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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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