数控技术---外文翻译.doc

NUMERICALCONTROLNumericalcontrolN／Cisaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers，letters，andothersymbols．Thenumbers，letters，andsymbolsarecodedinanappropriateformattodefineaprogramofinstructionsforaparticularworkpartorjob．Whenthejobchanges，theprogramofinstructionsischanged．ThecapabilitytochangetheprogramiswhatmakesN／Csuitableforlowandmediumvolumeproduction．Itismucheasiertowriteprogramsthantomakemajoralterationsoftheprocessingequipment．Therearetwobasictypesofnumericallycontrolledmachinetoolspointtopointandcontinuouspathalsocalledcontouring．Pointtopointmachinesuseunsynchronizedmotors，withtheresultthatthepositionofthemachiningheadCanbeassuredonlyuponcompletionofamovement，orwhileonlyonemotorisrunning．Machinesofthistypeareprincipallyusedforstraightlinecutsorfordrillingorboring．TheN／Csystemconsistsofthefollowingcomponentsdatainput，thetapereaderwiththecontrolunit，feedbackdevices，andthemetalcuttingmachinetoolorothertypeofN／Cequipment．Datainput，alsocalledmantocontrollink，maybeprovidedtothemachinetoolmanually，orentirelybyautomaticmeans．Manualmethodswhenusedasthesolesourceofinputdataarerestrictedtoarelativelysmallnumberofinputs．Examplesofmanuallyoperateddevicesarekeyboarddials，pushbuttons，switches，orthumbwheelselectors．Thesearelocatedonaconsolenearthemachine．Dialsaleanalogdevicesusuallyconnectedtoasynchrotyperesolverorpotentiometer．Inmostcases，pushbuttons，switches，andothersimilartypesofselectorsayedigitalinputdevices．Manualinputrequiresthattheoperatorsetthecontrolsforeachoperation．Itisaslowandtediousprocessandisseldomjustifiedexceptinelementarymachiningapplicationsorinspecialcases．Inpracticallyallcases，informationisautomaticallysuppliedtothecontrolunitandthemachinetoolbycards，punchedtapes，orbymagnetictape．EightchannelpunchedpapertapeisthemostcommonlyusedformofdatainputforconventionalN／Csystems．Thecodedinstructionsonthetapeconsistofsectionsofpunchedholescalledblocks．Eachblockrepresentsamachinefunction，amachiningoperation，oracombinationofthetwo．TheentireN／Cprogramonatapeismadeupofanaccumulationofthesesuccessivedatablocks．Programsresultinginlongtapesallwoundonreelslikemotionpicturefilm．Programsonrelativelyshorttapesmaybecontinuouslyrepeatedbyjoiningthetwoendsofthetapetoformaloop．Onceinstalled，thetapeisusedagainandagainwithoutfurtherhandling．Inthiscase，theoperatorsimplyloadsandunloadstheparts．Punchedtapesalepreparedontypewriterswithspecialtapepunchingattachmentsorintapepunchingunitsconnecteddirectlytoacomputersystem．Tapeproductionisrarelyerrorfree．Errorsmaybeinitiallycausedbythepartprogrammer，incardpunchingorcompilation，orasaresultofphysicaldamagetothetapeduringhandling，etc．Severaltrialrunsareoftennecessarytoremoveallerrorsandproduceanacceptableworkingtape．Whilethedataonthetapeisfedautomatically，theactualprogrammingstepsaledonemanually．Beforethecodedtapemaybeprepared，theprogrammer，oftenworkingwithaplanneroraprocessengineer,mustselecttheappropriateN／Cmachinetool，determinethekindofmaterialtobemachined，calculatethespeedsandfeeds，anddecideuponthetypeoftoolingneeded.Thedimensionsonthepartprintarecloselyexaminedtodetermineasuitablezeroreferencepointfromwhichtostarttheprogram．Aprogrammanuscriptisthenwrittenwhichgivescodednumericalinstructionsdescribingthesequenceofoperationsthatthemachinetoolisrequiredtofollowtocuttheparttothedrawingspecifications．Thecontrolunitreceivesandstoresallcodeddatauntilacompleteblockofinformationhasbeenaccumulated．Ittheninterpretsthecodedinstructionanddirectsthemachinetoolthroughtherequiredmotions．Thefunctionofthecontrolunitmaybebetterunderstoodbycomparingittotheactionofadialtelephone，where，aseachdigitisdialed，itisstored．Whentheentirenumberhasbeendialed，theequipmentbecomesactivatedandthecalliscompleted．Siliconphotodiodes，locatedinthetapereaderheadonthecontrolunit，detectlightasitpassesthroughtheholesinthemovingtape．Thelightbeamsareconvertedtoelectricalenergy，whichisamplifiedtofurtherstrengthenthesignal．Thesignalsarethensenttoregistersinthecontrolunit,whereactuationsignalsarerelayedtothemachinetooldrives．Somephotoelectricdevicesarecapableofreadingatratesupto1000characterspersecond．Highreadingratesarenecessarytomaintaincontinuousmachinetoolmotionotherwisedwellmarksmaybegeneratedbythecutteronthepartduringcontouringoperations．Thereadingdevicemustbecapableofreadingdatablocksataratefasterthanthecontrolsystemcanprocessthedata．AfeedbackdeviceisasafeguardusedonsomeN／Cinstallationstoconstantlycompensateforerrorsbetweenthecommandedpositionandtheactuallocationofthemovingslidesofthemachinetool．AnN／Cmachineequippedwiththiskindofadirectfeedbackcheckingdevicehaswhatisknownasaclosedloopsystem．Positioningcontrolisaccomplishedbyasensorwhich，duringtheactualoperation，recordsthepositionoftheslidesandrelaysthisinformationbacktothecontrolunit．Signalsthusreceivedalecomparedtoinputsignalsonthetape，andanydiscrepancybetweenthemisautomaticallyrectified．Inanalternativesystem，calledanopenloopsystem，themachineispositionedsolelybysteppingmotordrivesinresponsetocommandsbyacontrollers．TherearethreebasictypesofNCmotions,asfollowsPointtopointorPositionalControlInpointtopointcontrolthemachinetoolelementstools,table,etc.aremovedtoprogrammedlocationsandthemachiningoperationsperformedafterthemotionsarecompleted.Thepathorspeedofmovementbetweenlocationsisunimportantonlythecoordinatesoftheendpointsofthemotionsareaccuratelycontrolled.Thistypeofcontrolissuitablefordrillpressesandsomeboringmachines,wheredrilling,tapping,orboringoperationsmustbeperformedatvariouslocationsontheworkpiece.StraightLineorLinearControlStraightLinecontrolsystemsareabletomovethecuttingtoolparalleltooneofthemajoraxesofthemachinetoolatacontrolledratesuitableformachining.Itisnormallyonlypossibletomoveinonedirectionatatime,soangularcutsontheworkpiecearenotpossible,Consequently,formillingmachines,onlyrectangularconfigurationscanbemachinedorforlathesonlysurfacesparallelorperpendiculartothespindleaxiscanbemachined.Thistypeofcontrolledmotionisoftenreferredtoaslinearcontrolorahalfaxisofcontrol.Machineswiththisformofcontrolarealsocapableofpointtopointcontrol.ContinuousPathorContouringControlIncontinuouspathcontrolthemotionsoftwoormoreofthemachineaxesarecontrolledsimultaneously,sothatthepositionandvelocityofthecanbetoolarechangedcontinuously.Inthiswaycurvesandsurfacescanbemachinedatacontrolledfeedrate.Itisthefunctionoftheinterpolatorinthecontrollertodeterminetheincrementsoftheindividualcontrolledaxesofthemachinesnecessarytoproducethedesiredmotion.Thistypeofcontrolisreferredtoascontinuouscontrolorafullaxisofcontrol.SometerminologyconcerningcontrolledmotionsforNCmachineshasbeenintroduced.Forexample,somemachinesarereferredtoasfourorfiveorevensixaxismachines.Foraverticalmillingmachinethreeaxesofcontrolarefairlyobvious,thesebeingtheusualX,Y,Zcoordinatedirections.Afourthorfifthaxisofcontrolwouldimplysomeformofrotarytabletoindextheworkpieceorpossiblytoprovideangularmotionoftheworkhead.Thus,inNCterminologyanaxisofcontrolisanycontrolledmotionofthemachineelementsspindles,tables,etc.Afurthercomplicationisuseofthetermhalfaxisofcontrolforexample,manymillingmachinesarereferredtoas2.5axismachine.Thismeansthatcontinuouscontrolispossiblefortwomotionsaxesandonlylinearcontrolispossibleforthethirdaxis.Appliedtoverticalmillingmachines,2.5axiscontrolmeanscontouringintheX,YplaneandlinearmotiononlyintheZdirection.Withthesemachinesthreedimensionalobjectshavetobemachinedwithwaterlinesaroundthesurfaceatdifferentheights.Withanalternativeterminologythesamemachinecouldbecalleda2CLmachineCforcontinuous,Lforlinearcontrol.Thus,amillingmachinewithcontinuouscontrolintheX,Y,Zdirectionscouldbetermedbeathreeaxismachineora3cmachine,Similarly,lathesareusuallytwoaxisor2Cmachines.Thedegreeofworkprecisiondependsalmostentirelyupontheaccuracyoftheleadscrewandtherigidityofthemachinestructure．Withthissystem．thereisnoselfcorrectingactionorfeedbackofinformationtothecontrolunit．Intheeventofanunexpectedmalfunction，thecontrolunitcontinuestoputoutpulsesofelectricalcurrent．If，forexample，thetableonaN／Cmillingmachineweresuddenlytobecomeoverloaded，noresponsewouldbesentbacktothecontroller．Becausesteppingmotorsarenotsensitivetoloadvariations，manyN／Csystemsaredesignedtopermitthemotorstostallwhentheresistingtorqueexceedsthemotortorque．Othersystemsareinuse，however，whichinspiteofthepossibilityofdamagetothemachinestructureortothemechanicalsystem，aledesignedwithspecialhightorquesteppingmotors．Inthiscase，themotorshavesufficientcapacitytooverpowerthesystemintheeventofalmostanycontingency．TheoriginalN／Cusedtheclosedloopsystem．Ofthetwosystems，closedandopenloop，closedloopismoreaccurateand，asaconsequence，isgenerallymoreexpensive．Initially，openloopsystemswereusedalmostentirelyforlightdutyapplicationsbecauseofinherentpowerlimitationspreviouslyassociatedwithconventionalelectricsteppingmotors．Recentadvancesinthedevelopmentofelectrohydraulicsteppingmotorshaveledtoincreasinglyheaviermachineloadapplications．MILLINGMillingisabasicmachiningprocessinwhichthesurfaceisgeneratedbytheprogressiveformationandremovalofchipsofmaterialfromtheworkpieceasitisfedtoarotatingcutterinadirectionperpendiculartotheaxisofthecutter．Insomecasestheworkpieceisstationaryandthecutterisfedtothework．Inmostinstancesamultipletoothcutterisusedsothatthemetalremovalrateishigh，andfrequentlythedesiredsurfaceisobtainedinasinglepassofthework．Thetoolusedinmillingisknownasamillingcutter．Itusuallyconsistsofacylindricalbodywhichrotatesonitsaxisandcontainsequallyspacedperipheralteeththatintermittentlyengageandcuttheworkpiece．Insomecasestheteethextendpartwayacrossoneorbothendsofthecylinder．Becausethemillingprincipleprovidesrapidmetalremovalandcanproducegoodsurfacefinish，itisparticularlywellsuitedformassproductionwork，andexcellentmillingmachineshavebeendevelopedforthispurpose．However，veryaccurateandversatilemillingofageneralpurposenaturealsohavebeendevelopedthatarewidelyusedinjobshopandtoolanddiework．Ashopthatisequippedwithamillingmachineandanenginelathecanmachinealmostanytypeofproductofsuitablesize．TypesofMillingOperations．Millingoperationscanbeclassifiedintotwobroadcategories，eachofwhichhasseveralvariations1．Inperipheralmillingasurfaceisgeneratedbyteethlocatedintheperipheryofthecutterbodythesurfaceisparallelwiththeaxisofrotationofthecutter．Bothflatandformedsurfacessanbeproducedbythismethod．Thecrosssectionoftheresultingsurfacecorrespondstotheaxialcontourofthecutter．Thisprocedureofteniscalledslabmilling．2．Infacemillingthegeneratedflatsurfaceisatrightanglestothecutteraxisandisthecombinedresultoftheactionsoftheportionsoftheteethlocatedonboththeperipheryandthefaceofthecutter．Themajorportionofthecuttingisdonebytheperipheralportionsoftheteethwiththefaceportionsprovidingafinishingaction．ThebasicconceptsofperipheralandfacemillingareillustratedinFig．161．Peripheralmillingoperationsusuallyareperformedonmachineshavinghorizontalspindles，whereasfacemillingisdoneonbothhorizontalandverticalspindlemachines.SurfaceGenerationinMilling．Surfacescanbegeneratedinmillingbytwo