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NUMERICALCONTROLNumericalcontrol(NC)isaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,letters,andothersymbolsThenumbers,letters,andsymbolsarecodedinanappropriateformattodefineaprogramofinstructionsforaparticularworkpartorjobWhenthejobchanges,theprogramofinstructionsischangedThecapabilitytochangetheprogramiswhatmakesNCsuitableforlow-andmedium-volumeproductionItismucheasiertowriteprogramsthantomakemajoralterationsoftheprocessingequipmentTherearetwobasictypesofnumericallycontrolledmachinetools:pointtopointandcontinuouspath(alsocalledcontouring)Pointtopointmachinesuseunsynchronizedmotors,withtheresultthatthepositionofthemachiningheadCanbeassuredonlyuponcompletionofamovement,orwhileonlyonemotorisrunningMachinesofthistypeareprincipallyusedforstraightlinecutsorfordrillingorboringTheNCsystemconsistsofthefollowingcomponents:datainput,thetapereaderwiththecontrolunit,feedbackdevices,andthemetalcuttingmachinetoolorothertypeofNCequipmentDatainput,alsocalled“mantocontrollink”,maybeprovidedtothemachinetoolmanually,orentirelybyautomaticmeansManualmethodswhenusedasthesolesourceofinputdataarerestrictedtoarelativelysmallnumberofinputsExamplesofmanuallyoperateddevicesarekeyboarddials,pushbuttons,switches,orthumbwheelselectorsThesearelocatedonaconsolenearthemachineDialsaleanalogdevicesusuallyconnectedtoasyn-chro-typeresolverorpotentiometerInmostcases,pushbuttons,switches,andothersimilartypesofselectorsayedigitalinputdevicesManualinputrequiresthattheoperatorsetthecontrolsforeachoperationItisaslowandtediousprocessandisseldomjustifiedexceptinelementarymachiningapplicationsorinspecialcasesInpracticallyallcases,informationisautomaticallysuppliedtothecontrolunitandthemachinetoolbycards,punchedtapes,orbymagnetictapeEightchannelpunchedpapertapeisthemostcommonlyusedformofdatainputforconventionalNCsystemsThecodedinstructionsonthetapeconsistofsectionsofpunchedholescalledblocksEachblockrepresentsamachinefunction,amachiningoperation,oracombinationofthetwoTheentireNCprogramonatapeismadeupofanaccumulationofthesesuccessivedatablocksProgramsresultinginlongtapesallwoundonreelslikemotion-picturefilmProgramsonrelativelyshorttapesmaybecontinuouslyrepeatedbyjoiningthetwoendsofthetapetoformaloopOnceinstalled,thetapeisusedagainandagainwithoutfurtherhandlingInthiscase,theoperatorsimplyloadsandunloadsthepartsPunchedtapesalepreparedontypewriterswithspecialtapepunchingattachmentsorintapepunchingunitsconnecteddirectlytoacomputersystemTapeproductionisrarelyerror-freeErrorsmaybeinitiallycausedbythepartprogrammer,incardpunchingorcompilation,orasaresultofphysicaldamagetothetapeduringhandling,etcSeveraltrialrunsareoftennecessarytoremoveallerrorsandproduceanacceptableworkingtapeWhilethedataonthetapeisfedautomatically,theactualprogrammingstepsaledonemanuallyBeforethecodedtapemaybeprepared,theprogrammer,oftenworkingwithaplanneroraprocessengineer,mustselecttheappropriateNCmachinetool,determinethekindofmaterialtobemachined,calculatethespeedsandfeeds,anddecideuponthetypeoftoolingneeded.ThedimensionsonthepartprintarecloselyexaminedtodetermineasuitablezeroreferencepointfromwhichtostarttheprogramAprogrammanuscriptisthenwrittenwhichgivescodednumericalinstructionsdescribingthesequenceofoperationsthatthemachinetoolisrequiredtofollowtocuttheparttothedrawingspecificationsThecontrolunitreceivesandstoresallcodeddatauntilacompleteblockofinformationhasbeenaccumulatedIttheninterpretsthecodedinstructionanddirectsthemachinetoolthroughtherequiredmotionsThefunctionofthecontrolunitmaybebetterunderstoodbycomparingittotheactionofadialtelephone,where,aseachdigitisdialed,itisstoredWhentheentirenumberhasbeendialed,theequipmentbecomesactivatedandthecalliscompletedSiliconphotodiodes,locatedinthetapereaderheadonthecontrolunit,detectlightasitpassesthroughtheholesinthemovingtapeThelightbeamsareconvertedtoelectricalenergy,whichisamplifiedtofurtherstrengthenthesignalThesignalsarethensenttoregistersinthecontrolunit,whereactuationsignalsarerelayedtothemachinetooldrivesSomephotoelectricdevicesarecapableofreadingatratesupto1000characterspersecondHighreadingratesarenecessarytomaintaincontinuousmachinetoolmotion;otherwisedwellmarksmaybegeneratedbythecutteronthepartduringcontouringoperationsThereadingdevicemustbecapableofreadingdatablocksataratefasterthanthecontrolsystemcanprocessthedataAfeedbackdeviceisasafeguardusedonsomeNCinstallationstoconstantlycompensateforerrorsbetweenthecommandedpositionandtheactuallocationofthemovingslidesofthemachinetoolAnNCmachineequippedwiththiskindofadirectfeedbackcheckingdevicehaswhatisknownasaclosed-loopsystemPositioningcontrolisaccomplishedbyasensorwhich,duringtheactualoperation,recordsthepositionoftheslidesandrelaysthisinformationbacktothecontrolunitSignalsthusreceivedalecomparedtoinputsignalsonthetape,andanydiscrepancybetweenthemisautomaticallyrectifiedInanalternativesystem,calledanopenloopsystem,themachineispositionedsolelybysteppingmotordrivesinresponsetocommandsbyacontrollersTherearethreebasictypesofNCmotions,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.ThedegreeofworkprecisiondependsalmostentirelyupontheaccuracyoftheleadscrewandtherigidityofthemachinestructureWiththissystemthereisnoself-correctingactionorfeedbackofinformationtothecontrolunitIntheeventofanunexpectedmalfunction,thecontrolunitcontinuestoputoutpulsesofelectricalcurrentIf,forexample,thetableonaNCmillingmachineweresuddenlytobecomeoverloaded,noresponsewouldbesentbacktothecontrollerBecausesteppingmotorsarenotsensitivetoloadvariations,manyNCsystemsaredesignedtopermitthemotorstostallwhentheresistingtorqueexceedsthemotortorqueOthersystemsareinuse,however,whichinspiteofthepossibilityofdamagetothemachinestructureortothemechanicalsystem,aledesignedwithspecialhightorquesteppingmotorsInthiscase,themotorshavesufficientcapacityto“overpowerthesystemintheeventofalmostanycontingencyTheoriginalNCusedtheclosedloopsystemOfthetwosystems,closedandopenloop,closedloopismoreaccurateand,asaconsequence,isgenerallymoreexpensiveInitially,openloopsystemswereusedalmostentirelyforlight-dutyapplicationsbecauseofinherentpowerlimitationspreviouslyassociatedwithconventionalelectricsteppingmotorsRecentadvancesinthedevelopmentofelectrohydraulicsteppingmotorshaveledtoincreasinglyheaviermachineloadapplicationsMILLINGMillingisabasicmachiningprocessinwhichthesurfaceisgeneratedbytheprogressiveformationandremovalofchipsofmaterialfromtheworkpieceasitisfedtoarotatingcutterinadirectionperpendiculartotheaxisofthecutterInsomecasestheworkpieceisstationaryandthecutterisfedtotheworkInmostinstancesamultipletoothcutterisusedsothatthemetalremovalrateishigh,andfrequentlythedesiredsurfaceisobtainedinasinglepassoftheworkThetoolusedinmillingisknownasamillingcutterItusuallyconsistsofacylindricalbodywhichrotatesonitsaxisandcontainsequallyspacedperipheralteeththatintermittentlyengageandcuttheworkpieceInsomecasestheteethextendpartwayacrossoneorbothendsofthecylinderBecausethemillingprincipleprovidesrapidmetalremovalandcanproducegoodsurfacefinish,itisparticularlywellsuitedformass-productionwork,andexcellentmillingmachineshavebeendevelopedforthispurposeHowever,veryaccurateandversatilemillingofageneral-purposenaturealsohavebeendevelopedthatarewidelyusedinjo
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