先进制造技术第五单元

5NumericalControlNumericalcontrol(NC)isamethodofcontrollingthemovementsofmachinecomponentsbydirectlyinsertingcodedinstructions,intheformofnumbersandletters,intothesystem.Thesystemautomaticallyinterpretsthesedataandconvertsthemtooutputsignals.Thesesignals,inturn,controlvariousmachinecomponents-forexample,byturningspindlesonandoff,changingtools,movingtheworkpieceorthetoolsalongspecificpaths,orturningcuttingfluidsonandoff.Inordertoappreciatetheimportanceofnumericalcontrolofmachines,let'sbrieflyreviewhowaprocesssuchasmachininghastraditionallybeencarriedout.Afterstudyingtheworkingdrawingsofapart,theoperatorsetsuptheappropriateprocessparameters(suchascuttingspeed,feed,depthofcut,cuttingfluid,andsoon),determinesthesequenceofthemachiningoperationstobeperformed,clampstheworkpieceinaworkholdingdevice(suchasachuckorcollet),andproceedswiththemakingofthepart.Dependingonpartshapeandonthedimensionalaccuracyspecified,thisapproachusuallyrequiresskilledoperators.Themachiningprocedurefollowedmaydependontheparticularoperator;becauseofthepossibilitiesofhumanerror,evenpartsproducedbythesameoperatormaynotallbeidentical.Partqualitymay,therefore,dependontheparticularoperatoror(evenwiththesameoperator)onthedayoftheweekorthehouroftheday.Becauseofincreasedconcernwithimprovingproductqualityandreducingmanufacturingcosts,suchvariability(anditseffectsonproductquality)arenolongeracceptable.Thissituationcanbeeliminatedbynumericalcontrolofthemachiningoperation.Theimportanceofnumericalcontrolcanbefurtherillustratedbythefollowingexample.AssumethatseveralholesaretobedrilledonapartinthepositionsshowninFig.5.1.Inthetraditionalmanualmethodofmachiningthispart,theoperatorpositionsthedrillbitwithrespecttotheworkpiece,usingreferencepointsgivenbyanyofthethreemethodsshowninthefigure.Theoperatorthenproceedstodrilltheholes.Let'sfirstassumethat100parts,allhavingexactlythesameshapeanddimensionalaccuracy,aretobedrilled.Obviously,thisoperationisgoingtobetedious,becausetheoperatorhastogothroughthesamemotionsrepeatedly.Moreover,theprobabilityishighthat,forvariousreasons,someofthepartsmachinedwillbedifferentfromothers.Let'snowassumethatduringthisproductionrun,theorderforthesepartsischanged,andtenofthepartsnowrequireholesindifferentpositions.Themachinistnowhastorepositiontheworktable;thisoperationwillbetimeconsumingandissubjecttoerror.Suchoperationscanbeperformedeasilybynumericalcontrolmachinesthatarecapableofproducingpartsrepeatedlyandaccuratelyandofhandlingdifferentparts(bysimplyloadingdifferentpartprograms,aswillbedescribedlater).Inoperationsundernumericalcontrol,dataconcerningallaspectsofthemachiningoperation,suchaslocations,speeds,feeds,andcuttingfluids,canbestoredonmagneticmedia,changingovertimefromtapestoharddisks.TheconceptofNCcontrolisthatspecificinformationcanberelayedfromthesestoragedevicestothemachinetool'scontrolpanel.Onthebasisofinputinformation,relaysandotherdevices(hardwiredcontrols)canbeactuatedtoobtainadesiredmachinesetup.Complexoperations(suchasturningaparthavingvariouscontours,ordiesinkinginamillingmachine)canbecarriedouteasily.Numericalcontrolhashadamajorimpactonallaspectsofmanufacturingoperations.NCmachinesarenowusedextensivelyinsmallandmedium-quantityproduction(typically500partsorless)ofawidevarietyofparts,bothinsmallshopsandinlargemanufacturingfacilities.Oldermachinescanoftenberetrofittedwithnumericalcontrol.5.1HistoricalBackThebasicconceptbehindnumericalcontrolapparentlywasimplementedintheearly1800s,whenpunchedholesinsheet-metalcardswereusedtoautomaticallycontrolthemovementsofweavingmachines.Needleswereactivatedbythesensingofthepresenceorabsenceofaholeinthecard.Thisinventionwasfollowedbyautomaticpianoplayers(Pianola),inwhichthekeyswereactivatedbyairflowingthroughholespunchedinaperforatedrollofpaper.Theprincipleofnumericallycontrollingthemovementsofmachinetoolswasfirstconceivedinthe1940sbyJ.Parsonsinhisattempttomachinecomplexhelicopterblades.ThefirstprototypeNCmachinewasbuiltin1952attheMassachusettsInstituteofTechnology.Itwasavertical-spindle,two-axiscopymillingmachineretrofittedwithservomotors,andthemachiningoperationsperformedconsistedofendmillingandfacemillingonathickaluminumplateThenumericaldatatobepunchedintothepapertapesweregeneratedbyadigitalcomputer,anotherinventionwhichwasbeingdevelopedatthesametimeatMIT.Intheexperiments,partsweremachinedsuccessfully,accurately,andrepeatedlywithoutoperatorintervention.Onthebasisofthissuccess,themachine-toolindustrybegandesigning,building,andmarketingNCmachinetools.Later,thesemachineswereequippedwithcomputernumericalcontrols(CNC)yieldinggreaterflexibility,accuracy,versatility,andeaseofoperation.5.2ComputerNumericalControlInthenextstepinthedevelopmentofnumericalcontrol,thecontrolhardware(mountedontheNCmachine)wasconvertedtolocalcomputercontrolbysoftware.Twotypesofcomputerizedsystemsweredeveloped:directnumericalcontrol,andcomputernumericalcontrol.Indirectnumericalcontrol(DNC),asoriginallyconceivedanddevelopedinthe1960s,severalmachinesaredirectlycontrolled,stepbystep,byacentralmainframecomputer.Inthis