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IntJAdvManufTechnol199915171–1811999SpringerVerlagLondonLimitedDevelopmentofAutomatedFixturePlanningSystemsW.Ma,J.LiandY.RongDepartmentofMechanicalEngineering,WorcesterPolytechnicInstitute,Worcester,MA,USAFixturingisanimportantmanufacturingactivity.Thecomputeraidedfixturedesigntechniqueisbeingrapidlydevelopedtoreducetheleadtimeinvolvedinmanufacturingplanning.Anautomatedfixtureconfigurationdesignsystemhasbeendevelopedtoselectautomaticallymodularfixturecomponentsandplacetheminpositionwithsatisfactoryassemblyrelationships.Inthispaper,anautomatedfixturingplanningsystemispresentedinwhichfixturingsurfacesandpointsareautomaticallydeterminedbasedonworkpiecegeometryandoperationalinformation.Fixturingsurfaceaccessibility,featureaccuracy,andfixturingstabilityarethemainconcernsinthefixtureplanning.Thesystemdevelopment,thefixtureplanningdecisionprocedure,andanimplementationexamplearepresentedinthepaper.KeywordsAccuracyClampingFixtureplanningLocating1.IntroductionFixturingisanimportantmanufacturingactivityintheproductioncycle.AcomputeraidedorautomatedfixturedesignCAFDtechniquehasbeendevelopedaspartofCAD/CAMintegration1.ThedevelopmentofCAFDcontributestothereductionofmanufacturingleadtime,theoptimisationofmanufacturingoperations,andtheverificationofmanufacturingprocessdesigns2.CAFDplaysanimportantroleinflexiblemanufacturingsystemsFMSandcomputerintegratedmanufacturingsystemsCIMS3.Figure1outlinestheactivitiesforfixturedesigninmanufacturingsystemswhichincludethreemajoraspectssetupplanning,fixtureplanning,andfixtureconfigurationdesign4.Theobjectiveofsetupplanningistodeterminethenumberofsetups,thepositionandorientationoftheworkpieceineachsetup,andalsothemachiningsurfacesineachsetup.Fixtureplanningdeterminesthelocatingandclampingpointsonworkpiecesurfaces.ThetaskoffixtureconfigurationdesignistoselectfixturecomponentsandplacethemintoafinalconfigurCorrespondenceandoffprintrequeststoDrKevinRong,DepartmentofMechanicalEngineering,WorcesterPolytechnicInstitute,Worcester,MA016092280,USA.EmailrongKwpi.eduFig.1.Fixturedesigninmanufacturingsystems.ationtofulfilthefunctionsoflocatingandclampingtheworkpiece.Anautomatedmodularfixtureconfigurationdesignsystemhasbeendevelopedinwhich,whenfixturingsurfacesandpointsareselectedontheworkpiecemodel,fixtureunitsareautomaticallygeneratedandplacedintopositionwiththeassistanceoffixturecomponentassemblyrelationships4,5.Thispaperdealswithfixtureplanningwhenthefixturingsurfacesandpositionsontheworkpieceareselectedautomatically.Previouspapersonfixturedesignanalysishavebeenpublished,butacomprehensivefixtureplanningsystemwhichcanbeusedtogeneratefixtureplansforindustrialapplicationshasnotbeendeveloped.Previousworkincludesamethodfortheautomateddeterminationoffixturelocationandclampingderivedfromamathematicalmodel6analgorithmfortheselectionoflocatingandclampingpositionswhichprovidethemaximummechanicalleverage7kinematicanalysisbasedfixtureplanning8,9afixturinggradeanddependencygradebasedfixturabilityanalysis10automatedselectionofsetups172W.MaandY.Rongwithconsiderationoftolerancefactorsoforientationerrorsinfixturedesign11,andfinallyageometricanalysisbased2Dfixtureplanningsystem12.Inourpreviousresearch,fixturingfeatures13,fixturingaccuracy14,15,geometricconstraints16,andfixturingsurfaceaccessibility17havebeenstudied.Aframeworkhasbeendevelopedforsetupplanningandfixturedesign18.Inthispaper,anautomatedfixtureplanningsystem,FixPlanning,ispresentedwherefixturingsurfacesandpointsaredeterminedwhentheworkpiecemodelandsetupplanninginformationisinputtothesystem.2.BasicRequirementsofFixturePlanningInengineeringpractice,fixtureplanningisgovernedbyanumberoffactors,includingworkpiecegeometricinformationandtolerancesetupplanninginformationsuchasmachiningfeatures,themachinetoolandcuttingtoolstobeusedineachsetupinitialandresultingformsoftheworkpieceineachsetupandavailablefixturecomponents.Toensurethatthefixturecanholdtheworkpieceinanacceptablepositionsothatthemanufacturingprocesscanbecarriedoutaccordingtothedesignspecifications,thefollowingconditionsshouldbesatisfiedforafeasiblefixtureplan.1.ThedegreesoffreedomDOFoftheworkpiecearetotallyconstrainedwhentheworkpieceislocated.2.Machiningaccuracyspecificationscanbeensuredinthecurrentsetup.3.Fixturedesignisstabletoresistanyeffectsofexternalforceandtorque.4.Fixturingsurfacesandpointscanbeaccessedeasilybyavailablefixturecomponents.5.Thereisnointerferencebetweentheworkpieceandthefixture,andbetweenthecuttertoolandthefixture.Inthisinvestigation,wefocusonthefirstfourrequirements.Fixtureplanningiscarriedoutbasedonthefollowingconsiderations1.Althoughtheworkpiecegeometrycanbecomplexinindustrialproduction,inmostfixturedesigns,planarandcylindricalsurfacesinternalandexternalareusedasthelocatingandclampingsurfacesbecauseoftheeaseofaccessandmeasurementofthesefeatureswhentheworkpieceisfixed.Inthisinvestigation,planarandcylindricalsurfacesareusedinfixtureplanning.2.ManyCNCmachines,especiallymachiningcentres,canbeusedtoperformvariousoperationswithinonesetup.Inmostcases,thecuttingtoolaxisofthemachinetoolisfixed.Whenconsideringfixturingstability,thelocatingsurfacesarepreferablythosewithnormaldirectionsoppositeto,orperpendicularto,thecuttingtoolaxis.Forclampingfeatures,thenormaldirectionsshouldbeinlinewith,orperpendicularto,thecuttingtoolaxis,because,infixturedesign,clampingforcesshouldbeagainstlocators.3.Forthesurfacestobemachined,thereshouldexistdatumsurfaceswhichserveaspositionandorientationreferencesfromwhichotherdimensionsandtolerancesaremeasured.Infixtureplanning,surfaceswithhighaccuracygradesshouldbeselectedpreferentiallyaslocatingsurfacessothattheinheritedmachiningerrorisminimisedandtherequiredtolerancesofthemachiningfeaturesareeasilyattained.4.Infixtureplanning,morethanoneworkpiecesurfacemustbeselectedforthelocatingandclampingsurfacesforrestrictingtheDOFoftheworkpieceinasetup.Therefore,besidestheconditionsforindividualsurfaces,thecombinationstatusoftheavailablelocatingsurfacesisalsoimportantfortheaccuratelocationoftheworkpiece.5.Sincethelocatorsandclampsareincontactwiththeworkpiece,thedistributionoffixturingpointsplaysacriticalroleinensuringfixturingstability.6.Forafeasiblefixturedesign,thefixturingsurfacesmustbeaccessibletothefixturecomponents.Theusableeffectiveareaofthefixturingsurfaceshouldbelargeenoughtoaccommodatethefunctionalsurfacesofthelocatorsandclamps.Besidesconsideringafixturingsurface,theaccessibilityofpotentialfixturingpointsonthesurfaceisalsoimportantforthedeterminationofthefinalfixturingpointdistribution.3.FixturingSurfacesTheconceptoffeatureshasbeenwidelyusedindesignandmanufacturing.Aworkpiecetobemachinedcanbeviewedasacombinationoffeaturessuchasplanes,steps,pockets,slots,andholes.Inaparticularoperationsetup,featuresusedforfixturingtheworkpiececanbedefinedasfixturingfeaturesorfixturingsurfaces.Inpractice,mostfixturingfeaturesareplanarandcylindricalsurfaces.Accordingtothefixturingfunctions,thefixturingsurfacescanbeclassifiedintolocating,clamping,andsupportingfeatures.Unlikedesignandmanufacturingfeatures,fixturingsurfacesareorientationdependent.Theydonotplaythesamerolethroughoutthemanufacturingprocesses.Asetofsurfacesmayserveasfixturingsurfacesinasetup,butmaynotbeusedforfixturingorhavedifferentfixturingfunctionsinanothersetup.Theconceptoffixturingfeaturesallowsthefixturingrequirementstobeassociatedwiththeworkpiecegeometry.Featureinformationinafeaturebasedworkpiecemodelcanalsobeuseddirectlyforfixturedesignpurposes.Formanufacturingfeatures,theinformationnecessaryfordescribingafixturingfeaturecontainsgeometricandnongeometricaspects.Theformerincludesfeaturetype,shapeanddimensionalparameters,andpositionandorientationoftheworkpiece.Thelatterincludesthesurfacefinish,accuracylevelandrelationshipswithmachiningfeatures,andsurfaceaccessibility.3.1DiscretisationofFixturingSurfacesInmostfixturedesigns,thefixturingfeatures,especiallythelocatingsurfaces,areplanarandcylindricalsurfaces.Inordertoevaluatefixturingsurfaceaccessibilityanddeterminelocating/clampingpointsonfixturingsurfaces,acandidatefixturingsurfaceissampledintogridarrayeddiscretepointswithDevelopmentofAutomatedFixturePlanningSystems173equalintervalT.IfTissmallenough,thediscretesamplepointswillbealmostcontinuous.Inordertomakethesamplingalgorithmgeneric,anouterboundingrectangleonthesurfaceisusedasthesamplingregion.Sinceinmostcases,theprimarylocatingsurfaceisperpendiculartotheotherlocatingsurfaces,especiallyinmodularfixturedesigns,thefixturingsurfacesareconsideredasbottomlocating,topclamping,sidelocating,andsideclampingsurfaces.Forabottomlocating/topclampingsurfacewithanormalZor2Zdirection,twoedgesoftheouterboundingrectanglemustbeparalleltotheXaxisandtwootheredgesparalleltotheYaxis.Forasidelocating/clampingsurface,theremustbetwoedgesparalleltotheZaxis,whiletheothertwoedgesmustbeperpendiculartothefirsttwoedges.Figure2showsanexampleofsampledcandidatefixturingsurfaceswiththeouterboundingrectangle.WiththeassumptionthattheZaxisisnormaltothesurfaceinthesurfacelocalcoordinatesystem,thepointswithintheouterboundingrectanglecanberepresentedasxXminTu,u1,2,,NuyYminTv,v1,2,,Nv1whereNuandNvarethenumbersofpointsintheXandYdirections,respectively,whichareNuintXmax2Xmin/TandNvintYmax2Ymin/T.3.2FixturingSurfaceAccessibilityFixturingsurfaceaccessibilityisameasureofwhetheracandidatefixturingsurfaceisaccessibletoaregularfixturecomponent.Threemajorfactorsmustbetakenintoaccount1.Thegeometryofthefixturingsurfacewhichinvolvestheeffectiveareaandshapeofthesurface.2.Possibleobstructionoftheworkpiecegeometryalongthenormaldirectionand/oraroundthegeometricregionofthefixturingsurface.3.Thesizeandshapeofthefunctionalfixturecomponents.Inpracticalsituations,itispossiblethataplanarsurfaceoftheworkpiecehasacomplexshapeandhasafull/partialobstructionalongitsnormaldirectionand/orarounditsgeoFig.2.Samplingofacandidatefixturingsurfacewithanouterboundingrectangle.metricregion.Itisthusrequiredthattheaccessibilitymodelshouldcomprehensivelyreflectthesefactssothatareasonablycomparableaccessibilityvaluecanbeappliedforeverycandidatefixturingsurface.ThesurfaceaccessibilityisdefinedasastatisticalvaluebasedonthepointaccessibilityPAofeveryvalidsamplepointonthesurface,wherePAconsistsoftwopartsthepointselfindividualaccessibilitySIAandthepointneighbourrelatedaccessibilityNRA.TheSIAcorrespondsmainlytotheisolatedaccessibilityofthefixturingpoint,whereastheNRAreflectstheextendedaccessibilityofthefixturingpoint.TheSIAofasamplepointisdefinedonthebasisofthreeattributetags.Thetags1isusedtoindicatewhetherthesquaretestgridwithitscentreatthecurrentsamplepointisinside,on,oroutsidetheouterloopofthefixturingsurface.Threediscretevaluesareassignedtorepresentitsstatus,i.e.0,1,and2,respectively.Ifthereexistsobstructiveworkpiecegeometryinthesurfacenormaldirectionorsurroundingthesamplepoint,thisaffectsthesurfaceaccessibilityatthesamplepoint.Forexample,asshowninFig.3a,onacandidatebottomlocatingsurfaceofaworkpiece,samplepointp1isnotaccessiblebecauseoftheobstructivegeometryoftheworkpiecealongthebottomlocatingdirection,andp2isnotaccessibleeitherbecauseoftheobstructionssurroundingit.Toevaluateautomaticallywhetheranobstructionexistsinthesurfacenormaldirection,avirtualvolumeisgeneratedbyextrudingthesquaretestgridtoasolidentityinthesurfacenormaldirection.Byemployingatechniquefordetectingtheinterferencebetweentwosolidentities,theobstructioncanbeidentified,asshowninFig.3b.TheextrudingmethodisalittledifferentforthesquareFig.3.Obstructioncheckingatvirtualsamplepointsonabottomlocatingsurface.Kpimeanstheextrusioniscarriedoutatpointpialongitsaccessibledirection.174W.MaandY.Rongtestgridonthesidelocating/clampingsurface,wherethesquaretestgridisfirststretchedalongthebottomlocatingdirection,andthenthestretchedgridisextrudedalongthesidelocating/clampingdirectionasillustratedinFig.4.Theattributetags2isusedforrecordingtheresultofobstructioncheckingatasamplepoint.Whensuchanobstructionisdetected,s21,otherwise,s20.Ifthetestgridatthesamplepointisfoundtobenotobstructed,itsindividualaccessibilityislargelydependentonthecontactareabetweenthetestsurfaceandthefixturecomponents,whichisrepresentedbytheattributetags3.Thedefinitionofs3iss3Area1T2,s3P0,12whereAreaIisthecontactareaandTistheedgelengthofthetestgrid.Onthebasisofabovethreeattributetags,theSIAofasamplepointpu,vcanbegivenbyanumericalvalueaccordingtothefollowingrulesifs1OutsideOuterLoop,SIA21inaccessibleifs1–OutsideOuterLoopANDs2Obstructed,SIA21inaccessibleifbottomlocating/topclampingANDS1–OutsideOuterLoopANDs2NotObstructed,SIAs3Fig.4.Obstructioncheckingatsamplepointsonasidelocating/clampingsurface.ifsidelocating/clampingANDs1–OutsideOuterLoopANDs2NotObstructed,SIA0.5vs3wherevreflectstheheighteffectofthepointinsidelocating/clamping.Theaccessibilityinthesurroundingareaofthesamplepointalsoaffectstheaccessibilityofthepoint.Onafixturingsurface,thepositionalrelationshipbetweenthecurrentsamplepointandalltheneighbouringsamplepointscanberepresentedbya33mapwherePcisthecurrentsamplepointwithadiscretepositionofu,v,P1|P8are8neighboursamplepoints,andtheirlocationsarealllabelledinFig.5.TheNRAatsamplepointpu,vcanbecalculatedusingtheequationNRAu,vO8k1Fk83whereFkistherelatedaccessfactorofkthneighbour,whichcanbedeterminedbasedontheSIAaswellasitsmeasures1,s2,s3.Forbottomlocating/topclamping,F9k521,s2pk10,s1pk2ands2pk0IApk,s1pk–2ands2pk04Fk5F9k,k1,3,5,7F9k,k2,4,6,8,F9k210andF9k1021,k2,4,6,8,F9k2121orF9k1215Forsidelocating/clamping,F9k521,s2pk1,k1,5,6,820.5,s2pk1,k2,3,40,s1pk2ands2pk0SIAPk,s1pk–2ands2pk06Fk5F9k,k1,3,5,6,7,8F9k,k2,4,F93020.5,k2,4,F9320.57Foravalidsamplepoint,oncetheSIAandNRAareobtained,thePAcanalsobecalculatedaccordingtotheequationFig.5.33positionmapofcurrentpointPcand8neighboursamplepointsP1|P8.
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