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专业
机械设计制造及其自动化
班级
B842115
学号
B84211520
姓名
王俊博
指导教师
刘冰
沈阳航空航天大学北方科技学院
2012年6月
原文:
COMPUTERAIDEDGEOMETRICMODELINGAND5-AXISMILLINGOFASCREWPROPELLERINASINGLESETUP:
ACASESTUDY
Abstract
Thispaperpresentsageometricmodelingmethodand5-axisCNCmachiningalgorithmforthemanufactureofscrewpropellersinonesinglesetup.Anovelapproachfor5-axisroughingisdevelopedandimplementedinadditiontofurtherstreamlinefinishingsequences.Thetoolpathisgeneratedbydividingthepropellermodelintomillingregionsviz.thefront&
rearbladefaces,theleading&
trailingbladeedgesandthelateralhubsurfacesbetweenadjacentblades.Cuttingtoolsforeachregionarethenselectedalongwiththeappropriatetoolorientationfor5-axisflankmilling.TheCL-dataisacquiredusingUnigraphicsTMandtranslatedintoNCcodeusingapostprocessorfortheMahoMH600Emillingmachine.Theviabilityofthe
proposedmethodisverifiedbyvirtualmachiningonVericutTMandactualmachiningonMahoMH600E.
Keywords:
5-AxisCNCmachining,CAD/CAM,sculpturedsurfacemachining.
1INTRODUCTION
Sculpturedsurfacemachininghassignificantlydevelopedeversinceitsinceptioninthe1950sunderthehistoricprojectcalledAutomaticProgrammedToolLanguage(APT).Theterm‘sculptured’hasearnedpopularityin
machiningasNCprogrammershavegainedmorecontrolofthecuttingtoolthusresemblingthemovementofanartist’schisel.Machiningoffree-formsurfacescalledforadvancedCNCmulti-axismachineswhichhaveahigherdegreeofflexibilityandprecisionthanconventional3-axistypes.ItsimplementationalsodemandedevenmoresophisticatedCAD/CAMsystemstoeasedesignerworkinmodelingandprogramming.CAMtechnologyhasassisteddesignersinselectingcuttingparametersinadditiontopreparingNCdatabasedontherequireddesignsurfacetolerance.Theselectionofcuttingvariablesinvolvesspecifyingcuttingtoolsthataregeometricallycompatiblewiththedesignsurfaceaswellaschoosingtheappropriatemillingtechnique.Countlessresearchhasbeendevotedtoharnessthefullpotentialofmulti-axisNCmachininginbothhardwareandsoftwareaspects[1].
ManufacturingpartswithcomplexgeometryrequiresflexiblemethodsofCNCprogrammingandmachining
especiallywhenthedesignpartcoversanareaofseveralmeterssuchasgasturbinebladesandmarinepropellers.Amongthenumerousadvantagesof5-axismachining,thethreemostsignificantare:
reducedprocesstimeduetohighermaterialremovalrates,reducedsetuptimeforintricateprismaticpartsandimprovedsurfacequalitythusminimizingthetimerequiredforsubsequentfinishing[2,3].Theinherentabilityof5-axismachinestopositionthetoolandworkpieceatanygivenrelativepointandangleallowsthemtoproducethedesignpartusingseveralapproaches[2,4]]thatwhichisevidentlyashortcomingof3-axismachines.Incontrasttotheirpredecessors,5-axismachineshaveconsiderableadvantageintermsofaccessibilityandproductivity.Forexample,theeffectofemploying5-axismachinesinthemanufactureofdiemoldshasresultedin10-20timesmorethantheefficiencysetby3-axismachines[5,6].Moreover,partswithirregularshapessuchasturboimpellerscanbemachinedusingasinglesetupsinceareaspreviouslyinaccessibleto3-axismachinesaremadeworkablewithaddeddegressoffreedomalthoughundercertainconstraints[7,8].
Apartfromtheirbenefitsinsculpturedsurfacemachining,5-axismachineshavealsointroducedbothcomputationalandfunctionaldifficulties.First,currentCAMsystemsstilldonotprovideadequatesupportfortoolpathgenerationandverificationsuchthatdesignersstillrelyoniterativemethods[8,9,10,11].Apparentlytherearestillhugenumbersofresearchconcerningtheeffectivecontrolofscallopheightsbasedontoolgeometryandpositioning.Second,consideringtherigoroustaskofdevelopingcomplicatedalgorithmsforinterferenceandcollisiondetectioninadditiontopositioncorrection[12],5-axisispronetomachiningerrorsofwhichmanyareclassifiedasNCprogrammingrelated[5].5-axisoperationscanbecategorizedaseither‘pointmilling’or‘flankmilling’[8].Inconventionalpointmilling,materialisremovedusingthetipofthetool.Althoughtheprocesscanbeappliedtomachineanycomplexsurface,themaindrawbackinusingpointmillingisthatisittime-consumingandthemilledsurfacewouldrequirepolishinginordertoremovescallops[13].Theprocessofflankmillingontheotherhandremovesmaterialusingthesideofthetool,whichthenleadstohighermachiningefficiencyandtoagreatextenteliminatesthepresenceofsurfacescallops[14].Yetithasdisadvantagesinvolvinglargeovercutsandundercutswithincreasedchancesofcutterinterferenceandcollision.
Flankmillingcanbefurtherclassifiedaseither‘ruledmilling’or‘skivecut’[5].Ruledmillingreferstothemachiningofflatruledsurfacesorthemoreconvolutedhyperbolicparaboloidsurfacesbothofwhichareboundbytwoguidestrings.Commonapplicationsofruled-millingincludethemanufactureoffan,compressorandimpellerbladesurfaces.Themajordrawbackofruledmillingincludesrelativelylargedeflectionswhenslendertoolsareemployedaswellasgougingforthecaseofconcaveorsharpcorneredfeatures.Whilethecutalsomillswiththesideofthecutter,itispreferredforconvexsurfacessuchastheleadingandtrailingedgesofairfoilsfoundingasturbineblades.Screwpropellershavebeentheprimaryproductsof5-axisCNCmachiningsincethebeginning.Withtheirvisiblycomplexgeometry,themanufactureofpropellerspresentedNCprogrammersthedifficultyofguidingthetoolthroughnarrowareasbetweenadjacentbladesurfaceswithoutcausinggougingorinterference.Researchon5-axismachiningofpropellershoweverhavemostlyfocusedonthesemi-finishingandfinishingsequences[15,16].Roughingisstillwidelyperformedon3-axismachinesfortworeasonsmainly,cost-effectivenessaswellashighmaterialremovalrate[6,8].Themotivationofthisresearchistodeviatefromsuchcommonpracticewhereroughingwouldbeperformedstraight-awayin5-axismodethusfurthercomplimentingtheprocedurewithasignificantreductioninsetuptimeandoverallmachining
time.
2GEOMETRICMODELOFSCREWPROPELLER
Thegeometryofapropellerisgenerallyderivedfromthefollowingparameters:
chordlength,pitch,camber,skew,rakeandtheprofilethickness[17].Fromsuchdimensionalandothernon-dimensionalparameters,theefficiencyandaero-hydrodynamicperformanceofthepropellerisestimateddependingonitsspecificapplication.Sincethemainfocusoftheworkonpropellermodelingisnotintendedtosupporthydrodynamictestingsuchasinacavitationtunnelthereforegreateremphasishasbeenlaidongeometricmodelingandsubsequent5-axismachining.Forthisreasonamoresimplifiedapproachisadoptedtogeneratethe3DpropellermodelwithUnigraphicsTM.Thesuggestedmethodwouldfirsttakeintoaccounttheairfoilcoordinates,overalldiameter,meanpitchandpitchratio.Thereafterthespecificpitchangleandprofilethicknessdistributionsoftheselectedpropellerclasscanbeappliedbyformingtheairfoilsateachlocalpropellerradiiusingaseriesofaffinetransforms[18,19].Thesurfaceforthebladewasthencreatedemployingtheairfoilsassectionstrings.Tocompletetheprocedure,theblendsurfacewasgeneratedabouttherootsectiontomakeafillet.Aftertheconstructionofasingleblade,itwasduplicatedinto4copies,whichwasthenrotatedaboutthehubcenterlineat72º
intervaltomakeatotalof5blades.Thecompleted3Dmodelofthescrewpropelleralongwiththeresultingmechanicaldrawingisdepicted.Theproposedmodelingapproachissummarized.
35-AXISMACHININGOFSCREWPROPELLER
3.1ProcessPlanningandSetup
Afourprongedprocessplanwasfollowedtomillthescrewpropeller[4].Thestrategystartedoffwiththegeometricidentificationofthepartsurfaceswhichclassifiedthemintoeitherconvex,concaveorsaddle.Followingisthegroupingoftheidentifiedsurfacesintomillingregionsdependingontheircurvatureproperties.Thenthemaximumallowabletooldiameterwasdeterminedforeachregionafterwhichthemillingdirectionisselected.Consequentlythedrivesurfacetomachinethecollectivemillingregionsarecreated.Inthiscasetheminimumdistancebetweentwoadjacentbladesdeterminedthemaximumtooldiameter.Theflatendmillwasusedduetoitswiderrangeofeffectivecuttingradiusincontrasttoballnosecutters.ThesetupoftheblankpartontheMH600Ealongwithalgorithmforcollision.
3.2ToolpathPlanningandGeneration
Priortothetoolpathgenerationfortheroughingphase,apreformgeometryofthemodeledpropellerwasfirstconstructed.Thegeometrycoinedthe‘boundingboxes’isacrudeapproximationofthepropellerwhichiscomposedofruledsurfacesthatenvelopeachbladetoformapolygon.Theconceptofusingsuchpreformgeometryissimilartothatofacastpropelleratthenearnetshapestagewhichwouldhavetoundergofinishingsothattheassignedtoleranceisachieved[20].Sincetheperformgeometryisrelativelylesscomplexthanthefinalpartgeometry,amorestraightforwardmachiningstrategyfortheroughingphasecanbeemployedasaresult.Thebladepolygonasshownwasdividedintothreemillingregionsnamelyfrontface,leadingsideandbackface.Withtheseregions,thedrivesurfaceswereassignedaccordingly.Theroughingtoolpathforeachmillingregionofthe‘boundingboxes’geometryisdepicted.
Thenumberoftoolpass
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