文献翻译.docx
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文献翻译.docx
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文献翻译
Electrical-DischargeMachining
1.Electrical-dischargeMachining
Electrical-dischargemachining(EDM),orsparkmachining,asitisalsocalled,removesmaterialwithrepetitivesparkdischargesfromapulsatingDCpowersupply,withadielectricflowingbetweentheworkpieceandthetool.
TheprincipleoftheEDMprocessisillustratedbythesimplifieddiagram.Thetoolismountedonthechuckattachedtothemachinespindlewhosemotioniscontrolledbyaservo-controlledfeeddrive.Theworkpieceisplacedinatankfilledwithadielectricfluid;adepthofatleast50mmovertheworksurfaceismaintainedtoeliminatetheriskoffire.ThetoolandworkpieceareconnectedtoapulsatingDCpowersupply.Dielectricfluidiscirculatedunderpressurebyapump,usuallythroughaholeorholesinthetoolelectrode.Asparkgapofabout0.025to0.05mmismaintainedbytheservomotor.
InpowersuppliesforEDMtheinputpowerisfirstconvertedintocontinuousDCpowerbyconventionalsolid-staterectifiers.TheflowofthisDCpoweristhencontrolledbyabankofpowertransistorswhichareswitchedbyadigitalmultivibratoroscillatorcircuit.Thehigh-powerplusesoutputisthenappliedtothetoolsandworkpiecetoproducethesparksresponsibleformaterialremoval.
Eachsparkgeneratesalocalizedhightemperatureontheorderof12000℃initsimmediatevicinity.Thisheatcausedpartofthesurroundingdielectricfluidtoevaporate;italsomeltsandvaporizesthemetaltoformasmallcraterontheworksurface.Sincethesparkalwaysoccursbetweenthepointsofthetoolandworkpiecethatareclosesttogether,thehighspotsoftheworkaregraduallyeroded,andtheformofthetoolisreproducedonthework.Thecondensedmetalglobules,formedduringtheprocess,arecarriedawaybytheflowingdielectricfluid.Asthemetaliseroded,thetoolisfedtowardtheworkpiecebyaservo-controlledfeedmechanism.
EachpulseintheEDMcyclelastsforonlyafewmicroseconds.Repeatedpulses,atratesupto100000persecond,resultinuniformerosionofmaterialfromtheworkpieceandfromtheelectrode.Astheprocessprogressed,theelectrodeisadvancedbytheservodrivetowardtheworkpiecetomaintainaconstantgapdistanceuntilthefinalcavityisproduced.
ApplicationElectrical-dischargemachiningcanbeusedforallelectricallyconductingmaterialsregardlessofhardness.Theprocessismostsuitedtothesinkingofirregularlyshapedholes,slots,andcavities.Fragileworkpiecescanbemachinedwithoutbreakage.Holescanbeofvariousshapesandcanbeproducedatshallowanglesincurvedsurfaceswithoutproblemsoftoolwander.
TheEDMprocessfindsgreatestapplicationatpresentintoolmarking,particularlyinthemanufactureofpresstools,extrusiondies,forgingdies,andmolds.Graphiteelectrodesproducedbycopymillingfrompatternsareoftenused.
AgreatadvantageofEDMisthatthetoolordiecanbemachinedafteritishardenedandhencegreataccuracycanbeachieved.Toolsofcementedcarbidecanbemachinedafterfinalsintering,whicheliminatestheneedforanintermediatepartialsinteringstage,thuseliminatingtheinaccuraciesresultingfromfinalsinteringafterholes,slots,andsoon,aremachined.
Electrical-dischargemachiningcanbeusedeffectivelytodrillsmallhigh-aspect-ratioholes.Diametersassmallas0.3mminmaterial20mmormoreinthicknesscanbereadilyachieved.Withefficientflushing,holeswithaspectratiosashighas100:
1havebeenproduced.Theprocesshasbeenusedsuccessfullytoproducevery-small-diameterholesinhardenedfuel-injectornozzles.Varyingnumbersofholesinaprecisepattencanbedrilledaroundtheinjectortip.
2.NumericalControl
Numericalcontrol(NC)isamethodofcontrollingthemovementsofmachinecomponentsbydirectlyinsertingcodedinstructionsintheformofnumericaldata(numbersanddata)intothesystem.Thesystemautomaticallyinterpretsthesedataandconvertsittooutputsignals.Thesesignals,inturncontrolvariousmachinecomponents,suchasturningspindlesonandoff,changingtools,movingtheworkpieceorthetoolsalongspecificpaths,andturningcuttingfluidsonandoff.
Inordertoappreciatetheimportanceofnumericalcontrolofmachines,let’sbrieflyreviewhowaprocesssuchasmachininghasbeencarriedouttraditionally.Afterstudyingtheworkingdrawingofapart,theoperatorsetsuptheappropriateprocessparameters(suchascuttingspeed,feed,depthofcut,cuttingfluid,andsoon),determinesthesequenceofoperationstobeperformed,clampstheworkpieceinaworkholdingdevicesuchasachuckorcollet,andproceedstomakethepart.Dependingonpartshapeandthedimensionalaccuracyspecified,thisapproachusuallyrequiresskilledoperators.Furthermore,themachiningprocedurefollowedmaydependontheparticularoperator,andbecauseofthepossibilitiesofhumanerror,thepartsproducedbythesameoperatormaynotallbeidentical.Partqualitymaythusdependontheparticularoperatororeventhesameoperatorondifferentdaysordifferenthoursoftheday.Becauseofourincreasedconcernwithproductqualityandreducingmanufacturingcosts,suchvariabilityanditseffectsonproductqualityarenolongeracceptable.Thissituationcanbeeliminatedbynumericalcontrolofthemachiningoperation.
Wecanillustratetheimportanceofnumericalcontrolbythefollowingexample.Assumethatholeshavetobedrilledonapartinthepositionsshowninthepicture.Inthetraditionalmanualmethodofmachiningthispart,theoperatorpositionsthedrillwithrespecttotheworkpiece,usingasreferencepointsanyofthethreemethodshown.Theoperatorthenproceedstodrilltheseholes.Let’sassumethat100parts,havingexactlythesameshapeanddimensionalaccuracy,havetobedrilled.Obviously,thisoperationisgoingtobetediousbecausetheoperatorhastogothroughthesamemotionsagainandagain.Moreover,theprobabilityishighthat,forvariousreasons,someofthepathsmachinedwillbedifferentfromothers.Let’sfurtherassumethatduringthisproductionrun,theorderforthesepathsischanged,sothat10ofthepathsnowrequireholesindifferentpositions.Themachinistnowhastoresetthemachine,whichwillbetimeconsumingandsubjecttoerror.Suchoperationscanbeperformedeasilybynumericalcontrolmachinesthatarecapableofproducingpartsrepeatedlyandaccuratelyandofhandlingdifferentpartsbysimplyloadingdifferentpartprograms.
Innumericalcontrol,dataconcerningallaspectsofthemachiningoperation,suchaslocations,speeds,feeds,andcuttingfluid,arestoredonmagnetictape,cassetts,floppyorharddisks,orpaperorplastic(Mylar,whichisathermoplasticpolyester)tape.Dataarestoredonpunched25mmwidepaperorplastictape,asoriginallydevelopedandstillused.TheconceptofNCcontrolisthatholesinthetaperepresentspecificinformationintheformofalphanumericcodes.Thepresence(on)orabsence(off)oftheseholesisreadbysensingdevicesinthecontrolpanel,whichthenactuaterelaysandotherdevices(calledhard-wiredcontrols).Thesedevicescontrolvariousmechanicalandelectricalsystemsinthemachine.Thismethodeliminatedmanualsettingofmachinepositionsandtoolpathsortheuseoftemplatesandothermechanicalguidesanddevices.Complexoperations,suchasturningaparthavingvariouscontoursanddiesinkinginamillingmachine,canbecarriedout.
Numericalcontrolhashadamajorimpactonallaspectsofmanufacturingoperations.Itisawidelyappliedtechnology,particularlyinthefollowingareas:
a)Machiningcenters.
b)Milling,turning,boring,drilling,andgrinding.
c)Electrical-discharge,laser-beam,andelectron-beammachining.
d)Water-jetcutting.
e)Punchingandnibbling.
f)Pipebendingandmetalspinning.
g)Spotweldingandotherweldingandcuttingoperation.
h)Assemblyoperations.
Numericalcontrolmachinesarenowusedextensivelyinsmall-and-medium-quantity(typically500partsorless)ofawidevarietyofpartsinsmallshopsandlargemanufacturefacilities.Oldermachinescanberetrofittedwithnumericalcontrol.
AdvantagesandLimitationsNumericalcontrolhasthefollowingadvantagesoverconventionalmethodofmachinecontrol:
1.Flexibilityofoperationandabilitytoproducecomplexshapeswithgooddimensionalaccuracy,repeatability,reducedscraploss,andhighproductionrates,productivity,andproductquality.
2.Toolingcostsarereduced,sincetemplatesandotherfixturesarenotrequired.
3.Machineadjustmentsareeasytomakewithminicomputeranddigitalreadout.
4.Moreoperationscanbeperformedwitheachsetup,andlessleadtimeforsetupandmachiningisrequiredcomparedtoconventionalmethods.Designchangesarefacilitated,andinventoryisreduced.
5.Programscanbepreparedrapidlyandcanberecalledatanytimeutilizingmicroprocessors.Lesspaperworkisinvolved.
5.Fasterprototypeproductionispossible
6.Requiredoperatorskillisless,andtheoperatorhasmoretimetoattendtoothertasksintheworkarea.
ThemajorlimitationsofNCaretherelativelyhighcostoftheequipmentandtheneedforprogrammingandspecialmaintenance,requiringtrainedpersonal.BecauseNCmachinesarecomplexsystems,breakdownscanbeverycostly,sopreventivemaintenanceisessential.However,theselimitationsareofteneasilyoutweighedbytheoveralleconomicadvantagesofNC.
3.ScopeofCAD/CAM
Computer-aideddesignistheuseofcomputersystemstofacilitatethecreation,modification,analysis,andoptimizationofadesign.Inthiscontextthetermcomputersystemmeansacombinationofhardwareandsoftware.Comp
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