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1、NumericalControlNumericalcontrol(N/C)isaformofprogrammableautomationinwhichtheprocessingequipmentiscontrolledbymeansofnumbers,lettersandothersymbols.Thenumbers,letters,andsymbolsarecodedinanappropriateformattodefineaprogramofinstructionsforaparticularworkpartorjob.Whenthejobischanged,
2、theprogramofinstructionsmustbechanged.ThecapabilitytochangetheprogramiswhatmakesN/Csuitableforlow-volumeandmedium-volumeproduction.Itismucheasiertowriteprogramsthantomakemajoralterationsoftheprocessingequipment.Therearetwobasictypesofnumericallycontrolledmachinetools:point—to—pointand
3、continuous—path(alsocalledcontouring).Point—to—pointmachinesuseunsynchronizedmotors,withtheresultthatthepositionofthemachiningheadCanbeassuredonlyuponcompletionofamovement,orwhileonlyonemotorisrunning.Machinesofthistypeareprincipallyusedforstraight—linecutsorfordrillingorboring.TheN/C
4、systemconsistsofthefollowingcomponents:datainput,thetapereaderwiththecontrolunit,feedbackdevices,andthemetal—cuttingmachinetoolorothertypeofN/Cequipment.Datainput,alsocalled“man—to—controllink”,maybeprovidedtothemachinetoolmanually,orentirelybyautomaticmeans.Manualmethodswhenusedasthe
5、solesourceofinputdataarerestrictedtoarelativelysmallnumberofinputs.Examplesofmanuallyoperateddevicesarekeyboarddials,pushbuttons,switches,orthumbwheelselectors.Thesearelocatedonaconsolenearthemachine.Dialsaleanalogdevicesusuallyconnectedtoasynchronization-typeresolverorpotentiometer.I
6、nmostcases,pushbuttons,switchesandothersimilartypesofselectorsaredigitalinputdevices.Manualinputrequiresthattheoperatorsetthecontrolsforeachoperation.Itisaslowandtediousprocessandisseldomjustifiedexceptinelementarymachiningapplicationsorinspecialcases.Inpracticallyallcases,information
7、isautomaticallysuppliedtothecontrolunitandthemachinetoolbycards,punchedtapes,orbymagnetictape.Eight—channelpunchedpapertapeisthemostcommonlyusedformofdatainputforconventionalN/Csystems.Thecodedinstructionsonthetapeconsistofsectionsofpunchedholescalledblocks.Eachblockrepresentsamachine
8、funct
