预加工数控轮廓确认英文.docx

预加工数控轮廓确认英文.docx

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预加工数控轮廓确认英文

PRE-MACHININGCNCCONTOURVALIDATION

TonySchmitz,JohnZiegert

UniversityofFlorida,MachineToolResearchCenter

Gainesville,FL32611

Abstract

ThecurrentprocedurefollowedtomanufactureanewpartbyCNCmachiningistowritethepartprogram,

machineatestpartandmeasurethetestpartforconformancetotherequireddimensionsandtolerances.Ifthetest

partdimensionsarenotcorrect,thepartprogramismodifiedandtheprocessrepeateduntilasuccessfulpartis

machined.Inmanyapplications,suchastheaerospaceindustry,wherematerialcostandmachiningtimearehigh,

thisiterativeprocessbecomeseconomicallyunacceptable.

ResearchhasbeenconductedtotestthefeasibilityofusingtheLaserBallBar（LBB）,aspatialcoordinate

measuringdevice,tomeasuredynamiccontinuous-pathcontoursofCNCpartprogramstomicrometeraccuracy

priortomachining.Inthisway,avirtualtestpartcanbemeasuredandcomparedtothedesigndrawingsto

validatetheCNCpartprogram.Thisreducesoreliminatesthecostlyandtime-consumingstepsinvolvedinthe

machiningofphysicaltestparts.

ThispaperoutlinesthetestingmethodandresultsacquiredusingoneLBBtomeasuredynamicpartpaths

employingsequentialtrilateration.Acircularcontourwasmeasuredusinganencodertriggerfordatacapture.

Theradialerrormotionsofthespindleusedtogeneratethecircularcontourwerealsomeasuredusinga

capacitanceprobenesttoverifytheLBBresults.ComparableerrorwaveformsbetweentheLBBandcapprobe

measurementsverifiedthepossibilityofusingtheLBBtomeasuredynamiccontinuous-pathcontours.Futurework

usingthreeLBBssimultaneouslyisalsooutlined.

Keywords:

LaserBallBar;CNCmachining;dynamicmeasurements

Introduction

OneofthemostimportantusesofCNCmachinetoolsisthecuttingofcomplex

continuouspartpaths,orcontours.Incontinuous-pathnumericalcontrolsystems,thereis

contactbetweenthecuttingtoolandworkpiecethroughoutthepartpathwhileuptofiveaxesare

inmotion.Therefore,thefinalworkpiecedimensionsaredirectlyrelatedtothepositional

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relationshipbetweenthetoolandworkpiece.Theabilitytomonitorthisrelationshipand

predictthefinalpartdimensionsisimportantfortoday’smanufacturingengineer.

ForanewpartorproductionruntobemanufacturedwiththeuseofCNCmachinetools,

thecurrentprocedureistowritetheCNCpartprogramusingtheengineeringdrawings,execute

thepartprogramtomachineatestpartorprototype,andtheninspectthetestpart,normallywith

theaidofacoordinatemeasuringmachine,tocheckforconformancetodesigntolerances.This

feedbackoftheactualpartdimensions（withanadequatedegreeofprecision）iscurrentlythe

onlywaytocertifytheperformanceoftheCNCprogram.Ifthetestpartdoesnotmeetthe

specifiedtolerances,asisoftenthecaseforafirsttrial,theCNCprogramismodified,another

testpartismachinedandtheprocessrepeated.

Thisiterativeprocessmaybeacceptable,althoughnotefficient,insituationswherethe

materialisinexpensiveandmachiningtimeisshort.However,inmanycases,suchasthe

aerospaceindustry,thematerialiscostly（expensiveforgingstobemachined）andmachining

timeisdramaticallyincreased.Insuchinstances,thisiterativeprocessofproducing

dimensionallycorrectpartsbecomeseconomicallyunacceptable.

Bothtimeandmoneycouldbesavediftherewereawaytodirectlymeasurethemachine

tool’sdynamiccontouringaccuracyoveranarbitrary3-Dpathwithoutthenecessityof

machininganexpensivetestpart.Atthesametime,theefficiencyoftheCNCmachining

processcouldbegreatlyincreased.TheuseoftheLaserBallBar（LBB）,aspatialcoordinate

measuringdevice,totakedynamicmeasurementsofapathcouldreduceorreplacetheneedto

machineandinspectatestpart.Ineffect,thespatialcoordinatesofthedynamictoolpath

measuredbytheLBBcouldfunctionasavirtualtestpart1.

Thisparticularapproachmustbedifferentiatedfromtheconventionalapproachto

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machinetoolmetrology.Atthistime,thenormalprocedureistomeasurethemachinetoolerror

motionsincludinggeometric,thermalandperhapsprocesserrors.Theseerrorsmaythenbeused

aspre-machiningcompensationinthemachinetoolcontrollerinanefforttogivetheoretically

perfectmotions.Themachine’saccuracyand,therefore,thepartdimensionsarebasedlargely

onthesuccessofthistime-consumingprocess.Althoughthisresearchiscertainlynota

replacementforthisbodyofwork,itisalsonotthesame.Thepurposeofthisworkistotryand

predictthefinalpartdimensionsfortheexecutionofaspecificCNCpartprogramonagiven

machinetoolpriortocuttingthepart,nottomeasurethegeometricorservoerrorsofthe

machinetool.

Background

OneofthemaincategoriesinthemodernevaluationofCNCmachiningcentersisthe

assessmentofthecontouringcapabilitiesofthemachinetool.Theevaluationofthiscontouring

accuracycanbedividedintotwomainclassifications:

post-processandin-processtesting.

Post-processtestingincludesthosetestswhichareperformedaftermachininghasbeen

completed.Themostpopularpost-processinspectiontoolisthecoordinatemeasuringmachine

（CMM）.Otherpost-processmethodsusedtoevaluateaCNCmachinetool’scontouring

accuracyinvolvetheuseofeithermasterpartsorwell-definedcontours.Oneexampleofthis

techniqueistheuseofstandardpartpaths,suchasthepartprogramcorrespondingtothe

NationalAerospaceStandardtestpart979（NAS979）,tomachineamasterpart.Thispartcan

thenbemeasuredtoevaluateflatness,squareness,parallelism,roundness,etc.Asimilar,but

somewhatmoreefficientmethodhasbeentermedmasterparttracing.Thisproceduresimulates

machiningbyreplacingthetoolwithagageandtheworkpiecewithamasterpartofknown

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accuracy.Themachineisthenprogrammedtofollowtheidealpathgivenbythemasterpart.

Deviationsfromtheidealpathregisteredbythetoolgagerepresentthecontouringerror2,3.

Theothermaintypeofcontourmeasurementisin-processtesting.Thiscategorycan

thenbesubdividedintoin-processandin-cyclegaging.In-processgagingreferstotesting

carriedoutduringtheactualmachiningprocess,whilein-cyclegagingdefinesmeasurements

takenafterthepartisfinished,butbeforeitisremovedfromthemachine.

In-processgagingisgenerallyachievedbyaddinganalogtransducerstothemachinetool

todirectlyassessthetestpart’ssizeduringthecuttingoperation.Anexampleofin-process

measuremententitledWorkpiece-ReferredFormAccuracyControl,orWORFAC,hasbeen

suggestedbyUdaetal.4Itconsistsofahighlysensitiveopticalsurfacesensorandamicrotool

servotoadjustforchangesinthedistancebetweenthetoolholderandworkpiece.Although

accuracyimprovementswereshownwiththeuseofthisfeedbacksystem,itisstilllimitedto

simplegeometries（cylindricalturningoperations）andrequirescomplextoolingandset-up.

In-cyclegagingcanbeaccomplishedbyreplacingoneormoreofthetoolsinaCNC

machiningcenter’sturretwithmeasuringprobes.Theseprobescanthenbeindexedintothetool

positiontoprobekeyfeaturesofthetestpartwhileitremainsinthemachineand,unfortunately,

isstillinathermallyunstablestate.

AnadaptiveerrorcorrectionmethodhasbeenproposedbyMouetal.whichcombines

bothin-cycleandpost-processtestingwithageometric-thermalerrormodel5,6.Thistechnique

proposestheuseofin-cycleandpost-processgagingtointeractivelydeterminethechangesin

thegeometric-thermalmodelovertime.

AlthoughtheseprocessesprovideworthwhilemeansofevaluatingaCNCmachinetool’s

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contouringaccuracy,theyallsufferfromthenecessityofmachiningandinspectingatestpart

forconformancetorequiredtolerances.Thepurposeofthisresearchwastoinvestigatethe

possibilityofusingtheLaserBallBarasameasurementtooltodeterminethisCNCpath

accuracywithouttherequirementofproducingacostlytestpart.Commercialproductswhich

aremovingtowardthisgoalincludetheHeidenhain2-DgridplateandRenishawBallBar.

Thesetools,however,arepathlimited.TheBallBarallowsonlycircularorhemisphericalpaths

andrecordsjustradialdeviation.Thegridplatecanmeasureonlyplanarpartpaths.For3or5-

axispartpaths,theLaserBallBarsystemisrequiredtodynamicallymeasurethesecontours.

LaserBallBar

TheLBBisaprecisionlineardisplacementmeasuringdevicedevelopedbyZiegertand

MizeattheUniversityofFlorida7.Itconsistsofatwo-stagetelescopingtubewithaprecision

spheremountedateachend.Aheterodynedisplacementmeasuringinterferometerisaligned

insidethetelescopingtubeandmeasurestherelativedisplacementbetweenthetwospheres.See

Figure1.TheLBBhasbeenshowntobeaccuratetosub-micrometerlevelsduringstatic

measurementsofspatialcoordinates8.

Onceinitialized,theLBBusestrilaterationtomeasurethespatialcoordinatesofpoints

alongaCNCpartpath.Thesixsidesofthetetrahedronformedbythreebasesockets（attached

tothemachinetable）andatoolsocket（mountedinthespindle）aremeasuredand,bygeometry,

thecoordinatesofthetoolpositioncanbecalculated.

Insequentialtrilateration,thesamepartpathistraversedthreetimes,measuringthe

lengthsofoneofthebase-to-toolsocketlegsatafinitenumberofpointsduringeachrepetition.

SeeFigure2.Notethatthetoolsocketmustbeinexactlythesameposition（foragivenpoint）

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foreachofthethreemeasurements.Ifthetoolsocketwere