThesurfaceproducedbyEDMsparkingisnon-directionalandnon-reflectiveandismadeupofmanysphericalcavities.Theaffectingfactorsare:

Pulseenergyofpowersupply

Workpiecephysicalcharacteristic

Electrodesurface

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PulseEnergyofPowerSupply

Thesizeofthesphericalcavitiesdependsonthemeltingtemperatureoftheworkpiece,theelectricalpowercontainedinthespark,andhowlongtheelectricalpowerisappliedtotheworkpiecesurface.

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WorkpiecePhysicalCharacteristic

Workpieceswithhighmeltingtemperatureandthermalconductivitywillhavelessmaterialremovedforeachsparkthanmaterialswithlowerones.

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ElectrodeSurfaceImperfections

Theelectrode’ssparkingsurfaceisreproducedintheexactoppositeformasthatoftheworkpiece.Ifthissurfaceisscratched,araisedlinewillbeproducedintothemachinedsurfaceoftheworkpiece.

TheEDMsurfaceisalsoaffectedbythestructureoftheelectrodematerial.Ifitisacoarse-grainsubstancewithvoidsbetweenthegrains,thissurfaceconditionwillbereproducedonthemachinedsurfaceoftheworkpiece.

TocreatethefinestEDMsurface,ametallicelectrodematerialshouldbeused,suchascopper,withanelectrodesurfacefinishatleastequaltothedesiredworkpiecesurfacefinish.

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GraphiteGrainSizeSurface

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Spark Energy

Sparkenergyisdeterminedbytheamountofelectricalpowercontainedineachspark,multipliedbytheamountoftimetheelectricalpowerisflowing.Theequationfordeterminingsparkenergyis:

Wt=E×I×t

Where:

Wt:（joule，J）E=sparkingvoltageI=peakamperest=spark-ONtime

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Foranyparticularapplication,thesparkingvoltageisconsideredconstantsinceitisprimarilydeterminedbythecharacteristicsofthedielectricfluidused.Whensparkingvoltageismaintainedatasetvalue,thepeaksparkamperes,multipliedbythespark-Ontimeinmicroseconds,determinesparkenergy.Thetwoprimaryconditionsthatdeterminesparkenergyarewhen:

Maintainingspark-ONtime,sparkenergyisincreasedaspeaksparkamperesareincreased.

Maintainingpeaksparkamperes,sparkenergyisincreasedasspark-ONtimeisincreased.

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Figure12-1.Increasingsparkenergybyincreasingspark-ONtime.

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Figure12-2.Increasingsparkenergyincreasessurfaceroughness.

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DETERMININGSURFACEFINISH

Sincepeaksparkamperesandspark-ONtimeareprincipalfactorsindeterminingsparkenergy,thefollowingeffectsexist:

anincreaseineitherpeakamperesorspark-ONtimeincreasessparkenergy;

anincreaseinsparkenergyincreasesthevolumeofworkpiecematerialremovedbythespark;and

anincreaseinthevolumeofworkpiecematerialremovedbythesparkresultsinacoarserEDMsurfacefinish.

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VARIATIONINSPARK-CAVITYSIZE

WhileEDMsurfacefinishsparkingisnormallythoughttoproducesphericalcavitiesofuniformdiameteranddepth,thisisnotatruerepresentationofmostactualEDMmachinedsurface.Spark-ONtimeisnotalwaysequalforallsparksbecausethespark-ONtimesetatthepower-supplycontrolmaybequitedifferentthantheactualspark-ONtime.

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Sinceallsparksdonothavethesameactualspark-ONtime,thesurfacefinishproducedconsistsofsparkcavitiesofdifferentsizes.Thelargestsparkcavity,however,willbethesizeofafullON-timesparkasdeterminedbythepower-supplycontrol’speakampereandspark-ONsettings.Allothersparkcavitieswillbesmaller.

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Figure12-3.Actualspark-ONtimedeterminedbythedielectricfluid'sionizationpoint.

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EQUALSPARK-ENERGYPOWERSUPPLIES

Onepower-supplydesigndoesproducesparkswithequalenergy.Itisbasedontheactualspark-ONtimestartingfromthepointofdielectricfluidionization.

Spark-ONtimeandpeaksparkamperesaresetatthepowersupplycontrol,priortostartingthemachiningoperation.Peakamperesandspark-ONtimedeterminesparkenergy,thus,usingthispower-supplydesignmeansthateachsparkremovesthesamevolumeofworkpiecematerialandproducesasurfacefinishofuniformlysized,sphericalsparkcavities.

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Figure12-4.Equalspark-ONtimeandvariablespark-OFFtime

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2．RecastandHeat-AffectedZone

TheEDMprocesscreatesthreelayersofsurfaces.Thetopsurfacecontainsathinlayerofspatteredmaterialthathasbeenformedfromthemoltenmetalandthesmallamountsofelectrodematerial.ThissurfacelayerofspatteredEDMresidueiseasilyremoved.

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Underneaththespatteredmaterialistherecast(white)layer.WhenthecurrentfromtheEDMprocessmeltsthematerial,itheatsuptheunderlyingsurfaceandaltersthemetallurgicalstructure.

Thisrecastlayerisformedbecausesomeofthemoltenmetalhasnotbeenexpelledandhasinsteadbeenrapidlyquenchedbythedielectricoil.Dependingonthematerial,therecastlayersurfacecanbealteredtosuchanextentthatitbecomesahardenedbrittlesurfacewheremicrocrackscanappear.Thislayercanbereducedsubstantiallybyfinishingoperations.

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Thenextlayeristheheat-affectedzone.Thisareaisaffectedbytheamountofcurrentappliedintheroughingandfinishingoperations.Thematerialhasbeenheatedbutnotmeltedasintherecastlayer.Theheat-affectedzonemayaltertheperformanceofthematerial.

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DifferencesBetweenWire-cutandRamEDMHeatAffectedZones

WhenroughingwithramEDM,muchmoreenergycanbesuppliedthanwithWire-cutEDM.Thisgreatlyincreasestheheat-affectedzonewithramEDM.Onthinwebsitcancreateseriousproblemsbecausethematerialwillbeheattreatedandquenchedinthe

dielectricoil.Thiscancausethinwebstobecomebrittle.

Whendielectricoilisheated,thehydrocarbonintheoilbreaksdownandcreatesanenrichedcarbonareainthecuttingzone.Thiscarbonbecomesimpregnatedintothesurfaceandalterstheparentmaterial.Oftenthissurfacebecomeshardandmakespolishingmoredifficult.ToavoidheatproblemswhenEDMingthinwebs,partsshouldbepremachinedandEDMedwithlowerpowersettings.

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White(Recast)Layer

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3SurfaceMechanicalProperties

Becauseofthetensilestresscausedbyinstantaneoushightemperatureheatingandrapidcooling,therearemicrocracksonworkpiece'ssurfaceprocessedbyEDM.

ThereareresidualtensilestressandmicrocrackinsurfaceofworkpiecemachinedbyEDM,whichmakesthefatigueperformancemanytimeslowerthanthatmachinedbymechanicalprocessing.

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Micro-crack

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MachiningDefectbyMaterialStress

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SEMPhotos

Grinding

EDM

Wire-cutEDM

Zh

MachiningEfficiency&SurfaceRoughness

VSMmm3/min

4,5

4

3,5

3

2,5

2

1,5

1

0,5

0

CCD0.25

CCA0.25

Ra

25µm 18µm 07µm 05µm 03µm 02µm 015µm012µm

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SurfacePolishing

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6.2Near-mirror-finishingUsingPowderSuspendedinDielectric

Itisdifficulttoobtainsurfacewithsmallroughness(verysmooth)byEDM,especiallywhenthesurface’sareaisbig(forexamplebiggerthan10×10cm²).Soforthediewithsmallroughness,afterbeingmachinedbyEDM,itshouldbepolishedbyhuman,whichislabourconsumingandtimeconsuming.Furthermore,itisverydifficulttopolishsomenarrowslits,deeptroughsanddieswithcomplexshapes.

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1.ReasonsofBeingDifficulttoFinishLargeNear-mirror-finishingSurfacebyEDM

ThereasonisthatsurfacesprocessedbyEDMismadeupofnumeroussmalloverlappingcavitiesandflanging.Thesmallercavities’diameteranddepthare,thesmallersurfaceroughnessis.Onlywhenthepower(pulsewidth×pulsecurrent)ofasinglepulseisverysmall,dischargecavitiesareverysmallandshallow.

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产

ThoughtheoreticallyaverynarrowpulsewidthandaverysmallpulsecurrentcanbedesignedinEDMpowersupply,atoolelectrodeandaworkpieceareimmergedintheinsulatingdielectricoilandthetoolelectrodeandtheworkpiece’smachinedsurfaceareequivalenttopositiveandnegativeelectrodesofacapacitor.

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Twoelectrodesofthecapacitoronlycanbechargedbysmallsinglepulseenergywhichcannotionizethedielectricoilbetweentwoelectrodes.Afterseveralordozensofpulsesareaccumulatedonthetwoelectrodes,thegapvoltagecanjustriseto60～100v.Whenthevoltagereachtoionizationvoltage,sparkdischargehappens.

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Oncesparkdischarge,allelectricalenergyaccumulatingonsurfacesofthetoolelectrodeandtheworkpieceisquicklyreleased.Becauseofthatabig,deepandroughdischargecavityisproducedonthesurface.Therefore,eventhoughminimumsinglepulseenergyisapplied,asurfacewithsmallroughnesscannotbeobtained.

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ReasonsofBeingDifficulttoFinishLargeNear-mirror-finishingSurfacebyEDM

Mirrorsurfacecavity

Pulsepowerenergy

高频电源

Capacitorchargedbysmallpulseenergy

accumulated

sparking

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Twoelectrodeascapacitor

2.EffectofPowder-MixedEDM(PowderSuspendedinDielectric)

AdvancesinthecontrolsandthedielectricfluidhavedramaticallyimprovedsurfacefinishinJapan.Somemachinesuseaspeciallyformulateddielectricfluidforfinishingoperationsthatproducesmirrorfinishesoflessthan1.5µmRmax.Somemachinescontaintwodielectricfluidtanks,oneforconventionalroughingandsemi-finishingandtheotherforproducingnear-mirrorfinishes.

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3.MachiningPrincipleofPowder-MixedEDM

Siliconoraluminumpowdersaremixedinthedielectricoil.Thepowders’diametershouldbeabout1-2µm,otherwisewhicheasilysink.Powdersfirstlyaremixedandtheninjectedintotheoiltroughandthedischargegapbetweenthetoolelectrodeandtheworkpiece.Duringprocessing,thetoolelectrodeusuallycanliftandmoveorshakeandbythisprocessthemachinedsurfaceismoreuniformandsmoother.

Manufacturershavediscoveredthatafteraddingsilicon,graphite,oraluminumpowdertothedielectricoil,excellentsurfacefinishesareproduced.ThisprocessisalsocalledDiffusedDischargeMachining(DDM).

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Powder-mixedEDMcanreachthenear-mirrorwitharelativelylargearea.Themajorreasonsis：

(1)Conductivepowdermaketheinter-resistivitygreatlyreduced,processinggapincreased,inter-electrodecapacitancereduced,theactualsinglepulseenergycanbereducedtoverysmallvalue.Thiscanprocessasmallcavity.Inaddition,asthegapisincreased,chipremovaliseasy,soflowfliedgetbetter,stabilityalsoimproved.

(2)WhattranspiresinDDMistheelectricaldischargesfromtheelectrodedonotfirstbombardtheworkpiece,butbombardthesiliconorotherparticlesandgenerateminutedischarges.Theseminuteelectricaldischargesresultincavitiessoshallowthattheyproduceanear-mirrorfinish.

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(3)Withpositivepolaritymachining,toolelectrode(cathode)launchedelectronic,throughalongdischargegaptoreachsurfaceoftheworkpiece,thedischargechanneldiameterhasbeenexpanded,sosparkcavitiesaredishshallowwithlargerdiameterandsmallerdepth.

(4)Siliconpowder,aluminumpowdercanform"vitreous"orsolid-fluxingalloylayerontheworkpiecesurfaceattransienthightemperatureandhighpressuredischarge,thesurfaceismoresmooth,resistanttowearandcorrosion,soprolongthelifeofcavitymold.

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6.3DCArcing

Arcdischarge

Dischargecontinuouslyoccursinthesamelocationontheelectrodesurfacetoformastablearcdischarge,apulsevoltagewaveformcharacteristicsusuallyhavenotionizationpointdelayordischargesustainingvoltageisslightlylowerandthehigh-frequencycomponentsismorefewer.

Arcinthefirstfewsecondsshowsagreatdealharm,theelectrodeandtheworkpiecewillbeablatedintoadeepcrater,resultinginaseriousheat-affectedzoneasdeepasafewmillimeters,thezonemaygrowatarateofmorethan1mm/minandtoolelectrodemaybescrapped.

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(2)Shortcircuit

Althoughtheshort-circuitdoesn’tproducematerialablationorinjuryofelectrodes,butcancauseahotspotatshort-circuit,andaftertheshort-circuitmanytimes,theautomaticservosystemallowstoolelectrodefallbacktoeliminateshort-circuit,thiseasycausesarcing。

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NORMALSPARKINGPULSE

withapreciseEDMspark,theflowofsparkelectricitystartsatthebeginningofthespark-ONtimeandstopsatthebeginningofthespark-OFFtime.Spark-ONand-OFFtimeisdeterminedbythecontrolsettingattheEDM-powersupply.

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Figure13-1.EDM-sparktiming.

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ACTUALFLOWOFSPARKELECTRCITY

Inreality,manyEDMsparksdonotoccurasillustrated.Instead,conditionsexistinginthesparkingareamaynotallowtheflowofelectricitytostopimmediatelyatthesametimesetasthebeginningofthespark-OFFperiod.

ThepeakamperewaveforminthedrawingshowsthatthesparkelectricitydoesnotimmediatelyturnOFFatthetimecommandedbythepower-supplycontrol.

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Twopossibilitiesforthisare:

misuseofcapacitors(alsocalledcondensers)withanEDMpulse-typepowersupply.Thiscommentdoesnotapplytocapacitorsusedwithresistor-capacitor(R-C)-typeEDMpowersupplies;and

excessiveelectricalinductance,causedbyseparationoftheelectrodeandworkpiecesparkingwires.

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Figure13-2.Sparkelectricityflowduringspark-OFFtime.

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Figure13-3.Separationofsparkingwiresincreasesinductance.

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ifspark-OFFtimeisreducedtoapointwheresparkelectricitycontinuestoflowfromonesparkintothenext,therewillbenospark-OFFtimebetweentheelectrodeandworkpiece,eventhoughthepowersupplyhascommandedone.

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Figure13-4.Sparkelectricityextendingfromonesparktothenext.

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POORCHIPREMOVAL

Dielectricfluiddoesnotnecessarilyflowequallyoverallsparking-gapsurfaces.

Forasquareelectrode,thatwouldbefromthecenterholetothecenterpointoneachside.Sincethereisagreaterdistancefromthecenterholetotheelectrodecorners,thecomersreceivelessfluidflowand,consequently,havelessthanidealchip-removalconditions.EDMsparkingby-products,primarilycarbon,alsoha