机械毕业设计英文外文翻译用改变加工工具方向的方法来减少五轴联动数控加工中的非线性误差.docx
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机械毕业设计英文外文翻译用改变加工工具方向的方法来减少五轴联动数控加工中的非线性误差.docx
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机械毕业设计英文外文翻译用改变加工工具方向的方法来减少五轴联动数控加工中的非线性误差
英文翻译
ACutterOrientationModificationMethodfortheReductionofNon-linearityErrorsinFive-AxisCNCMachining
ABSTRACT
Inthemachiningofsculpturedsurfaces,five-axisCNCmachinetoolsprovidemoreflexibilitytorealizethecutterpositionasitsaxisorientationspatiallychanges.Conventionalfive-axismachiningusesstraightlinesegmentstoconnectconsecutivemachiningdatapoints,anduseslinearinterpolationtogeneratecommandsignalsforpositionsbetweenendpoints,Duetofive-axissimultaneousandcoupledrotaryandlinearmovements,theactualmachiningmotiontrajectoryisanon-linearpath.Thenon-linearcurvesegmentsdeviatefromthelinearlyinterpolatedstraightlinesegments,resultinginanon-linearitymachiningerrorineachmachiningstep.Thesenon-linearityerrors,inadditiontolinearityerror,commonlycreateobstaclestotheassuranceofhighmachiningprecision.Inthispaper,anovelmethodologyforsolvingthenon-linearityerrorsprobleminfive-axisCNCmachiningispresented.Theproposemethodisbasedonthemachinetype-specifickinematicsandthemachiningmotiontrajectory.Non-linearityerrorsarereducedbymodifyingthecutterorientationswithoutinsertingadditionalmachiningdatapoints.Anoff-lineprocessingofasetoftoolpathdataformachiningasculpturedsurfaceillustratesthattheproposedmethodincreasesmachiningprecision.
Keyword
Non-linearerror;Machinekinematics;Machiningmotiontrajectory.
INTRODUCTION
Inconventionalfive-axismachining,atoolpath,representedbythecutterlocationsdata(CLDATA),consistsofthespatiallyvaryingcutterpositionsanditsaxisorientations.TheseCLDATAaregeneratedbasedsolelyonthegeometricalpropertiesofthemachinedsurfacesandthecutter.TheseCLDATAarefurtherprocessedintoNC-codeswhichisspecifictoaparticularmachineconfiguration.Linearinterpolationisthenusedtogeneratetherequiredcommandsforpositionsalonglinesegmentconnectingthemachiningdatapoints.Thesimultaneouslinearandrotarymovementsareinvolvedinfive-axismachiningsinceevernewcutteraxisorientationrequiresthemotionatleastoneotheraxis.Therearealsocouplingeffectsofthecutteraxiswillaffectthepositionofthecutter.Thesesimultaneousandcoupledmovementscausethecuttercontractpoint(CCpoint)tomoveinanon-linearmanner.Asaresult,themachiningerrorineachmotionstepismadeupofnotonlythelinearsegmentationapproximationerrorbutalsoanadditionalmachiningerror.Asshowninfigure1formachiningiseitheraconcavesurfaceoraconvexsurface,alinesegmentisusedtoconnecttwoconsecutivemachiningdatapoints(thespindlechunkisthemachinecontrolpointMCP).Linearinterpolationgenerateintermediatepositionsalongthelinesegment.Thedesiresurfaceisdesigncurve(eitherconcaveorconvex).Thelinearsegmentapproximatestodesigncurveresultinginthelinearityerror,δt.Apartfromthelinearityerror.Thenon-linearCCpointtrajectorydeviatesfromthestraightlinesegment(thecuttergagelengthisconstantandMCPisinterpolatedalongthelinesegment)resultinanadditionalmachiningerror,referredtoasthenon-linearityerror,δn.Inthecasethatthedesiresurfaceisconcave(seefigure1a),thetotalmachiningerrorisdifferenceofthenon-linearityerrorandthelinearityerror:
δtotal=δt-δn.Thenon-linearityerror,inthiscase,compensateforthetotalmachiningerror(AIGPPost-processor,1996;Liu,1994).Onthecontrary,forthemachiningofconvexsurfaceasshowninfigure1b,thenon-linearityerroraddsontothelinearityerrorandenlargesthemachiningerror:
δtotal=δt+δn(AIGPPost-processor,1996;Liu,1994).
figure1.Themulti-axisCNCmachiningerror
Consequentlythenon-linearityerrorhavecauseddifficultiesforensuringultra-precisionmachiningrequirements.Inthemachiningofairfoilsurface,forexample,themachiningofthecontoursurfaceofairfoiltotheedgesisproblematic.Thesurfacecurvatureontheseareachangesabruptly,andthusthecutterorientationvariesinconsistentlyfromonecuttertothenext.Theseabruptcutterorientationvariationsinconsistentlyfromonecutterlocationtothenext.Theseabruptcutterorientationareatypicalnon-linearityerrorproblem.
Inordertosolvethefive-axisCNCmachiningerrorproblem,effortshavebeenmadetotreatnon-linearityerrorsingenerateNCcodes.Someresearchersandpostprocessorproducersused“linearizationprocesses”forthispurpose.Thebasicfunctionof“linearizationprocesses”areinsertingmachiningdatapointsbetweenNCcodeswherethetotalmachiningerrorisoutofthespecifiedtolerancerange.Takeuchietal.(1990)insertedpointsbysubdividingthelinesegmentwithequallyspacedinterval.Choetal.(1993)inserteddatapointsbylimitingthemaximummachiningerrorwithinthelineintervalfromthestartpointtotheinsertedpointtobethetolerance.And,bothofthemsetthecutterorientationsvaryinglinearlyinsuccessivepositions.IntheAutomationIntelligenceGeneralizationPostprocessor(AIGP)(1996),a“linearizationprocesses”calculatesthemiddlepoint(MP)betweenadjacentNC-codesandinsertstheMPasanadditionaldataintheNCcode.TheinsertioncanbeperformedfurtherbetweentheconsecutiveNC-codeduntileitherallpointsarewithinthemachiningtoleranceoruntilamaximumof63pointsareinsertedbetweentheconsecutivedatapoint.Thecurrentpost-processors,suchastheVanguardCustomPost-processorGenerator(1996),theOminimillCustomPostprocessor(1992),theAIXNumericalControlPostGenerator(1996),areallhavingthesimilar“linearizationprocesses”asintheAIGP.InthecurrentCAD/CAMsoftware.Unigraphics(2001),theUG/postpostprocessorsinsertsdatapointsbetweenadjacentNC-codes,therebysimulatingastraightlinewithseriesofsmallcurves.Thenumberoftheinsertedpointsisdeterminedbasedonthemaximumallowabledeviationandaniterationmethodisusedtosegmentthemove.Intheextremecase,namelyafterlooping20times,ifthedeviationbetweenthesegmentedarcsandthelinearestilloutofthespecifiedtolerancelimit,theprocessisaborted.
“linearizationprocesses”discussedabovemanipulateNC-codesbyinsertingextramachiningdatapoints.AlthoughtheproducedNC-codessatisfythemachiningrequirement,theymaycontaindensesetsofnon-equallyspaceddatawithconstantorlinearlyvaryingcutterorientation.Consequently,thelinearizationprocesshasraisedthefollowingproblems.
Inthemachiningofcomplexcontoursurface,thecutterorientationvariesfromonecutterlocationtothenext.Thecutterpositionchangesinthiscasecannotbetoosmallsincethemachinewillproduceeitherjerkmotionorrandomrotarymovements.Asinanindustrialprocedureofmachiningairfoilsurfaceofanimpeller,alinearizationprocesswasusedtoreducethenon-linearityerrors.ManydatapointswereinsertedbetweenapairofNC-codes.Theinsertionofmanydatapointscausedthecutterpositionchangetobenearlyequaltozerowhilethecutterorientationchangedabruptly.Asaconsequence,themachinerotarymovementswererapidwithinfinitefeedrate.Randomrotarymovementsresultedandtheworkpiecewasdamaged.
Theinsertionofmachiningdatapointscanalsocausenon-constantfederatealongthecuttingcurve.Theinsertionofadditionaldataresultsinnon-equallyspacedsegment,whileaccelerationanddecelerationstepsarerequiredforeachsegment.Thus,thefeederatevariesineachsegmentandmayneverreachthedesiredvalue.Theresultofvaryingfeederatecausesanonsmoothsurfacefinishandtheunreachablefeedrateincreasesoverallmachiningtime.Inaddition,theinsertionofconstantcutterorientationvariationalsocausessevereroughnessaroundtheendpointsalongthesurface.Linearlyinaccuratelysincethechangeincutterorientationisnotnecessarilylinear.
Thenon-linearityerrorproblemarisesfromthefactthatfive-axismachiningmotiontrajectoriesarenon-linearcurvesegments.Thesimultaneousandcoupledrotaryandtranslationmovementsgeneratethenon-linearitymotiontrajectory,andthelinearinterpolationtechniqueisnotcapabletocurvefitthenonlinearpath.Onesolutiontoistodesignnewinterpolationmethods.Liangetal.(2002)presentedacombine3Dlinearandcircular(3DL&C)interpolationtechnique.Thenew3DL&Cinterpolationcanon-linedrivetherotationmovementpivotalongapre-designed3Dcurvepath,sothattheCCpointmotiontrajectoryisaviaastraightlineconnectingmachiningdatapoints,thus,thenon-linearityerrorcanbeeliminated.Five-axismachiningmovementsarekinematicallyrelatedtothecutterlocationdata.Inotherword,thenon-linearmotiontrajectorydependsonthecutterorientationchangesandnon-linearityerrorsarerelatedtothetoolpathgeneration.Thus,anothersolutiontothenon-linearityerrorproblemcanbeapproachedfromtoolpath(CLDATA)generationwiththerequirementsthatthemachiningerrorsareminimizedandthereisnointerferencebetweentheworkpieceandthecutter.
Intoolpathgeneration,varioustechniquesfordifferentsurfacerepresentationshavebeenusedbytheCAD/CAMpackageproducers(CLDATA,1996;Unigraphics,1990)andresearchers.HuangandOliver(1992).Bedietal.(1997)presentedaprinciplecurvaturealignmenttechniqueforfive-axismachiningusingatoroidalshapedtool.Liu(1995)presentedthesinglepointoffsetandthedoublepointoffsetalgorithmsforfive-axisflankmillingtoolpathgenerationbasedondifferentialgeometryandanalyticalgeometry.Morishgeetal.(1999)presentedatoolpathgenerationmethodforfive-axisCNCmachi
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- 机械 毕业设计 英文 外文 翻译 改变 加工 工具 方向 方法 减少 联动 数控 中的 非线性 误差