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READINGMATERIALUnit20TEXTTheEDMProcessItisknowntoeverymachinistthatelectricalenergyisconvertedintomotionbyanelectricmotoronstandardmachinetools.Recently,wehavediscoveredthatelectricalenergycanbedirectlyemployedinmetalremoval.Themainadvantageisthatmetalofanyhardnesscanbemachinedbyelectricalmachiningprocesseswithgoodsurfacefinish.

Electricaldischargemachiningisaformofmetalremovalinwhichpulsatingdirectcurrentisappliedtoashapedtool(electrode)andawork-piece,bothofwhicharecapableofconductingelectricity.Thetwoareheldclosetoeachotherwithanonconductingfluidservingasaninsulatorbetweenthem.Whenavoltagehighenoughtobreakdowntheinsulatorisreached,asparkjumpsthegapbetweenthetoolandthework-piece.Asaresult,thissparkremovesasmallportionofmaterial.ThesurfacefinishofapartproducedonanEDMmachinedependsonthesizeofthesparksgeneratedbetweenitandtheelectrode.Big,powerfulsparksproducearoughsurfaceandsmall,lesspowerfulsparksproduceasmoothsurface.Itisreportedthatmetalremovalratesandsurfacefinisharecontrolledbythefrequencyandintensityofthespark.Highfrequency,low-intensitysparksresultinalowmetalremovalrateandproduceasmoothfinish.Lowfrequency,high-intensitysparksresultinrapidmetalerosionandacoarsefinish.LEARNTOSPEAKMr.Zhang：Hello,Mr.Cheng.Mr.Cheng：Hi,Mr.Zhang.Mr.Zhang：Whatareyoudoing?Mr.Cheng：Iammachiningawork-piece.Mr.Zhang：Butthereisaflameonthework-piece.Isthisakindofmethodofwelding?Mr.Cheng：ItisEDMprocess.Mr.Zhang：WhatdoestheEDMmean?Mr.Cheng：Itmeanselectricaldischargemachining,anditusestheelectricalenergyremovedirectlythemetalofwork-piece.Mr.Zhang：Howdoesitwork?Mr.Cheng：Whenmachiningpulsatingdirectcurrentisappliedtoashapedtool(electrode)andawork-piece,bothofwhicharecapableofconductingelectricity.Whenthetwoareclosetoeachother,apowerfulsparkswillremoveasmallportionofmaterial.Mr.Zhang：Whatisitsadvantage?Mr.Cheng：Themainadvantageisthatitcanmachinemetalofanyhardnessandformagoodsurfacefinish.Mr.Zhang：Oh,howniceitis.Thanksalot.Mr.Cheng：Notatall.NEWWORDSANDEXPRESSIONSelectrical [I5lektrIk(E)l]adj.電的，電力的electrically [IlektrIkElI]adv.用電，電學上discharge [5dIstFB:dV]vt.放電discover [dI5skQvE(r)]v.發(fā)現(xiàn)pulsate [pQl5seIt;(?@)5pQlseIt] vi.脈動，波動current [5kQrEnt] n.電流electrode [I5lektrEJd] n.電極close [klEJs] v.關(guān)閉closely [5klEJzlI] adv.接近地，緊密地nonconducting[nBnkEndQktIN] adj.不導電的fluid [5flu:Id] n.流體，液體insulator [5InsjJleItE(r)] n.絕緣體voltage [5vEJltIdV] n.電壓reach [ri:tF] vt.到達，達到spark [spB:k] n.火花，電火花jump [dVQmp] n.&v.跳gap [^Ap] n.間隙result [rI5zQlt] n.&v.結(jié)果portion [5pC:F(E)n] n.部分powerful [5paJEfJl] adj.強有力report [rI5pC:t] v.&n.報告，通報frequency [5fri:kwEnsI] n.頻率，頻繁erosion [I5rEJV(E)n] n.腐蝕，侵蝕electricaldischargemachining(EDM) 電火花加工directcurrent=D.C. 直流電breakdown 破壞，毀壞，擊穿closetoeachother 相互靠近asaresult 結(jié)果，因此asresultof ……結(jié)果，由于resultfrom 起因，由來resultin 導致，結(jié)果形成asmallportionof 小部分NOTES1．Itisknowntoeverymachinistthat…

每一個機械師都知道……

Itis…that…句型中，it是形式主語，that…引出的從句是本句真正的主語。2．…thatmetalofanyhardnesscanbemachinedbyelectricalmachiningprocesseswithgoodsurfacefinish.

用電加工法能夠加工任何硬度的金屬，而且表面粗糙度小。

That引出的是表語從句，withgoodsurfacefinish是從句的狀語。3．Electricaldischargemachiningisaformofmetalremovalinwhichpulsatingdirectcurrentisappliedtoashapedtool(electrode)andawork-piece,bothofwhicharecapableofconductingelectricity.

電火花加工是一種去除金屬的方式。在此過程中，脈沖直流電通到能導電的成形刀具(電極)和工件上。其中inwhichpulsatingdirectcurrentisappliedtoashapedtool(electrode)andawork-piece,bothofwhicharecapableofconductingelectricity是主句的定語從句，修飾表語aformofmetalremoval，而bothofwhicharecapableofconductingelectricity則是該從句中的從句，修飾ashapedtool(electrode)andawork-piece。EXERCISES1．DiscussthefollowingquestionsinEnglish:(1)?Whatistheelectricaldischargemachining(EDM)?(2)?WhichworkcantheEDMdo?Givesomeexamples.(3)?CantheEDMmachinetheplasticwork-piece?Why?2．UseEnglishreplacingChineseoruseChinesereplacingEnglish:(1)用電火花加工的方法可以加工很硬的金屬工件。(2)電火花加工所用的刀具就是電極。(3)電火花加工可以得到高質(zhì)量的表面。(4)?TheEDMusedirectcurrenttomachinework-piece.(5)?Thework-pieceandelectrodeareheldclosetoeachotherwithanon-conductingfluidservingasaninsulatorbetweenthem.3．Placea“T”aftersentencesthataretrueandan“F”afterthosethatarefalse:(1)?Onstandardmachinetools,electricalenergyisdirectlyemployedinmetalremoval.()(2)?Metalofanyhardnesscanbemachinedbyelectricalmachiningprocesseswithcoarsesurfacefinish.()(3)?ThesurfacefinishofapartproducedonanEDMmachinedependsonthesizeofpart.()(4)?Big,powerfulsparksproduceasmoothsurfaceandsmall,lesspowerfulsparksproducearoughsurface.()(5)?Metalmaterialremovalratesandsurfacefinishcanbecontrolledbythefrequencyandintensityofthespark.()(6)?Highfrequency,low-intensitysparksresultinalowmetalremovalrateandproduceasmoothsurfacefinish.()READINGMATERIALElectricalMachiningMostelectricalmachiningprocesseshavecomeaboutduetothedevelopmentofspace-agemetalalloysthataretoughandhardtomachine.Grindingcutting,andshapingwithcuttingtoolsareexacting,time-consuming,andexpensiveprocessesonsuchalloys.Insomecases,machiningwithconventionalequipmentisalmostimpossible.Amaterialliketungstencarbide,forexample,isverydifficulttomachineevenwithdiamondcuttingtools.

Onstandardmachinetools,electricalenergyisconvertedbyanelectricmotorintomotionthatistransmittedthroughaseriesofshafts,pulleys,belts,andgearstoacuttingtool.Incontrast,theelectricalmachiningprocessesmakedirectuseofelectricalenergy.Thisresultsincertainabilitiesthatareuniquetotheseprocesses.Theseincludemachiningmetalsofanyhardness,machiningwithoutcuttingforces,machiningwithouttoolrotationorwit