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