毕业论文外文翻译齿轮的加工方法.docx
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毕业论文外文翻译齿轮的加工方法.docx
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毕业论文外文翻译齿轮的加工方法
英文原文
Gearmanufacturingmethods
Therearetwobasicmethodsofmanufacturinggearteeth:
thegeneratingprocessandtheformingprocess.whenageartoothisgenerated,theworkpieceandthecuttingorgrindingtoolareincontinuousmeshandthetoothformisgeneratedbythetool.Inotherwords,theworkandthetoolareconjugatedtoeachother.hobbing:
machines,shapercutters,shavingmachines,andgrindersusethisprinciple.
Whenageartoothisformed,thetoolisintheshapeofthespacethatisbeingmachinedout.Somegrindingmachinesusethisprinciplewithanindexingmechianismwhichallowsthegearteethtobeformedtoothbytooth.Broachesareexamplesofformtoolsthatmachineallthegearteethsimultaneously.
shaping
Shapingisinherentlysimilartoplanningbutusesacircularcuttrerinsteadofrackandtheresultingreductioninthereciprocatinginertiaallowsmuchhigherstrokingspeeds:
modernshaperscuttingcargearscanrunat2,000cuttingstrokesperminmute.Theshapeofthecutterisroughlythesameasaninvolutegearbutthetipsoftheteetharerounded.
Thegeneratingdrivebetweencutterandworkpiecedoesnotinvolvearackorleadscrewsinceonlycircularmotionininvolved.Thetoolandworkpiecemovetangentialtypically0.5mmforeachstrokeofthecutter.Onthereturnstrokethecuttermustberetractedabout1mmtogiveclearanceotherwisetoolruboccursonthebackstrokeandfailureisrapid.Thespeedonthistypeofmachineislimitedbytherateatwhichsome50kgofcutterandbearingscanbemovedadistanceof1mm.theaccelerationsinvolvedtequireforcesoftheorderof5000Nyethighaccuracymustbemaintained.
Theadvantagesofshapingarethatproductionratesarerelativelyhighandthatitispossibletocutrightuptoashoulder.Unfortunately,forhelicalgears,ahelicalguideisrequiredtoimposearotationalmotiononthestrokingmotion;suchhelicalguidescannotbeproducedeasilyorcheaplysothemethodisonlysuitableforlongrunswithhelicalgearssincespecialcuttersandguidesmustbemanufacturedforeachdifferenthelixangle.Agreatadvantageofshapingisitsabilitytoannulargearssuchasthoserequiredforlargeepicycliedrives.
Whenveryhighaccuracyisofimportancetheinaccuraciesintheshapingcuttermattersincetheymaytransfertothecutgear.Itisobviousthatprofileerrorswilltransferbutitislessobviousthananeccentricallymountedorgroundcutterwillgiveacharacteristic“droppedtooth”.Thereareseveralcausesfor“droppedtooth”butitoccursmostcommonlywhenthediameteroftheworkpieceisabouthalf,oneandhalf,twoandahalf,etc,timesthecutterdiameter.Ifthecutterstartsonahighpointandfinishesonalowpointduringthefinalfinishingrevolutionofthegearthepeaktopeakeccentricityerrorsinthecutteroccursbetweenthelastandthefirsttoothofthefinalrevolutionofthecutgear;asthecumulativepitcherrorofthecuttermaywellbeover25micronsthereisasuddenpitcherrorofthisamountonthecutgear.Thenextgearcutonthemachinemayhoweverbeverygoodonadjacentpitchifthefinalcuthappenedtostartinafavorablepositiononthecutter.
Variousattemptshavebeenmadetopreventthiseffect,inparticularbycontinuingrotationwithoutanyfurthercutterinfeedbutiftheshapingmachineisnotveryrigidandthecutterverysharpthennofurthercuttingwilloccurandtheerrorwillnotberemoved.
hobbing
hobbing,themostusedmetalcuttingmethod,usestherackgeneratingprinciplebutavoidsslowreciprocationbymountingmany“racks”onarotatingcutter.The“racks”aredisplacedaxiallytoformagashedworm.The“racks”donotgeneratethecorrectinvoluteshapeforthewholelengthoftheteethsincetheyaremovingonacircularpathandsothehobisfedslowlyalongtheteetheitheraxiallyinnormalorinthedirectionofthehelixin“oblique”hobbing.
Metalremovalratesarehighsincenoreciprocationofhoborworkpieceisrequiredandsocuttingspeedsof40m/mincanbeusedforconventionalhobsandupto150m/minforcarbidehobs.Typicallywitha100mmdiameterhobtherotationspeedwillbe100rpmandsoatwentytoothworkpiecewillrotateat5rpm.Eachrevolutionoftheworkpiecewillcorrespondto0.75mmfeedsothehobwilladvancethroughtheworkpieceatabout4mmperminute.Forcarproductionroughingmultiplestarthobscanbeusedwithcoarsefeedsof3mmperrevolutionsothat100rpmonthecutter,atwo-starthobanda20toothgearwillgiveafeedrateof30mm/minute.
Thedisadvantageofacoarsefeedrateisthataclearmarkingisleftontheworkpiece,particularlyintheroot,showingapatternataspacingofthefeedrateperrevolution.Thissurfaceundulationislessmarkedontheflanksthanintherootandisnotimportantwhenthereisasubsequentfinishingoperationsuchasshavingorgrinding.Whentherearenofurtheroperationsthefeedperrevolutionmustberestrictedtokeeptheundulationsbelowalimitwhichisusuallydictatedbylubricationconditions.Theheightoftheundulationsintherootofthegearisgivenbysquaringthefeedperrevolutionanddividingbyfourtimesthediameterofthehob;1mmfeedand100mmdiametergives2.5micronhighundulationsintheroot.Onthegearflanktheundulationisroughlycos70aslarge,i.e.,about0.85micron.
Accuracyofhobbingisnormallyhighforpitchandforhelix,providedmachinesaremaintained;involuteisdependentsolelyontheaccuracyofthehobprofile.Astheinvoluteformisgeneratedbyasmanycutsastherearegashesonthehobtheinvoluteisnotexact,butifthereare,say,14tangentsgeneratingaflankof20mmradiuscurvatureabout4mmhighthedivergencefromatrueinvoluteisonlyabouthalfamicron;hobmanufacturingandmountingerrorscanbeabove10microns.Useoftwostarthobsorobliquehobbinggivesincreasederrorlevelssincehoberrorsofpitchingtransfertothecutgear.
broaching
Broachingisnotusedforhelicalgearsbutisusefulforinternalspurgears;theprincipaluseofbroachinginthiscontextisforinternalsplineswhichcannoteasilybemadebyanyothermethod.Aswithallbroachingthemethodisonlyeconomicforlargequantitiessincesetupcostsarehigh.
ThemajorapplicationofbroachingtechniquestohelicalexternalgearsisthatusedbyGleasonsintheirG-TRACmachine.thismachineoperatesbyincreasingtheeffectiveradiusofahobbingcuttertoinfinitysothateachtoothofthecutteristravelinginastraightlineinsteadofonaradius.Thisallowsthecuttingactiontoextendoverthewholefacewidthofagearinsteadofthetypical0.75mmfeedperrevolutionofhobbing.Theresultingprocessgivesaveryhighproductionrate,moresuitableforU.S.A.productionvolumesthanfortherelativelylowEuropeanvolumesandso,despiteahighinitialcost,isverycompetitive.
Broachinggivehighaccuracyandgoodsurfacefinishbutlikeallcuttingprocessesislimitedto“soft”materialswhichmustbesubsequentlycasehardenedorheattreated,givingdistortion.
Shaving
Ashavingcuttingcutterlookslikeagearwhichhasextraclearanceattherootandwhosetoothflankshavebeengroovedtogivecuttingedges.Itisruninmeshwiththeroughgearwithcrossedaxessothatthereisintheorypointcontactwitharelativevelocityalongtheteethgivingscrapingaction.Theshavingcutterteetharerelativelyflexibleinbendingandsowillonlyoperateeffectivelywhentheyareindoublecontactbetweentwogearteeth.Thegearandcutteroperateathighrotationalspeedswithtraversingoftheworkfaceandabout100mmmicronofmaterialisremoved.Cycletimescanbelessthanhalfaminuteandthemachinesarenotexpensivebutcuttersaredelicateanddifficulttomanufacture.Itiseasytomakeadjustmentsofprofileattheshavingstageandcrowningcanbeapplied.Shavingcanbecarriedoutnearashoulderbyusingacutterwhichisplungedintodepthwithoutaxialmovement;thismethodisfastbutrequiresmorecomplexcutterdesign.
grinding
Grindingisextremelyimportantbecauseitisthemainwayhardenedgeararemachined.Whenhighaccuracyisrequireditisnotsufficienttopre-correctforheattreatmentdistortionandgrindingisthennecessary.
Thesimplestapproachtogrinding,oftentermedtheOrcuttmethod.Thewheelprofileisdressedaccuratelytoshapeusingsinglepointdiamondswhicharecontrolledbytemplatescuttotheexactshaperequired;6:
1scalingwithapantographisoftenused.Theprofilewheelisthenreciprocatedaxiallyalongthegearwhichrotatestoallowforhelixangleeffects;whenonetoothshapehasbeenfinished,involvingtypically100micronmetalremovalthegearisindexedtothenexttoothspace.Thismethodisfairlyshowbutgiveshighaccuracyconsistently.Settingupislengthybecausedifferentdressingtemplatesareneededifmodule,numberofteeth,helixangle,orprofilecorrectionarechanged.
Thefastestgrindingmethodusesthesameprincipleashobbingbutreplacesagashedandrelievedwormbyagrindingwheelwhichisarackinsection.Sincehighsurfacespeedsareneededthewheeldiameterisincreasedsothatwheelsof0.5mdiametercanrunatover2000rpmtogivethenecessary1000m/min.onlysinglestartwormsarecutonthewheelbutgearrotationspeedsarehigh,100rpmtypically,soitisdifficulttodesignthedrivesystemtogiveaccuracyandrigidity.Accuracyoftheprocessisreasonablyhighalthoughthereisatendencyforwheelandworkpiecetodeflectvariablyduringgrindingsothewheelformmayrequirecompensationformachinedeflectioneffects.Generationofawormshapeonthegrindingwheelisa
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