中英文翻译NUMERICAL CONTROL数控技术.doc中英文翻译NUMERICAL CONTROL数控技术.doc

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NUMERICALCONTROLNumericalcontrol(N/C)isaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,letters,andothersymbols.Thenumbers,letters,andsymbolsarecodedinanappropriateformattodefineaprogramofinstructionsforaparticularworkpartorjob.Whenthejobchanges,theprogramofinstructionsischanged.ThecapabilitytochangetheprogramiswhatmakesN/Csuitableforlow-andmedium-volumeproduction.Itismucheasiertowriteprogramsthantomakemajoralterationsoftheprocessingequipment.Therearetwobasictypesofnumericallycontrolledmachinetools:point—to—pointandcontinuous—path(alsocalledcontouring).Point—to—pointmachinesuseunsynchronizedmotors,withtheresultthatthepositionofthemachiningheadCanbeassuredonlyuponcompletionofamovement,orwhileonlyonemotorisrunning.Machinesofthistypeareprincipallyusedforstraight—linecutsorfordrillingorboring.TheN/Csystemconsistsofthefollowingcomponents:datainput,thetapereaderwiththecontrolunit,feedbackdevices,andthemetal—cuttingmachinetoolorothertypeofN/Cequipment.Datainput,alsocalled“man—to—controllink”,maybeprovidedtothemachinetoolmanually,orentirelybyautomaticmeans.Manualmethodswhenusedasthesolesourceofinputdataarerestrictedtoarelativelysmallnumberofinputs.Examplesofmanuallyoperateddevicesarekeyboarddials,pushbuttons,switches,orthumbwheelselectors.Thesearelocatedonaconsolenearthemachine.Dialsaleanalogdevicesusuallyconnectedtoasyn-chro-typeresolverorpotentiometer.Inmostcases,pushbuttons,switches,andothersimilartypesofselectorsayedigitalinputdevices.Manualinputrequiresthattheoperatorsetthecontrolsforeachoperation.Itisaslowandtediousprocessandisseldomjustifiedexceptinelementarymachiningapplicationsorinspecialcases.Inpracticallyallcases,informationisautomaticallysuppliedtothecontrolunitandthemachinetoolbycards,punchedtapes,orbymagnetictape.Eight—channelpunchedpapertapeisthemostcommonlyusedformofdatainputforconventionalN/Csystems.Thecodedinstructionsonthetapeconsistofsectionsofpunchedholescalledblocks.Eachblockrepresentsamachinefunction,amachiningoperation,oracombinationofthetwo.TheentireN/Cprogramonatapeismadeupofanaccumulationofthesesuccessivedatablocks.Programsresultinginlongtapesallwoundonreelslikemotion-picturefilm.Programsonrelativelyshorttapesmaybecontinuouslyrepeatedbyjoiningthetwoendsofthetapetoformaloop.Onceinstalled,thetapeisusedagainandagainwithoutfurtherhandling.Inthiscase,theoperatorsimplyloadsandunloadstheparts.Punchedtapesalepreparedontypewriterswithspecialtape—punchingattachmentsorintapepunchingunitsconnecteddirectlytoacomputersystem.Tapeproductionisrarelyerror-free.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.Highreadingratesarenecessarytomaintaincontinuousmachine—toolmotion;otherwisedwellmarksmaybegeneratedbythecutteronthepartduringcontouringoperations.Thereadingdevicemustbecapableofreadingdatablocksataratefasterthanthecontrolsystemcanprocessthedata.AfeedbackdeviceisasafeguardusedonsomeN/Cinstallationstoconstantlycompensateforerrorsbetweenthecommandedpositionandtheactuallocationofthemovingslidesofthemachinetool.AnN/Cmachineequippedwiththiskindofadirectfeedbackcheckingdevicehaswhatisknownasaclosed-loopsystem.Positioningcontrolisaccomplishedbyasensorwhich,duringtheactualoperation,recordsthepositionoftheslidesandrelaysthisinformationbacktothecontrolunit.Signalsthusreceivedalecomparedtoinputsignalsonthetape,andanydiscrepancybetweenthemisautomaticallyrectified.Inanalternativesystem,calledanopen—loopsystem,themachineispositionedsolelybysteppingmotordrivesinresponsetocommandsbyacontrollers.TherearethreebasictypesofNCmotions,asfollows:Point-to-pointorPositionalControlInpoint-to-pointcontrolthemachinetoolelements(tools,table,etc.)aremovedtoprogrammedlocationsandthemachiningoperationsperformedafterthemotionsarecompleted.Thepathorspeedofmovementbetweenlocationsisunimportant;onlythecoordinatesoftheendpointsofthemotionsareaccuratelycontrolled.Thistypeofcontrolissuitablefordrillpressesandsomeboringmachines,wheredrilling,tapping,orboringoperationsmustbeperformedatvariouslocationsontheworkpiece.Straight-LineorLinearControlStraight-Linecontrolsystemsareabletomovethecuttingtoolparalleltooneofthemajoraxesofthemachinetoolatacontrolledratesuitableformachining.Itisnormallyonlypossibletomoveinonedirectionatatime,soangularcutsontheworkpiecearenotpossible,consequently,formillingmachines,onlyrectangularconfigurationscanbemachinedorforlathesonlysurfacesparallelorperpendiculartothespindleaxiscanbemachined.Thistypeofcontrolledmotionisoftenreferredtoaslinearcontrolorahalf-axisofcontrol.Machineswiththisformofcontrolarealsocapableofpoint-to-pointcontrol.ContinuousPathorContouringControlIncontinuouspathcontrolthemotionsoftwoormoreofthemachineaxesarecontrolledsimultaneously,sothatthepositionandvelocityofthecanbetoolarechangedcontinuously.Inthiswaycurvesandsurfacescanbemachinedatacontrolledfeedrate.Itisthefunctionoftheinterpolatorinthecontrollertodeterminetheincrementsoftheindividualcontrolledaxesofthemachinesnecessarytoproducethedesiredmotion.Thistypeofcontrolisreferredtoascontinuouscontrolorafullaxisofcontrol.SometerminologyconcerningcontrolledmotionsforNCmachineshasbeenintroduced.Forexample,somemachinesarereferredtoasfour-orfive-orevensix-axismachines.Foraverticalmillingmachinethreeaxesofcontrolarefairlyobvious,thesebeingtheusualX,Y,Zcoordinatedirections.Afourthorfifthaxisofcontrolwouldimplysomeformofrotarytabletoindextheworkpieceorpossiblytoprovideangularmotionoftheworkhead.Thus,inNCterminologyanaxisofcontrolisanycontrolledmotionofthemachineelements(spindles,tables,etc).Afurthercomplicationisuseofthetermhalf-axisofcontrol;forexample,manymillingmachinesarereferredtoas2.5-axismachine.Thismeansthatcontinuouscontrolispossiblefortwomotions(axes)andonlylinearcontrolispossibleforthethirdaxis.Appliedtoverticalmillingmachines,2.5axiscontrolmeanscontouringintheX,YplaneandlinearmotiononlyintheZdirection.Withthesemachinesthree-dimensionalobjectshavetobemachinedwithwaterlinesaroundthesurfaceatdifferentheights.Withanalternativeterminologythesamemachinecouldbecalleda2CLmachine(Cforcontinuous,Lforlinearcontrol).Thus,amillingmachinewithcontinuouscontrolintheX,Y,Zdirectionscouldbetermedbeathree-axismachineora3cmachine,Similarly,lathesareusuallytwoaxisor2Cmachines.Thedegreeofworkprecisiondependsalmostentirelyupontheaccuracyoftheleadscrewandtherigidityofthemachinestructure.Withthissystem.thereisnoself-correctingactionorfeedbackofinformationtothecontrolunit.Intheeventofanunexpectedmalfunction,thecontrolunitcontinuestoputoutpulsesofelectricalcurrent.If,forexample,thetableonaN/Cmillingmachineweresuddenlytobecomeoverloaded,noresponsewouldbesentbacktothecontroller.Becausesteppingmotorsarenotsensitivetoloadvariations,manyN/Csystemsaredesignedtopermitthemotorstostallwhentheresistingtorqueexceedsthemotortorque.Othersystemsareinuse,however,
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