机械毕业设计英文外文翻译数控机床Word文档格式.docx
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ThesecondmajorbenefitofCNCtechnologyisconsistentandaccurateworkpieces.Today'
sCNCmachinesboastalmostunbelievableaccuracyandrepeatabilityspecifications.Thismeansthatonceaprogramisverified,two,ten,oronethousandidenticalworkpiecescanbeeasilyproducedwithprecisionandconsistency.
AthirdbenefitofferedbymostformsofCNCmachinetoolsisflexibility.Sincethesemachinesarerunfromprograms,runningadifferentworkpieceisalmostaseasyasloadingadifferentprogram.Onceaprogramhasbeenverifiedandexecutedforoneproductionrun,itcanbeeasilyrecalledthenexttimetheworkpieceistoberun.Thisleadstoyetanotherbenefit,fastchangeover.Sincethesemachinesareveryeasytosetupandrun,andsinceprogramscanbeeasilyloaded,theyallowveryshortsetuptime.Thisisimperativewithtoday'
sjust-in-time(JIT)productrequirements.
Motioncontrol-theheartofCNC
ThemostbasicfunctionofanyCNCmachineisautomatic,precise,andconsistentmotioncontrol.Ratherthanapplyingcompletelymechanicaldevicestocausemotionasisrequiredonmostconventionalmachinetools,CNCmachinesallowmotioncontrolinarevolutionarymanner2.AllformsofCNCequipmenthavetwoormoredirectionsofmotion,calledaxes.Theseaxescanbepreciselyandautomaticallypositionedalongtheirlengthsoftravel.Thetwomostcommonaxistypesarelinear(drivenalongastraightpath)androtary(drivenalongacircularpath).
Insteadofcausingmotionbyturningcranksandhandwheelsasisrequiredonconventionalmachinetools,CNCmachinesallowmotionstobecommandedthroughprogrammedcommands.Generallyspeaking,themotiontype(rapid,linear,andcircular),theaxestomove,theamountofmotionandthemotionrate(feedrate)areprogrammablewithalmostallCNCmachinetools.
ACNCcommandexecutedwithinthecontroltellsthedrivemotortorotateaprecisenumberoftimes.Therotationofthedrivemotorinturnrotatestheballscrew.Andtheballscrewdrivesthelinearaxis(slide).Afeedbackdevice(linearscale)ontheslideallowsthecontroltoconfirmthatthecommandednumberofrotationshastakenplace3.Refertofig.1.
Fig.1
Thougharathercrudeanalogy,thesamebasiclinearmotioncanbefoundonacommontablevise.Asyourotatethevisecrank,yourotatealeadscrewthat,inturn,drivesthemovablejawonthevise.Bycomparison,alinearaxisonaCNCmachinetoolisextremelyprecise.Thenumberofrevolutionsoftheaxisdrivemotorpreciselycontrolstheamountoflinearmotionalongtheaxis.
Howaxismotioniscommanded-understandingcoordinatesystems
ItwouldbeinfeasiblefortheCNCusertocauseaxismotionbytryingtotelleachaxisdrivemotorhowmanytimestorotateinordertocommandagivenlinearmotionamount4.(Thiswouldbelikehavingtofigureouthowmanyturnsofthehandleonatablevisewillcausethemovablejawtomoveexactlyoneinch!
)Instead,allCNCcontrolsallowaxismotiontobecommandedinamuchsimplerandmorelogicalwaybyutilizingsomeformofcoordinatesystem.ThetwomostpopularcoordinatesystemsusedwithCNCmachinesaretherectangularcoordinatesystemandthepolarcoordinatesystem.Byfar,themorepopularofthesetwoistherectangularcoordinatesystem.
TheprogramzeropointestablishesthepointofreferenceformotioncommandsinaCNCprogram.Thisallowstheprogrammertospecifymovementsfromacommonlocation.Ifprogramzeroischosenwisely,usuallycoordinatesneededfortheprogramcanbetakendirectlyfromtheprint.
Withthistechnique,iftheprogrammerwishesthetooltobesenttoapositiononeinchtotherightoftheprogramzeropoint,X1.0iscommanded.Iftheprogrammerwishesthetooltomovetoapositiononeinchabovetheprogramzeropoint,Y1.0iscommanded.Thecontrolwillautomaticallydeterminehowmanytimestorotateeachaxisdrivemotorandballscrewtomaketheaxisreachthecommandeddestinationpoint.Thisletstheprogrammercommandaxismotioninaverylogicalmanner.Refertofig.2,3.
Fig.2
Fig.3
Alldiscussionstothispointassumethattheabsolutemodeofprogrammingisused6.ThemostcommonCNCwordusedtodesignatetheabsolutemodeisG90.Intheabsolutemode,theendpointsforallmotionswillbespecifiedfromtheprogramzeropoint.Forbeginners,thisisusuallythebestandeasiestmethodofspecifyingendpointsformotioncommands.However,thereisanotherwayofspecifyingendpointsforaxismotion.
Intheincrementalmode(commonlyspecifiedbyG91),endpointsformotionsarespecifiedfromthetool'
scurrentposition,notfromprogramzero.Withthismethodofcommandingmotion,theprogrammermustalwaysbeasking"
HowfarshouldImovethetool?
"
Whiletherearetimeswhentheincrementalmodecanbeveryhelpful,generallyspeaking,thisisthemorecumbersomeanddifficultmethodofspecifyingmotionandbeginnersshouldconcentrateonusingtheabsolutemode.
Becarefulwhenmakingmotioncommands.Beginnershavethetendencytothinkincrementally.Ifworkingintheabsolutemode(asbeginnersshould),theprogrammershouldalwaysbeasking"
Towhatpositionshouldthetoolbemoved?
Thispositionisrelativetoprogramzero,NOTfromthetoolscurrentposition.
Asidefrommakingitveryeasytodeterminethecurrentpositionforanycommand,anotherbenefitofworkingintheabsolutemodehastodowithmistakesmadeduringmotioncommands.Intheabsolutemode,ifamotionmistakeismadeinonecommandoftheprogram,onlyonemovementwillbeincorrect.Ontheotherhand,ifamistakeismadeduringincrementalmovements,allmotionsfromthepointofthemistakewillalsobeincorrect.
Assigningprogramzero
KeepinmindthattheCNCcontrolmustbetoldthelocationoftheprogramzeropointbyonemeansoranother.HowthisisdonevariesdramaticallyfromoneCNCmachineandcontroltoanother8.One(older)methodistoassignprogramzerointheprogram.Withthismethod,theprogrammertellsthecontrolhowfaritisfromtheprogramzeropointtothestartingpositionofthemachine.ThisiscommonlydonewithaG92(orG50)commandatleastatthebeginningoftheprogramandpossiblyatthebeginningofeachtool.
Another,newerandbetterwaytoassignprogramzeroisthroughsomeformofoffset.Refertofig.4.Commonlymachiningcentercontrolmanufacturerscalloffsetsusedtoassignprogramzerofixtureoffsets.Turningcentermanufacturerscommonlycalloffsetsusedtoassignprogramzeroforeachtoolgeometryoffsets.
Fig.4
Flexiblemanufacturingcells
Aflexiblemanufacturingcell(FMC)canbeconsideredasaflexiblemanufacturingsubsystem.ThefollowingdifferencesexistbetweentheFMCandtheFMS:
1.AnFMCisnotunderthedirectcontrolofthe
centralcomputer.Instead,instructionsfromthecentral
computerarepassedtothecellcontroller.
2.Thecellislimitedinthenumberofpartfamiliesit
canmanufacture.
ThefollowingelementsarenormallyfoundinanFMC:
•Cellcontroller
•Programmablelogiccontroller(PLC)
•Morethanonemachinetool
•Amaterialshandlingdevice(robotorpallet)
TheFMCexecutesfixedmachiningoperationswithpartsflowingsequentiallybetweenoperations.
Highspeedmachining
ThetermHighSpeedMachining(HSM)commonlyreferstoendmillingathighrotationalspeedsandhighsurfacefeeds.Forinstance,theroutingofpocketsinaluminumairframesectionswithaveryhighmaterialremovalrate1.Overthepast60years,HSMhasbeenappliedtoawiderangeofmetallicandnon-metallicworkpiecematerials,includingtheproductionofcomponentswithspecificsurfacetopographyrequirementsandmachiningofmaterialswithhardnessof50HRCandabove.Withmoststeelcomponentshardenedtoapproximately32-42HRC,machiningoptionscurrentlyinclude:
Roughmachiningandsemi-finishingofthematerialinitssoft(annealed)conditionheattreatmenttoachievethefinalrequiredhardness=63HRCmachiningofelectrodesandElectricalDischargeMachining(EDM)ofspecificpartsofdiesandmoulds(specificallysmallradiianddeepcavitieswithlimitedaccessibilityformetalcuttingtools)finishingandsuper-finishingofcylindrical/flat/cavitysurfaceswithappropriatecementedcarbide,cermet,solidcarbide,mixedceramicorpolycrystallinecubicboronnitride(PCBN)
Formanycomponents,theproductionprocessinvolvesacombinationoftheseoptionsandinthecaseofdiesandmouldsitalsoincludestimeconsuminghandfinishing.Consequently,productioncostscanbehighandleadtimesexcessive.
Itistypicalinthedieandmouldindustrytoproduceoneorjustafewtoolsofthesamedesign.Theprocessinvolvesconstantchangestothedesign,andbecauseofthesechangesthereisalsoacorrespondingneedformeas