毕业论文外文翻译齿轮的加工方法.docx

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