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IntJAdvManufTechnol200117649–6532001SpringerVerlagLondonLimitedRealTimePredictionofWorkpieceErrorsforaCNCTurningCentre,Part1.MeasurementandIdentificationX.LiDepartmentofManufacturingEngineering,CityUniversityofHongKong,HongKongThispaperanalysestheerrorsourcesoftheworkpieceinbarturning,whichmainlyderivefromthegeometricerrorofmachinetools,i.e.thethermallyinducederror,theerrorarisingfrommachine–workpiece–toolsystemdeflectioninducedbythecuttingforces.AsimpleandlowcostcompactmeasuringsystemcombiningafinetouchsensorandQsetterofmachinetoolsFTSFQisdeveloped,andappliedtomeasuretheworkpiecedimensions.Anidentificationmethodforworkpieceerrorsisalsopresented.Theworkpieceerrorswhicharecomposedofthegeometricerror,thermalerror,andcuttingforceerrorcanbeidentifiedaccordingtothemeasurementresultsofeachstep.ThemodelofthegeometricerrorofatwoaxisCNCturningcentreisestablishedrapidlybasedonthemeasurementresultsbyusinganFTSFQsetterandcoordinatemeasuringmachineCMM.ExperimentalresultsshowthatthegeometricerrorcanbecompensatedbymodifiedNCcommandsinbarturning.KeywordsDimensionmeasureErroridentificationGeometricerrorTurning1.IntroductionInrecentyears,ultraprecisionmachininghasmaderemarkableprogress.Somespeciallatheshavebeenabletomakeultraprecisionmachining,tolessthanasubmicronandnanomicrontolerancesapossibility.Acommonsecondapproachisthatthegrindingisusedtoachieveahighlevelofdimensionalaccuracyafterturning.However,theconditionofthecuttingtooldiamondandworkpiecealuminiumhaverestrictedtheapplicationofultraprecisionlathes.Thesecondapproachincreasesthenumberofmachinetoolsandmachiningprocessesused1,whichresultsinanincreaseinthemanufacturingcost.Atpresent,mostCNClathesareequippedwithapositioningresolutionof1mm.Variousmachiningerrorsinfinishturning,however,degradetheaccuracytoalevelofapproximatelyCorrespondenceandoffprintrequeststoDrXiaoliLi,DepartmentofManufacturingEngineering,CityUniversityofHongKong,HongKong.EmaillixiaoliFhityahoo.com10mm,sothatwhenturningcarbonsteel,amachiningerrorpredictablyarisesinexcessof20–30mm.Forimprovingmachiningaccuracy,themethodofcarefuldesignandmanufacturehasbeenextensivelyusedinsomeCNClathes.However,themanufacturingcostbasedontheabovemethodwillrapidlyincreasewhentheaccuracyrequirementsofthemachinetoolsystemareincreasedbeyondacertainlevel.Forfurtherimprovingmachineaccuracycosteffectively,realtimeerrorpredictionandcompensationbasedonsensing,modellingandcontroltechniqueshavebeenwidelystudied2,soultraprecisionandfinishtuningcanbeperformedononeCNClathe.Thepositioningresolutionofthecuttingtoolsandworkpieceisreducedsothatitcannotmaintainhighaccuracyduringmachiningbecauseofthecuttingforceinduceddeflectionofthemachine–workpiece–toolsystem,andthethermallyinducederror,etc.Ingeneral,apositioningdeviceusingapiezoeletricactuatorisusedtoimprovetheworkingaccuracy,butthemethodintroducessomeproblems,suchas,thefeedbackstrategy,andtheaccuracyofsensors,whichaddtothemanufacturingcostoftheproducts.However,iftheworkpieceerrorcanbemeasuredbyusingameasuringinstrument,orpredictedbyusingamodelling,theturningprogramproducedbymodifiedNCcommandscanbeexecutedsatisfactorilyonaCNCmachinetool.Thus,aCNCturningcentrecancompensateforthenormalmachiningerror,i.e.themachinetoolcanmachineaproductwithahighlevelofaccuracyusingmodifiedNCcommands,inrealtime.Theworkpieceerrorderivesfromtheerrorintherelativemovementbetweenthecuttingtoolandtheidealworkpiece.Foratwoaxisturningcentre,thisrelativeerrorvariesastheconditionofthecuttingprogresses,e.g.thethermaldeflectionofthemachinetoolistimevariant,whichresultsindifferentthermalerrors.Accordingtothevariouscharactersoftheerrorsourcesoftheworkpiece,theworkpieceerrorscanbeclassifiedasgeometricerror,thermallyinducederror,andcuttingforceinducederror.Themainaffectingfactorsincludethepositionerrorsofthecomponentsofthemachinetoolandtheangularerrorsofthemachinestructure,i.e.thegeometricerror.Thethermallyinducederrorsofthemachinetooli.e.thethermalerror,andthedeflectionofthemachiningsystemincludingthemachinetools,workpiece,andcuttingtoolsarisingfromcuttingforces,arecalledthecuttingforceerror.650X.LiThispaperanalysestheworkpieceerrorsourcesinturning.Theerrorsofamachinedworkpiecearemainlycomposedofthegeometricerrorofthemachinetools,thethermallyinducederror,andtheerrorarisingfrommachine–workpiece–toolsystemdeflectioninducedbythecuttingforces.Asimpleandlowcostmeasuringinstrumentfortheworkpiecedimensions,whichcombinesafinetouchsensorandmachinetoolQsetterFTSFQ,isdescribed,andappliedtomeasuretheworkpieceerror.Anewmethodforidentifyingthegeometricerror,thethermalerror,andthecuttingforceerrorisalsopresentedforatwoaxisturningcentre.Finally,themodellingofthegeometricerrorofaCNCturningcentreispresented,basedonthemeasurementresultsusingtheFTSFQandCMM.ThegeometricerrorcanbecompensatedbythemodifiedNCcommandmethod.2.ErrorSourcesinTurningThemachinetoolsystemiscomposedofthedriveservo,themachinetoolstructure,theworkpieceandthecuttingprocess.Themajorerrorsourcesderivefromthemachinetoolthermalerrors,geometricerrors,andforcedvibrations,thecontroldriveservodynamicsandprogrammingerrorsandthecuttingprocessmachinetoolandcuttingtooldeflection,workpiecedeflection,toolwear,andchatter3.Errorsderivedfromthemachinetoolincludethermalerrorsmachinethermalerrorandworkpiecethermalerrors,geometricerrors,andforcedvibrations,whichdominatemachiningaccuracy.Thethermalerrorsandgeometricerrorsarethedominantfactorswithrespecttomachiningaccuracyinfinecutting.However,machinetoolerrorscanbedecoupledfromtheothererrorsourcesandcompensated4.Theerrorderivedfromforcedvibrationcanbereducedthroughbalanceddynamiccomponentsandvibrationisolation3.Theerrorsderivedfromthecontroller/drivedynamicsarerelatedtothecuttingforcedisturbancesandtheinertiaofthedriveandthemachinetable.Theseerrorscanbereducedbyaninterpolatorwithadecelerationfunction5orbyanadvancedfeeddrivecontroller6,theseerrors,reducedbyusingtheabovemethods,aresmallwhencomparedwithothererrorsources.Owingtothedemandforhighproductivity,highfeedratesandlargedepthsofcutarerequired,whichresultinlargecuttingforces.Therefore,thecuttingforceinduceddeflectionsofthemachinetoolspindle,toolholder,workpiece,andcuttingtoolmakesignificantcontributionstomachiningaccuracyduringthecuttingprocess.Inaddition,toolwearandmachinetoolchatterarealsoimportanterrorsourcesinthecuttingprocess.However,theseeffectsareneglectedheresoastofocusonthemainerrorsources.Inshort,theerrorofamachinedworkpiece,i.e.thetotalmachiningerrordTot,iscomposedmainlyofthegeometricerrorsofthemachinetoolsdG,thethermallyinducederrordT,andtheerrordFarisingfromthedeflectionofthemachine–workpiece–toolsysteminducedbythecuttingforces.Hence,dTotdGdTdF1Inthenextsection,wepresentanovelcompactmeasuringinstrumentandanewanalyticalapproachformeasuringandidentifyingworkpieceerrorsinturning.3.ACompactMeasurementSystemContactsensors,suchastouchtriggerprobes,havebeenusedtomeasureworkpiecedimensionsinmachining.Inmachiningpractice,themeasuringinstrumentisattachedtooneofthemachinesaxestomeasureasurfaceontheworkpiece.ATP7MorMP3associatedwiththePH10MrangeofmotorisedprobeheadsoraPH6MfixedheadhavebeenusedwidelyintheautomatedCNCinspectionenvironmentowingtotheirhighlevelofreliabilityandaccuracyandintegralautojoint.Thoughtheprobeheadsareofadequateaccuracyunidirectionalrepeatabilityatstylustiphighsensitivity0.25mmpretravelvariation360highsensitivity60.25mm,andversatileinapplication,theyhavecleardrawbacks,includingcomplexityofconstruction,highprice4988,andtheneedforcarefulmaintenance.Toovercomethesedrawbacksoftouchtriggerprobes,Ostafievetal.7presentedanoveltechniqueofcontactprobingfordesigningafinetouchsensor.Thecuttingtoolitselfisusedasacontactprobe.Thesensoriscapableofyieldingmeasurementaccuracycomparabletothatofthebesttouchtriggerprobeinuse.Moreover,theprincipleofoperationandconstructionofthesensorisextremelysimple,thecostofthesensorislow,andthemaintenanceisveryeasy.Inthispaper,thissensorwillbeusedtomeasurethediameterofaworkpieceassociatedwiththeQsetter.AtouchsensorismountedonaCNCturningcentre.Whenwemanuallybringthetoolnoseintocontactwithit,aninterruptsignalisgeneratedfortheNCunittostopanaxis.Moreover,itcanwriteinanoffsetandaworkpiececoordinateshiftautomatically.Thisfunctionfacilitatessetupwhenreplacingatool,andthisconvenientfunctioniscalledtheQuickToolSetterorQsetter.Basedontheaboveprinciple,wecanoperateaswitch,whichiscontrolledbyfinetouchsensor,betweentheQsetterandNCunit.Whenthetooltiptouchestheworkpiecesurface,thefinetouchsensorcansendacontrolsignaltotheswitch,toturnittotheoffstate.SeeFig.1,thefinetouchsensorreplacestheQsetterfunction,tostopanaxisandwriteinanoffsetandaworkpiececoordinateshiftautomatically.Therefore,thefinetouchsensorassociatedwithFig.1.FlowdiagramofafinetouchsensorfixedonaCNCcontroller.RealTimePredictionofWorkpieceErrors.Part1651aQsetterFTSFQcanbeusedtoinspectthediameteroftheworkpiece,themethodisshowninFig.2.Whenthecuttingtooltiptouchestheworkpiecesurface,abeepsoundisheardandtheswitchingOFFsignalappearsandtheaxisstopsautomatically,asfartheQsetter.AnewtooloffsetXTWisobtainedbytheNCunitdisplayofCNC.Beforetouchingtheworkpiecesurface,thecuttingtooltiptouchestheQsetter,andthetooloffsetXTQisobtained.Thus,theonmachineworkpiecediameterDonmachineisgivenbythefollowingEq.Donmachine2HuXTQuuXTWu2whereXTQisthetooloffsetwhenthecuttingtoolcontactstheQsetterXTWisthetooloffsetwhenthecuttingtoolcontactstheworkpiecesurfaceHisthedistancefromthecentreoftheQsettertothecentreofthespindleinthexaxisdirectionandisprovidedbythemachinetoolmanufacturer,fortheSeikiSeicosLIITurningcentre,itis85.356mm.OstafievandVenuvinod8testedthemeasurementaccuracyofthefinetouchsensor,performingonmachineinspectionofturnedparts,andfoundthatthemethodwascapableofachievingameasurementaccuracyoftheorderof0.01mmundershopfloorconditions.However,themeasurementaccuracyofthefinetouchsensortogetherwiththeQsetterobtainedanaccuracyofaboutmmbecausetheresultsofthemeasurementsystemaredisplayedbytheCNCsystem,andthereadingsaccuracyoftheCNCsystemisupto1mm.4.IdentificationofWorkpieceErrorsFromtheaboveanalysisoferrorsourcesoftheworkpiece,thetotalerrordTotofmachinedpartsismainlycomposedofthefollowingerrorsinaturningoperationIdGthegeometricerrorsofmachinetools.IdTthethermallyinducederror.Fig.2.InspectionforthediameterofaworkpiecebyusingthefinetouchsensorwiththeQsetterofamachinetool.IdFthecuttingforceinducederror.Toanalysetheerrorsourcesofamachinedworkpiece,LiuVenuvinod9usedFig.3toillustratetherelationshipamongstdimensionsassociatedwithdifferenterrorcomponentsinturning.InFig.3,DdesisthedesireddimensionoftheworkpieceDomwisthedimensionobtainedbyonmachinemeasurementusingFTSFQimmediatelyafterthemachiningoperationDomcisthedimensionobtainedbyonmachinemeasurementusingFTSFQafterthemachinehascooleddownandDppisthedimensionobtainedbypostprocessprocessmeasurementusingaCMMaftertheworkpiecehasbeenremovedfromthemachine.Whentheworkpiecehasbeenmachined,andremovedfromthemachinetoolsystem,itisthensentforinspectionofthedimensionsusingaCMM.Thisprocedureiscalledpostprocessinspection,bywhichweobtainitDppvalue.AsthepositioningerroroftheCMMisverymuchsmallerthanthedesiredmeasurementaccuracy,thetotalerrorisdTotDppDdes/23ThedimensionDomwisobtainedthroughonmachinemeasurementusingFTSFQimmediatelyaftermachining,i.e.themachineisstillinthesamethermalstateasatthetimeofmachining.Themeasurementismadewiththesamepositioningerrorasthatwhichexistedduringmachining.Hence,thepositioningerrorinthisstatewouldbeequaltodG1dT,i.e.DppDomw/2dGdT4Whenthemachinehascompletelycooleddown,i.e.withoutthermalerror,thedimensionDomccanbeobtainedbyonmachinemeasurementusingFTSFQ.Themeasurementhasapositioningerrorequaltothegeometricerrorofthemachineatthelocationofmeasurement.Hence,thepositioningerrorinthisstatewouldbeequaltodG,i.e.DppDomc/2dG5CombiningEqs4and5,thethermallyinducederrordTisDomcDomw/2dT6Hence,takingEqs1,3,and4intoaccount,thecuttingforceinducederrorowningtothedeflectionofthemachine–workpiece–toolsystemdFisDomwDdes/2dF7Fig.3.Therelationshipsamongdimensions.652X.LiSofar,themachiningerroriscomposedofthegeometricerror,thethermalerror,andthecuttingforceinducederrorandcanbeidentifiedusingtheaboveprocedure.ThethermalerrorandtheforceinducederrormodellingsisaddressedinLi10.Here,thegeometricerrorofmachinetoolismeasuredandmodelled.5.ModellingofGeometricErrorThegeometricerrorofaworkpieceismainlyaffectedbytheoffsetofthespindle,andthelinearerrorandtheangularerrorsofthecrossslideforatwoaxisCNCturningcentre.Here,onlythegeometricerrorofworkpieceinthexaxisdirectionistakenintoaccountforabarworkpiece.Thisisexpressedbythefollowingformula.dGdsexhTQdxx8wheredsisthespindleoffsetalongthexaxisdirectionexistheangularerroryawofthecrossslideinthex,yplanedxxisthelineardisplacementerrorofthecrossslidealongthexaxisdirectionThespindleoffsetisaconstantvalueindependentofthethemachiningposition.Theangularerrortermandthelinearerrortermarefunctionsofthecrossslidepositionx.Inthispaper,theFTSFQismountedonaHitachiSeiiki,HITECTURN20SIItwoaxisturningcentre.TheFTSFQcalibrationinstrumentwasdevelopedtomeasurerapidlythedimensionoftheworkpieceinthexaxisdirectiononthetwoaxisCNCturningcentrewhenthemachinehascompletelycooleddown,i.e.withouttheeffectofthermalerror.ThegeometricerrorcanbecomputedbyusingEq.5accordingtothemeasuredresults.First,thediameterofaprecisiongroundtestbarismeasuredat10positions,20mmapart,byaCMM,theirvaluesDppii1,2,...,10arerecorded.Then,thetestbarismountedonthespindle,anditsdiameterisalsomeasuredat10positions,20mmapart,bytheFTSFQ.ThemeasurementarrangementisshowninFig.4,thereadingsareDomcli1,2,...,10.Thus,thegeometricerrorateachpointalongthexaxisforthebarworkpiecearecomputedasfollowsdGiDppiDGi/29FromstartingpointBtopointA,theresultsareshowninFig.5fordiametersof30,45,60,and75mm.TheworkpieceFig.4.DiagramofthegeometricerrormeasurementoftheworkpieceusingFTSFQ.Fig.5.Geometricerrorsoftheworkpiecealongthezaxis.geometricerrorsinthezaxisdirectionarethesame.Theworkpiecegeometricerrors,however,increasealongthexaxisdirection,asshowninFig.6.Theseaveragegeometricerrorsare27.1036,29.0636,210.7764,212.5955mmfordiameters30,45,60,and75mm,respectively.Hence,thegeometricerrorsofthetwoaxisCNCturningcentrecanbecalculatedbythefollowingEq.dGx0.121x3.51910wherexisthediameteroftheworkpiecemm,dGxmmisthegeometricerroroftheworkpiece.6.CompensationofGeometricErrorTocompensateforthegeometricerrorinthedirectionofthedepthofcut,thetoolpathcanbeshiftedinaccordancewiththeerror.TheNCcommandsinturningaremodified,ataminimumresolution1mm,inthedirectionofthedepthofcut.Thecalculatedgeometricerrorexceeded1mmaccordingtotheequation10,asillustratedinFig7.Figure8showsthattheworkpieceerrorsincludethegeometricerror,thethermalerrorandthecuttingforceerror.Thetoolpathdeterminedbythecalculatedgeometricerror,andtheworkpieceerrorarecompensatedforbythemodifiedNCcommandmethod.Inthisexample,weusedacuttingspeedof4ms21,afeedrateof0.2mmrev21,adepthofcutofFig.6.Theaveragegeometricerrorforthedifferentdiameters.
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