机械英语方面文章.docx
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机械英语方面文章.docx
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机械英语方面文章
Grindingisamanufacturingprocessthatinvolvestheremovalofmetalbyemployingarotatingabrasivewheel.Thelattersimulatesamillingcutterwithanextremelylargenumberofminiaturecuttingedges.Generally,grindingisconsideredtobeafinishingprocessthatisusuallyusedforobtaininghigh-dimensionalaccuracyandbettersurfacefinish.Grindingcanbeperformedonflat,cylindrical,oreveninternalsurfacesbyemployingspecializedmachinetools,whicharereferredtoasgrindingmachines.Obviously,grindingmachinesdifferinconstructionaswellascapabilities,andthetypetobeemployedisdeterminedmainlybythegeometricalshapeandnatureofthesurfacetobeground,e.g.,cylindricalsurfacesaregroundoncylindricalgrindingmachines.
Surfacegrinding.Asthenamesurfacegrindingsuggests,thisoperationinvolvesgrindingofflatorplanesurfaces.Fig.8.1indicatesthetwopossiblevariations,eitherahorizontalorverticalmachinespindle.Inthefirstcase(horizontalspindle),themachineusuallyhasaplaner-typereciprocatingtableonwhichtheworkpieceisheld.However,grindingmachineswithverticalspindlescanhaveeitheraplanertypetablelikethatofthehorizontal-spindlemachineorarotatingworktable.Also,thegrindingactioninthiscaseisachievedbytheendfaceofthegrindingwheel,contrarytothecaseofhorizontal-spindlemachines,wheretheworkpiecegroundbytheperipheryofthegrindingwheel.Fig.8.1aandbalsoindicatetheequationstobeusedforestimatingthedifferentparametersofthegrindingoperation,suchasthemachiningtimeandtherateofmetalremoval.Duringthesurface-grindingoperations,heavyworkpiecesareeitherheldinfixturesorclampedonthemachinetablebyscrapclampsandthelike,whereassmallerworkpiecesareusuallyheldbychucks.
Cylindricalgrinding.Incylindricalgrinding,theworkpieceisheldbetweencentersduringgrindingoperation,andthewheelrotationisthesourceandcausefortherotarycuttingmotion,asshownifFig.8.2.Infact,cylindricalgrindingcanbecarriedoutbyemployinganyofthefollowingmethods:
(1)Thetransversemethod,inwhichboththegrindingboththegrindingwheelandtheworkpiecerotateandlongitudinallinearfeedisappliedtoenablegrindingofthewholelength.Thedepthofcutisadjustedbythecrossfeedifthegrindingwheelintotheworkpiece.
(2)Theplunge-cutmethod,inwhichgrindingisachievedthroughthecrossfeedofthegrindingwheelandnoaxialfeedisapplied.Asyoucansee,thismethodcanbeappliedonlywhenthesurfacetobegroundisshorterthanthewidthofthegrindingwheelused.
(3)Thefull-depthmethod,whichissimilartothetransversemethodexceptthatthegrindingallowanceisremovedinasinglepass.Thismethodisusuallyrecommendedwhengrindingshortrigidshafts.
Internalgrinding.Internalgrindingisemployedforgrindingrelativelyshortholes,asshowninFig.8.3.Theworkpieceisheldinachuckoraspecialfixture.Boththegrindingwheelandtheworkpiecerotateduringtheoperationandfeedisappliedinthelongitudinaldirection.Anydesireddepthofcutcanbeobtainedbythecrossfeedofthegrindingwheel.Avariationfromthistypeisplanetaryinternalgrinding,whichisrecommendforheavyworkpiecesthatcannotbeheldinchucks.Inthatcase,thegrindingwheelnotonlyspinsarounditsownaxisbutalsorotatesaroundthecenterlineoftheholethatisbeingground.
Centerlessgrinding.Centerlessgrindinginvolvespassingacylindricalworkpiece,whichissupportedbyarestblade,betweentwowheels,i.e.thegrindingwheelandtheregulatingoffeedwheel.Thegrindingwheeldoestheactualgrinding,whiletheregulatingwheelisresponsibleforrotatingtheworkpieceaswellasgeneratingthelongitudinalfeed.Thisispossiblebecauseofthefrictionalcharacteristicsofthatwheel,whichisusuallymadeofrubber-bondedabrasive.AscanbeseeninFig.8.4,theaxisoftheregulatingwheelistiltedataslightanglewiththeaxisofthegrindingwheel.Consequently,theperipheralvelocityoftheregulatingwheelcanberesolvedintotwocomponents,namely,workpiecerotationalspeedandlongitudinalfeed.Thesecanbegivenbythefollowingequation:
Vworkpiece=Vregulatingwheel*cosа
Axialfeed=Vregulatingwheel*c*sinа
Wherecisaconstantcoefficienttoaccountfortheslipbetweentheworkpieceandtheregulatingwheel(c=0.94~0.98).
Thevelocityoftheregulatingwheeliscontrollableandisusedtoachieveanydesiredrotationalspeedoftheworkpiece.Theangleаisusuallytakingfrom10to50andthelargertheangle,thelargerthelongitudinalfeedwouldbe.Whenаistakenas00,i.e.,thetwoaxesofthegrindingandregulatingwheelsareparallel,thereisnolongitudinalfeedoftheworkpiece.
Grindingwheels
Grindingwheelsarecomposedofabrasivegrainshavingsimilarsizeandabinder.Theactualgrindingprocessisperformedbytheabrasivegrains.Poresbetweenthegrainswithinthebinderenablethegrainstoactasseparatesingle-pointcuttingtools.Theseporesalsoprovidespaceforthegeneratedchips,thuspreventingthewheelfromclogging.Inaddition,poresassisttheeasyflowofcoolantstoenableefficientandpromptremovaloftheheatgeneratedduringthegrindingprocess.
Grindingwheelsareidentifiedbasedontheirshapeandsize,kindofabrasive,grainsize,binder,grade(hardness),andstructure.
Shapeandsizeofgrindingwheels.Grindingwheelsdifferinshapeandsize,dependinguponthepurposeforwhichtheyaretobeused.VariousshapesareshowninFig.8.5andincludethefollowingtypes:
(1)Straightwheelsusedforsurface,cylindrical,internal,andcenterlessgrinding.
(2)Beveled-faceortaperedwheelsusedforgrindingthreads,gearteeth,andthelike.
(3)Straightrecessedwheelsforcylindricalgrindingandfacing.
(4)Abrasivedisksforcutoffandslottingoperations.
(5)Cylinders,straightcups,andflaringcupsareusedforsurfacegrindingwiththeendfaceofthewheel.
ThemaindimensionsofagrindingwheelaretheoutsidediameterD,theborediameterd,andtheheightH.Thesedimensionsvarywidely,dependinguponthegrindingprocessforwhichthewheelistobeused.
Kindofabrasive.Grindingwheelscanbemadeofnaturalabrasivessuchasquartz,emery,andcorundumorofindustriallypreparedchemicalcompoundssuchasaluminumoxideorsiliconcarbide(knownascarborundum).Generally,siliconcarbidegrindingwheelsareusedwhengrindinglow-tensile-strengthmaterialslikecastiron,whereasaluminumoxidewheelsareemployedforgrindinghigh-strengthmetalssuchasalloysteel,hardenedsteel,andthelike.
Grainsizeofabrasiveused.Asyoumayexpect,thegrainsizeoftheabrasiveparticlesofthewheelplaysafundamentalroleindeterminingthequalityofgroundsurfaceobtained.Thefinerthegrains,thesmootherthegroundsurfaceis.Therefore,course-grainedgrindingwheelsareusedforroughingoperations,whereasfine-grainedwheelsareemployedinfinaloperations.
Thegradeofthebond.Thegradeofthebondisactuallyanindicationoftheresistanceofthebondtopullingofftheabrasivegrainsfromthegrindingwheel.Generally,wheelshavinghardgradesareusedforgrindingsoftmaterialsandviceversa(反之亦然).Ifhard-gradewheelweretobeusedforgrindingahardmaterial,thedullgrainswouldnotbepulledofffromthebondquicklyenough,thusimpeding(妨碍)theself-dressingprocessofthesurfaceofthewheelandfinallyresultinginclogging(堵塞)ofthewheelandburnsonthegroundsurface.Infact,thecuttingpropertiesofallgrindingwheelsmustberestoredperiodicallybydressingwithacementedcarbide(硬质合金)rolleroradiamondtooltogivethewheeltheexactdesiredshapeandremoveallwornabrasivegrains.
Structure.Structurereferstotheamountofvoidspacebetweentheabrasivegrains.Whengrindingsoftermetals,largervoidspacesareneededtofacilitatetheflowoftheremovedchips.
Thebinder.Abrasiveparticlesarebondedtogetherinmanydifferentways.Theseincludebond,silicate,rubber,resinoid,shellac,andoxychloride.Nevertheless,thebondisthemostcommonlyusedone.
Infact,thestandardmarkingsystemisemployedfordistinguishinggrindingwheels,byprovidingalltheprecedingparametersinspecificsequence.
机械工程专业英语文章:
LappingandPolishing
专业英语、行业英语2010-07-0816:
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Lapping
Lappingisafinishingprocessoperationusedonflatandcylindricalsurfaces.Thelap,showninFig.9.1a,isusuallymadeofcastiron,copper,leather,orcloth.Theabrasiveparticlesareembeddedinthelap,ortheymaybecarriedthroughslurry.Dependingonthehardnessoftheworkpiece,lappingpressuresrangefrom7kPato140kPa.
Lappinghastwomainfunctions.Firstly,itproducesasuperiorsurfacefinishwithallmachiningmarksbeingremovedfromthesurface.Secondly,itisusedasamethodofobtainingveryclosefits(过盈配合、密配合)betweenmatingparts(配件)suchaspistonsandcylinders.
Thelappedworkpiecesurfacemaylooksmoothbutitisactuallyfilledwithmicroscopicpeaks,valleys,scratchesandpits(凹点).Fewsurfacesareperfectlyflat.Lappingminimizesthesurfaceirregularities,therebyincreasingtheavailablecontactarea.ThedrawinginFig.9.1ashowstwosurfaces.Theupperoneishowasurfacemightlookbeforelappingandtheloweroneafterlapping.Lappingremovesthemicroscopicmountaintopsandproducesrelativelyflatplateaus.Entiremicroscopicmountainrangesmayneedtobegrounddowninordertoincreasetheavailablecontactarea.
Productionlappingonflatorcylindricalpiecesisdoneonmachinessuchasthose
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