外文翻譯--數(shù)控車削中心工件誤差實(shí)時(shí)預(yù)報(bào) 英文版.pdf

IntJAdvManufTechnol(2001)17:6496532001Springer-VerlagLondonLimitedReal-TimePredictionofWorkpieceErrorsforaCNCTurningCentre,Part1.MeasurementandIdentificationX.LiDepartmentofManufacturingEngineering,CityUniversityofHongKong,HongKongThispaperanalysestheerrorsourcesoftheworkpieceinbarturning,whichmainlyderivefromthegeometricerrorofmachinetools,i.e.thethermallyinducederror,theerrorarisingfrommachineworkpiecetoolsystemdeflectioninducedbythecuttingforces.Asimpleandlow-costcompactmeasuringsystemcombiningafinetouchsensorandQ-setterofmachinetools(FTSFQ)isdeveloped,andappliedtomeasurethework-piecedimensions.Anidentificationmethodforworkpieceerrorsisalsopresented.Theworkpieceerrorswhicharecomposedofthegeometricerror,thermalerror,andcuttingforceerrorcanbeidentifiedaccordingtothemeasurementresultsofeachstep.Themodelofthegeometricerrorofatwo-axisCNCturningcentreisestablishedrapidlybasedonthemeasurementresultsbyusinganFTSFQsetterandcoordinatemeasuringmachine(CMM).ExperimentalresultsshowthatthegeometricerrorcanbecompensatedbymodifiedNCcommandsinbarturning.Keywords:Dimensionmeasure；Erroridentification；Geo-metricerror；Turning1.IntroductionInrecentyears,ultraprecisionmachininghasmaderemarkableprogress.Somespeciallatheshavebeenabletomakeultra-precisionmachining,tolessthanasubmicronandnanomicrontolerancesapossibility.Acommonsecondapproachisthatthegrindingisusedtoachieveahighlevelofdimensionalaccuracyafterturning.However,theconditionofthecuttingtool(diamond)andworkpiece(aluminium)haverestrictedtheapplicationofultraprecisionlathes.Thesecondapproachincreasesthenumberofmachinetoolsandmachiningprocessesused1,whichresultsinanincreaseinthemanufacturingcost.Atpresent,mostCNClathesareequippedwithapositioningresolutionof1mm.Variousmachiningerrorsinfinishturning,however,degradetheaccuracytoalevelofapproximatelyCorrespondenceandoffprintrequeststo:DrXiaoliLi,DepartmentofManufacturingEngineering,CityUniversityofHongKong,HongKong.E-mail:lixiaoliF10mm,sothatwhenturningcarbonsteel,amachiningerrorpredictablyarisesinexcessof2030mm.Forimprovingmach-iningaccuracy,themethodofcarefuldesignandmanufacturehasbeenextensivelyusedinsomeCNClathes.However,themanufacturingcostbasedontheabovemethodwillrapidlyincreasewhentheaccuracyrequirementsofthemachinetoolsystemareincreasedbeyondacertainlevel.Forfurtherimprov-ingmachineaccuracycost-effectively,real-timeerrorpredictionandcompensationbasedonsensing,modellingandcontroltechniqueshavebeenwidelystudied2,soultraprecisionandfinishtuningcanbeperformedononeCNClathe.Thepositioningresolutionofthecuttingtoolsandworkpieceisreducedsothatitcannotmaintainhighaccuracyduringmachiningbecauseofthecutting-force-induceddeflectionofthemachineworkpiecetoolsystem,andthethermallyinducederror,etc.Ingeneral,apositioningdeviceusingapiezo-eletricactuatorisusedtoimprovetheworkingaccuracy,butthemethodintroducessomeproblems,suchas,thefeedbackstrat-egy,andtheaccuracyofsensors,whichaddtothemanufactur-ingcostoftheproducts.However,iftheworkpieceerrorcanbemeasuredbyusingameasuringinstrument,orpredictedbyusingamodelling,theturningprogramproducedbymodifiedNCcommandscanbeexecutedsatisfactorilyonaCNCmachinetool.Thus,aCNCturningcentrecancompensateforthenormalmachiningerror,i.e.themachinetoolcanmachineaproductwithahighlevelofaccuracyusingmodifiedNCcommands,inrealtime.Theworkpieceerrorderivesfromtheerrorintherelativemovementbetweenthecuttingtoolandtheidealworkpiece.Foratwo-axisturningcentre,thisrelativeerrorvariesastheconditionofthecuttingprogresses,e.g.thethermaldeflectionofthemachinetoolistimevariant,whichresultsindifferentthermalerrors.Accordingtothevariouscharactersoftheerrorsourcesoftheworkpiece,theworkpieceerrorscanbeclassifiedasgeometricerror,thermallyinducederror,andcutting-force-inducederror.Themainaffectingfactorsincludethepositionerrorsofthecomponentsofthemachinetoolandtheangularerrorsofthemachinestructure,i.e.thegeometricerror.Thethermallyinducederrorsofthemachinetool(i.e.thethermalerror),andthedeflectionofthemachiningsystem(includingthemachinetools,workpiece,andcuttingtools)arisingfromcuttingforces,arecalledthecutting-forceerror.650X.LiThispaperanalysestheworkpieceerrorsourcesinturning.Theerrorsofamachinedworkpiecearemainlycomposedofthegeometricerrorofthemachinetools,thethermallyinducederror,andtheerrorarisingfrommachineworkpiecetoolsys-temdeflectioninducedbythecuttingforces.Asimpleandlow-costmeasuringinstrumentfortheworkpiecedimensions,whichcombinesafinetouchsensorandmachinetoolQ-setter(FTSFQ),isdescribed,andappliedtomeasuretheworkpieceerror.Anewmethodforidentifyingthegeometricerror,thethermalerror,andthecutting-forceerrorisalsopresentedforatwo-axisturningcentre.Finally,themodellingofthegeo-metricerrorofaCNCturningcentreispresented,basedonthemeasurementresultsusingtheFTSFQandCMM.ThegeometricerrorcanbecompensatedbythemodifiedNCcommandmethod.2.ErrorSourcesinTurningThemachinetoolsystemiscomposedofthedriveservo,themachinetoolstructure,theworkpieceandthecuttingprocess.Themajorerrorsourcesderivefromthemachinetool(thermalerrors,geometricerrors,andforcedvibrations),thecontrol(driveservodynamicsandprogrammingerrors)andthecuttingprocess(machinetoolandcuttingtooldeflection,workpiecedeflection,toolwear,andchatter)3.Errorsderivedfromthemachinetoolincludethermalerrors(machinethermalerrorandworkpiecethermalerrors),geo-metricerrors,andforcedvibrations,whichdominatemachiningaccuracy.Thethermalerrorsandgeometricerrorsarethedominantfactorswithrespecttomachiningaccuracyinfinecutting.However,machinetoolerrorscanbedecoupledfromtheothererrorsourcesandcompensated4.Theerrorderivedfromforcedvibrationcanbereducedthroughbalanceddynamiccomponentsandvibrationisolation3.Theerrorsderivedfromthecontroller/drivedynamicsarerelatedtothecuttingforcedisturbancesandtheinertiaofthedriveandthemachinetable.Theseerrorscanbereducedbyaninterpolatorwithadecelerationfunction5orbyanadvancedfeeddrivecontroller6,theseerrors,reducedbyusingtheabovemethods,aresmallwhencomparedwithothererrorsources.Owingtothedemandforhighproductivity,highfeedratesandlargedepthsofcutarerequired,whichresultinlargecuttingforces.Therefore,thecutting-forceinduceddeflectionsofthemachinetool(spindle),toolholder,workpiece,andcuttingtoolmakesignificantcontributionstomachiningaccu-racyduringthecuttingprocess.Inaddition,toolwearandmachinetoolchatterarealsoimportanterrorsourcesinthecuttingprocess.However,theseeffectsareneglectedheresoastofocusonthemainerrorsources.Inshort,theerrorofamachinedworkpiece,i.e.thetotalmachiningerror(dTot),iscomposedmainlyofthegeometricerrorsofthemachinetool(s)(dG),thethermallyinducederror(dT),andtheerror(dF)arisingfromthedeflectionofthemachineworkpiecetoolsysteminducedbythecuttingforces.Hence,dTotdG+dT+dF(1)Inthenextsection,wepresentanovelcompactmeasuringinstrumentandanewanalyticalapproachformeasuringandidentifyingworkpieceerrorsinturning.3.ACompactMeasurementSystemContactsensors,suchastouchtriggerprobes,havebeenusedtomeasureworkpiecedimensionsinmachining.Inmachiningpractice,themeasuringinstrumentisattachedtooneofthemachinesaxestomeasureasurfaceontheworkpiece.ATP7MorMP3associatedwiththePH10MrangeofmotorisedprobeheadsoraPH6MfixedheadhavebeenusedwidelyintheautomatedCNCinspectionenvironmentowingtotheirhighlevelofreliabilityandaccuracyandintegralautojoint.Thoughtheprobeheadsareofadequateaccuracy(unidirectionalrepeatabilityatstylustip(highsensitivity):0.25mm；pre-travelvariation360(highsensitivity):60.25mm),andversatileinapplication,theyhavecleardrawbacks,includingcomplexityofconstruction,highprice($4988),andtheneedforcarefulmaintenance.Toovercomethesedrawbacksoftouchtriggerprobes,Ostafievetal.7presentedanoveltechniqueofcontactprobingfordesigningafinetouchsensor.Thecuttingtoolitselfisusedasacontactprobe.Thesensoriscapableofyieldingmeasurementaccuracycomparabletothatofthebesttouchtriggerprobeinuse.Moreover,theprincipleofoperationandconstructionofthesensorisextremelysimple,thecostofthesensorislow,andthemaintenanceisveryeasy.Inthispaper,thissensorwillbeusedtomeasurethediameterofaworkpieceassociatedwiththeQ-setter.AtouchsensorismountedonaCNCturningcentre.Whenwemanuallybringthetoolnoseintocontactwithit,aninterruptsignalisgeneratedfortheNCunittostopanaxis.Moreover,itcanwriteinanoffsetandaworkpiececoordinateshiftautomatically.Thisfunctionfacilitatesset-upwhenreplac-ingatool,andthisconvenientfunctioniscalledthe“QuickToolSetter”or“Q-setter”.Basedontheaboveprinciple,wecanoperateaswitch,whichiscontrolledbyfinetouchsensor,betweentheQ-setterandNCunit.Whenthetooltiptouchestheworkpiecesurface,thefinetouchsensorcansendacontrolsignaltotheswitch,toturnittothe“off”state.SeeFig.1,thefinetouchsensorreplacestheQ-setterfunction,tostopanaxisandwriteinanoffsetandaworkpiececoordinateshiftautomatically.Therefore,thefinetouchsensorassociatedwithFig.1.FlowdiagramofafinetouchsensorfixedonaCNCcontroller.Real-TimePredictionofWorkpieceErrors.Part1651aQ-setter(FTSFQ)canbeusedtoinspectthediameteroftheworkpiece,themethodisshowninFig.2.Whenthecuttingtooltiptouchestheworkpiecesurface,a“beep”soundisheardandtheswitching“OFF”signalappearsandtheaxisstopsautomatically,asfartheQ-setter.Anew“tooloffset”XT-WisobtainedbytheNCunit(displayofCNC).Beforetouchingtheworkpiecesurface,thecuttingtooltiptouchestheQ-setter,andthe“tooloffset”XT-Qisobtained.Thus,theon-machineworkpiecediameterDon-machineisgivenbythefollowingEq.:Don-machine=2H+uXT-Qu-uXT-Wu(2)whereXT-QisthetooloffsetwhenthecuttingtoolcontactstheQ-setterXT-Wisthe“tooloffset”whenthecuttingtoolcontactstheworkpiecesurfaceHisthedistancefromthecentreoftheQ-settertothecentreofthespindleinthex-axisdirectionandisprovidedbythemachinetoolmanufacturer,fortheSeiki-SeicosLIITurningcentre,itis85.356mm.OstafievandVenuvinod8testedthemeasurementaccuracyofthefinetouchsensor,performingon-machineinspectionofturnedparts,andfoundthatthemethodwascapableofachiev-ingameasurementaccuracyoftheorderof0.01mmundershopfloorconditions.However,themeasurementaccuracyofthefinetouchsensortogetherwiththeQ-setterobtainedanaccuracyofaboutmmbecausetheresultsofthemeasurementsystemaredisplayedbytheCNCsystem,andthereadingsaccuracyoftheCNCsystemisupto1mm.4.IdentificationofWorkpieceErrorsFromtheaboveanalysisoferrorsourcesoftheworkpiece,thetotalerrordTotofmachinedpartsismainlycomposedofthefollowingerrorsinaturningoperation:IdGthegeometricerrorsofmachinetools.IdTthethermallyinducederror.Fig.2.InspectionforthediameterofaworkpiecebyusingthefinetouchsensorwiththeQ-setterofamachinetool.IdFthecuttingforceinducederror.Toanalysetheerrorsourcesofamachinedworkpiece,Liu&Venuvinod9usedFig.3toillustratetherelationshipamongstdimensionsassociatedwithdifferenterrorcomponentsinturning.InFig.3,Ddesisthedesireddimensionoftheworkpiece；Domwisthedimensionobtainedbyon-machinemeasurementusingFTSFQimmediatelyafterthemachiningoperation；Domcisthedimensionobtainedbyon-machinemeasurementusingFTSFQafterthemachinehascooleddown；andDppisthedimensionobtainedbypost-processprocessmeasurementusingaCMMaftertheworkpiecehasbeenremovedfromthemachine.Whentheworkpiecehasbeenmachined,andremovedfromthemachinetoolsystem,itisthensentforinspectionofthedimensionsusingaCMM.Thisprocedureiscalledpost-processinspection,bywhichweobtainitDppvalue.AsthepositioningerroroftheCMMisverymuchsmallerthanthedesiredmeasurementaccuracy,thetotalerrorisdTot=(Dpp-Ddes)/2(3)ThedimensionDomwisobtainedthroughon-machinemeasurementusingFTSFQimmediatelyaftermachining,i.e.themachineisstillinthesamethermalstateasatthetimeofmachining.Themeasurementismadewiththesamepositioningerrorasthatwhichexistedduringmachining.Hence,thepositioningerrorinthisstatewouldbeequalto(dG1dT),i.e.(Dpp-Domw)/2=dG+dT(4)Whenthemachinehascompletelycooleddown,i.e.withoutthermalerror,thedimensionDomccanbeobtainedbyon-machinemeasurementusingFTSFQ.Themeasurementhasapositioningerrorequaltothegeometricerrorofthemachineatthelocationofmeasurement.Hence,thepositioningerrorinthisstatewouldbeequalto(dG),i.e.(Dpp-Domc)/2=dG(5)CombiningEqs(4)and(5),thethermallyinducederrordTis(Domc-Domw)/2=dT(6)Hence,takingEqs(1),(3),and(4)intoaccount,thecutting-force-inducederrorowningtothedeflectionofthemachineworkpiecetoolsystemdFis(Domw-Ddes)/2=dF(7)Fig.3.Therelationshipsamongdimensions.652X.LiSofar,themachiningerroriscomposedofthegeometricerror,thethermalerror,andthecutting-force-inducederrorandcanbeidentifiedusingtheaboveprocedure.Thethermalerrorandtheforce-inducederrormodellingsisaddressedinLi10.Here,thegeometricerrorofmachinetoolismeasuredandmodelled.5.ModellingofGeometricErrorThegeometricerrorofaworkpieceismainlyaffectedbytheoffsetofthespindle,andthelinearerrorandtheangularerrorsofthecross-slideforatwo-axisCNCturningcentre.Here,onlythegeometricerrorofworkpieceinthex-axisdirectionistakenintoaccountforabarworkpiece.Thisisexpressedbythefollowingformula.dG=d(s)-e(x)hT-Q-dx(x)(8)whered(s)isthespindleoffsetalongthex-axisdirectione(x)istheangularerror(yaw)ofthecross-slideinthex,y-planedx(x)isthelineardisplacementerrorofthecross-slidealongthex-axisdirectionThespindleoffsetisaconstantvalueindependentofthethemachiningposition.Theangularerrortermandthelinearerrortermarefunctionsofthecross-slidepositionx.Inthispaper,theFTSFQismountedonaHitachiSeiiki,HITEC-TURN20SIItwo-axisturningcentre.TheFTSFQcali-brationinstrumentwasdevelopedtomeasurerapidlythedimen-sionoftheworkpieceinthex-axisdirectiononthetwo-axisCNCturningcentrewhenthemachinehascompletelycooleddown,i.e.withouttheeffectofthermalerror.ThegeometricerrorcanbecomputedbyusingEq.(5)accordingtothemeasuredresults.First,thediameterofaprecisiongroundtestbarismeasuredat10positions,20mmapart,byaCMM,theirvaluesDppi(i=1,2,.,10)arerecorded.Then,thetestbarismountedonthespindle,anditsdiameterisalsomeasuredat10posit