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数控技术---外文翻译.doc

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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

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