《機(jī)械專業(yè)英語》課件第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．電火花加工通常簡稱EDM，它是采用電火花放電腐蝕工件來控制金屬切削的加工方法。這樣加工出的形狀與切割刀具即電極的形狀相反?！窬渲嘘P(guān)系副詞“whereby”相當(dāng)于bywhich、bywhat，意思是“借以”?！皐hich”引出的非限定性定語從句修飾“theworkpiece”。

2.?Theseelectricalenergyimpulsesbecomesparkswhichjumpthegapbetweentheelectrodeand

thework-piecethroughthedielectricfluid．脈沖能量在電極與工件間的工作液中火花放電?！窬渲小皐hich”引導(dǎo)的定語從句修飾sparks?！癲ielectricfluid”可譯為“工作液”。

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

narrowslotthroughtheworkpiece．切割線被控制在X-Y平面內(nèi)沿工件輪廓曲線移動(dòng)，從而在工件上電蝕出一條狹縫?！窬渲小皌hatthewirefollows”為定語從句，修飾“Thepath”。句子的主干是Thepathiscontrolledalong…。“eroding”是現(xiàn)在分詞短語作伴隨狀語。

4.?Adielectricfluid，usuallydeionizedwaterwhichisconstantlybeingcirculated,carriesaway

theerodedparticlesofmetal．工作液通常是除去離子的水，它不斷循環(huán)流動(dòng)，沖走腐蝕的金屬微粒?！?deionizedwater：除去離子的水?！?“whichisconstantlybeingcirculated”為定語從句，修飾“deionizedwater”。句子主干是Adielectricfluidcarriesawaytheerodedparticlesofmetal.

5.?Thewireisfedfromasupplyspoolthroughaseriesofguidesandguiderollerswhichapply?tensiontothewire．新線從貯絲筒的供線端繞過一系列導(dǎo)輪與導(dǎo)柱，這些導(dǎo)輪和導(dǎo)柱給切割線提供一定張緊力?！?“which”引導(dǎo)的定語從句修飾“guidesandguiderollers”，說明它們所起的作用。●?supplyspool：貯絲筒?！?apply…to…：把……施加于……。

EXERCISES

1.?DiscussthefollowingquestionsinEnglish:

(1)?HowcanEDMremovemetalfromaworkpiece?

(2)?Whymustanappropriategapbemaintainedbetweentheelectrodeandthework?

(3)?Whatisthedielectricfluidusedfor?

(4)?Whatkindofmetalcanbeusedasanelectrode?

(5)?Howmanymainpartsdoesawire-cutEDMhave?

2.?UseEnglishreplaceChineseoruseChinesereplaceEnglish.

(1)電容器

(2)線切割電火花加工

(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