《机械专业英语》课件第20章

Unit20TEXTLEARNTOSPEAKNEWWORDSANDPHRASESNOTESEXERCISESREADINGMATERIALPROFESSIONALWORDSANDPHRASES

Wire-cutEDM(1)

PrincipleofEDM

Electricaldischargemachining，commonlyknownasEDM，isacontrolledmetalremovalprocesswherebyanelectricsparkisusedtocut(erode)theworkpiece，whichthentakestheshapeoppositetothatofthecuttingtoolorelectrode.Theelectrodeandtheworkpiecearebothsubmergedinadielectricfluid，whichisgenerallyalightlubricatingoil．Thisdielectricfluidshouldbeanonconductorofelectricity.Aservomechanismmaintainsagapofabout0.0005to0.001in.(0.01to0.02mm)betweentheelectrodeandthework,preventingthemfromcomingintocontactwitheachother.Adirectcurrentoflowvoltageandhighamperageisdeliveredtotheelectrodeattherateofapproximately20,000hertz(Hz)．Theseelectricalenergyimpulsesbecomesparkswhichjumpthegapbetweentheelectrodeandtheworkpiecethroughthedielectricfluid．Intenseheatiscreatedinthelocalizedareaofthesparkimpact；themetalmeltsandasmallparticleofmoltenmetalisexpelledfromtheworkpiece.Thedielectricfluid，whichisconstantlybeingcirculated，carriesawaytheerodedparticlesofmetalandalsohelpsindissipatingtheheatcausedbythespark．

TherearetwotypesofEDMmachinesusedinindustry：theverticalEDMmachineandthewire-cutEDMmachine(asshowninFig.20-l).Sincethewire-cutEDMisgenerallyusedformachiningcomplexformswhichrequireNCprogramming，onlythistypewillbediscussedindetail．

Wire-cutEDM

Thewire-cutEDMisadischargemachinewhichusesNCmovementtoproducethedesiredcontourorshapeonapart．Itdoesnotrequireaspecial-shapedelectrode；instead，itusesacontinuoustravelingwireundertensionastheelectrode．Theelectrodeorcuttingwirecanbemadeofbrass，copper，oranyotherelectricallyconductivematerialrangingindiameterfrom0.002to0.012in.(0.05to0.30mm).Thepaththatthewirefollowiscontrolledalongatwo-axis(XY)contour，eroding(cutting)anarrowslotthroughtheworkpiece．Thiscontrolledmovementiscontinuousandsimultaneousinincrementsof0.000,05in．(0.001mm).Anycontourmaybecuttoahighdegreeofaccuracyandisrepeatableforanynumberofsuccessiveparts.Adielectricfluid，usuallydemonizedwaterwhichisconstantlybeingcirculated,carriesawaytheerodedparticlesofmetal．Thedielectricfluidmaintainstheproperconductivitybetweenthewireandtheworkpieceandassistsinreducingtheheatcausedbythespark．Fig.20-1Electricaldischargemachines(a) VerticalEDM；(b) Wire-cutEDM

PartsoftheWire-cutEDM

Themainpartsofthewire-cutEDMarethebed，saddle，table，column，arm，UVaxishead，wirefeedanddielectricsystems，andmachinecontrolunit(MCU)(asshowninFig.20-2)．Fig.20-2Themainpartsofawire-cutEDM

Bed：Thebedisaheavy，ruggedcastingusedtosupporttheworkingpartsofthewire-cutEDM．Guiderails(“ways”)aremachinedonthetopsection，andtheseguideandalignmajorpartsofthemachine.

Saddle：ThesaddleisfittedontopoftheguiderailandmaybemovedintheXYdirectionbyfeedservomotorsandballleadscrews．

Table：Thetable，mountedontopofthesaddle，isU-shapedandcontainsaseriesofdrilledandtappedholesequallyspacedaroundthetopsurface．Theseareusedfortheworkpieceholdingandclampingdevices.

Column：Thecolumn，whichisattachedtothebed，supportsthewirefeedsystem，theUVaxes，andthecapacitorswitches．

Wirefeedsystem：Thewirefeedsystemisusedtoprovideacontinualfeedofnewwire(electrode)forthecuttingoperation．Thewireisfedfromasupplyspoolthroughaseriesofguidesandguiderollerswhichapplytensiontothewire.Thewiretravelsinacontinuouspathpasttheworkpiece，andtheusedwireisrewoundonatake-upspool．

Dielectricsystem：Thedielectricsystemcontainsfilters，anionexchanger，andacooler.Thissystemprovidesacontinuousflowofcleandeionizedwaterataconstanttemperature．Thedeionizedwaterstabilizesthecuttingoperation，flushesawayparticlesofelectrodeandworkpiecematerialthathavebeeneroded，andcoolstheworkpiece．

MCU：TheMCUcanbeseparatedasthreeindividualpanels．

(1) Thecontrolpanelforsettingthecuttingcondition．

(2) Thecontrolpanelformachinesetup．

(3) Thecontrolpanelformanualdatainput(MDI)andcathode-raytube(CRT)characterdisplay(asshowininFig.20-3)．Fig.20-3Controlpanel LEARNTOSPEAK

Mr.Zhang: Hi,Miss.Li.

Mr.Li: Hello,Mr.Cheng.

Mr.Zhang: Wouldyoumindaskingyousomequestions?

Miss.Li: Noproblem.Whatdoyouwanttoknow?

Mr.Zhang: HowmanytypesofEDMmachinesarethereusedinindustry?

MissLi: Therearetwo.

Mr.Zhang: Canyoudescribeitindetail?

MissLi: Yes.ThereareverticalEDMmachineandthewire-cutEDMmachine.And

thewire-cutEDMisgenerallyusedformachiningcomplexformswhich

requireNCprogramming.Mr.Zhang: Whatisthemainpartsofthewire-cutEDM?MissLi: Themainpartsofthewire-cutEDMarethebed，saddle，table，column，arm，

UVaxishead，wirefeedanddielectricsystems，andmachinecontrolunit

(MCU).Mr.Zhang: IstheMCUcanbeseparatedasthreeindividualpanels?MissLi: Yes.Mr.Zhang: Canyoudescribeitindetail?MissLi: Thecontrolpanelforsettingthecuttingcondition,thecontrolpanelformachinesetup．Andthecontrolpanelformanualdatainput(MDI)andcathode-raytube(CRT)characterdisplayMr.Zhang: You’resokindfortellingmeaboutthese.MissLi: Formypleasure.Mr.Zhang: Bye-bye.MissLi: Bye. NOTES

1. Electricaldischargemachining，commonlyknownasEDM，isacontrolledmetalremoval

processwherebyanelectricsparkisusedtocut(erode)theworkpiece，whichthentakesthe

shapeoppositetothatofthecuttingtoolorelectrode．电火花加工通常简称EDM，它是采用电火花放电腐蚀工件来控制金属切削的加工方法。这样加工出的形状与切割刀具即电极的形状相反。●句中关系副词“whereby”相当于bywhich、bywhat，意思是“借以”。“which”引出的非限定性定语从句修饰“theworkpiece”。

2. Theseelectricalenergyimpulsesbecomesparkswhichjumpthegapbetweentheelectrodeand

thework-piecethroughthedielectricfluid．脉冲能量在电极与工件间的工作液中火花放电。●句中“which”引导的定语从句修饰sparks。“dielectricfluid”可译为“工作液”。

3. Thepaththatthewirefollowsiscontrolledalongatwo-axis(XY)contour，eroding(cutting)a

narrowslotthroughtheworkpiece．切割线被控制在X-Y平面内沿工件轮廓曲线移动，从而在工件上电蚀出一条狭缝。●句中“thatthewirefollows”为定语从句，修饰“Thepath”。句子的主干是Thepathiscontrolledalong…。“eroding”是现在分词短语作伴随状语。

4. Adielectricfluid，usuallydeionizedwaterwhichisconstantlybeingcirculated,carriesaway

theerodedparticlesofmetal．工作液通常是除去离子的水，它不断循环流动，冲走腐蚀的金属微粒。● deionizedwater：除去离子的水。● “whichisconstantlybeingcirculated”为定语从句，修饰“deionizedwater”。句子主干是Adielectricfluidcarriesawaytheerodedparticlesofmetal.

5. Thewireisfedfromasupplyspoolthroughaseriesofguidesandguiderollerswhichapply tensiontothewire．新线从贮丝筒的供线端绕过一系列导轮与导柱，这些导轮和导柱给切割线提供一定张紧力。● “which”引导的定语从句修饰“guidesandguiderollers”，说明它们所起的作用。● supplyspool：贮丝筒。● apply…to…：把……施加于……。

EXERCISES

1. DiscussthefollowingquestionsinEnglish:

(1) HowcanEDMremovemetalfromaworkpiece?

(2) Whymustanappropriategapbemaintainedbetweentheelectrodeandthework?

(3) Whatisthedielectricfluidusedfor?

(4) Whatkindofmetalcanbeusedasanelectrode?

(5) Howmanymainpartsdoesawire-cutEDMhave?

2. UseEnglishreplaceChineseoruseChinesereplaceEnglish.

(1)电容器

(2)线切割电火花加工

(3)工作液

(4) EDM

(5) CRT

(6) MCU

3. Placea“T”aftersentencesthataretrueandan“F”afterthosethatarefalse.

(1) EDMusesanelectricsparktocuttheworkpieceintothesameshapeastheelectrodehas.

()

(2) Thedielectricfluidisusedonlytocarryawaytheerodedparticlesofmetalandassistin

reducingtheheatcausedbythespark.()

(3) Awire-cutEDMmachinecan’tcutacontourtoahighdegreeofaccuracy.()

(4) Thedielectricsystemofawire-cutEDMprovidesacontinuousflowofcleandeionized

waterataconstanttemperatureinordertokeeptheoperationdonesuccessfully.()READINGMATERIAL

Wire-cutEDM(2)

MachineOperatingSystems

Varioussystemsonwire-cutEDMmachinesplayanimportantpartintheefficientoperationofthemachinetool.Servosystems，dielectricfluid，electrode，andtheMCUarethemainoperatingcomponentsofwire-cutEDMmachines．

TheServomechanism

TheEDMpowersupplycontrolsthecuttingcurrentlevelsandthefeedrateofthedrivemotors.Italsocontrolsthetravelingspeedofthewire．EDMmachinesareequippedwithaservocontrolmechanismthatautomaticallymaintainsaconstantgapofapproximately0.001to0.002in.(0.02to0.05mm)betweenthewireandtheworkpiece．Itisimportantthattherebenophysicalcontactbetweenthewire(electrode)andtheworkpiece；otherwisearcingcoulddamagetheworkpieceandbreakthewire.Theservomechanismalsoadvancesthewireintotheworkpieceastheoperationprogressesandsensesthework-wirespacingandslowsorspeedsupthedrivemotorsasrequiredtomaintaintheproperarcgap.Precisecontrolofthegapisessentialtoasuccessfulmachiningoperation.Ifthegapistoolarge，ionizationofthedielectricfluiddoesnotoccurandmachiningcannottakeplace.Ifthegapistoosmall,thewirewilltouchtheworkpiece，causingittomeltandbreak.

Precisegapcontrolisaccomplishedbyacircuitinthepowersupplycomparingtheaveragegapvoltagetoapreselectedreferencevoltage.Thedifferencebetweenthetwovoltagesistheinputsignal,whichtellstheservomechanismhowfarandhowfasttofeedthewireandwhentoretractfromthework-piece.Thisisusuallyindicatedbyareverseservolight．

Whenchipsinthesparkgapreducethevoltagebelowacriticallevel,theservomechanismcausesthewiretoretractuntilthechipsareflushedoutbythedielectricfluid．Theservosystemshouldnotbetoosensitiveto“short-lived”voltagescausedbychipsbeingflushedout；otherwisethewirewouldbeconstantlyretracting,therebyseriouslyaffectingmachiningrates.

TheDielectricFluid

OneofthemostimportantfactorsinasuccessfulEDMoperationistheremovaloftheparticles(chips)fromtheworkinggap.Flushingtheseparticlesoutofthegapwiththedielectricfluidwillproducegoodcuttingconditions，whilepoorflushingwillcauseerraticcuttingandpoormachiningconditions.

Thedielectricfluidinthewire-cutEDMprocessisusuallydeionizedwater.Thisistapwaterthatiscirculatedthroughanion-exchangeresin.Thedeionizedwatermakesagoodinsulator,whileuntreatedwaterisaconductorandisnotsuitablefortheEDMprocess.Theionexchangerisacompoundofpositiveionexchangeresin(cation)andnegativeionexchangeresin(anion).Whenwaterisappliedtothiscompound,theionexchangereactionstartsandiscontinuouslyrepeateduntilalltheimpuritiesarecompletelyremovedfromthewater，thusproducingpurewater.

Theamountofdeionizationismeasuredbyitsspecificresistance.Formostoperations，thelowertheresistance，thefasterthecuttingspeed.However，theresistanceofthedielectricfluidshouldbemuchhigherwhencarbidesandhigh-densitygraphitearecut(ThedielectricfluidsystemasshowinFig.20-4).Fig.20-4Thedielectricfluidsystem

Thedielectricfluidinthewire-cutEDMprocessservesseveralfunctions:

(1) Ithelpstoinitiatethesparkbetweenthewire(electrode)andtheworkpiece.

(2) Itservesasaninsulatorbetweenthewireandtheworkpiece.

(3) Itflushesawaytheparticlesofdisintegratedwireandworkpiecetopreventshorting.

(4) Itactsasacoolantforboththewireandtheworkpiece.

Thedielectricfluidmustbecirculatedunderconstantpressureifitistoflushawaytheparticlesandassistinthemachiningprocess.Theflowofwaterisgovernedbytwocontrolvalves:onecontrollingthewaterflowingabovetheworkpieceandonecontrollingtheflowundertheworkpiece.Whenstartingtosupplythewaterforthecuttingprocess,applyasteadyamount“down-stream”andthenslowlyapplythewaterfromunderneathuntilthereisaflaringeffectatthetopsurfaceoftheworkpiece(asshowninFig.20-5).Fig.20-5Asteadystreamofdielectricmustbesuppliedtotheworkpiece

Ifredsparksoccurduringthecuttingoperation，thewatersupplyisinadequate．Toovercomethisproblem，increasetheflowofwateruntilbluesparksappear．Note:Excessivewaterflowcancausethewiretobedeflected，causingerraticcuttingandlossmachiningaccuracy.

TheElectrode

Theelectrodeinwire-cutEDMisaspoolofwirerangingfrom0.002to0.012in.(0.05to0.30mm)indiameterandfrom2to1001b(0.90to45.36kg)inweight．Thelengthofwireonaspoolcanprovideover500hofunattendedmachiningtime．Theelectrodeofthewire-cutmachinecontinuouslytravelsfromasupplyspooltoatake-upspoolsothatitisconstantlyrenewed.Whenthistypeofelectrodeisused，thewearonthewiredoesnotaffecttheaccuracyofthecutbecausenewwireisbeingfedpasttheworkpiececontinuouslyatratesfromafractionofaninchtoseveralinchesperminute．Boththeelectrodewireandthematerialremovalratefromtheworkpiecedependonsuchthingsasthematerial’sthermalconductivity，itsmeltingpoin