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外文翻译

外文翻译

系别

机械工程系

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

外文翻译

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,whichwasthenrotatedaboutthehubcenterlineat72ºintervaltomakeatotalof5blades.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