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外文翻译平面度测量.doc

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外文翻译平面度测量.doc

ProcedureforperformingflatnessmeasurementFlatnessmeasurementsareperformedtochecktheflatnessofCMMtablesandsurfaceplates.Itdetermineswhetheranysignificantpeaksortroughsexistandquantifiesthem.Iftheseerrorsaresignificantthenremedialwork,suchaslapping,mayberequired.TounderstandthebasicprinciplesandtechniquesofflatnessmeasurementrefertoTheprinciplesofflatnessmeasurement.Standardmethodsofassessingflatness.PrincipleofflatnessmeasurementTheflatnessmeasurementkitisusedtomeasureflatness.Theangularinterferometerisattachedtotheturningmirrorandtheangularreflectorisattachedontopoftheselectedflatnessbase.Theangularinterferometerisplacedinthepathbetweenthelaserheadandtheangularreflector.Figure1PrincipleofmeasurementThelaserbeamissplitintotwobythebeamsplitterinsidetheangularinterferometer.OnepartofthebeamthemeasurementbeamA1passesstraightthroughtheinterferometerandisreflectedbyoneofthetwinreflectorsoftheangularreflectorbackthroughtheinterferometerandintothelaserhead.TheotherbeammeasurementbeamA2passesthroughtheperiscopepartoftheangularinterferometertothesecondreflectorfromwhereitreturnsthroughtheinterferometerandintothelaserhead.AnangularmeasurementisproducedbycomparingthepathdifferencebetweenthebeamsA1andA2,i.e.themeasurementisindependentofthedistancebetweenthelaserandtheinterferometer.Theflatnessreadingdisplayedbythesoftwareistheincrementalheightbetweenthefrontandbackfeetoftheflatnessbaseplateonwhichtheangularreflectorisfitted.Thisincrementalheightiscalculatedfromtheangularmeasurementandknowledgeofthedistancebetweenthecentresofthefrontandbackfeetoftheflatnessbaseplate.Thisdistance,termedthefootspacing,mustbeenteredintothecalibrationsoftwarebeforemeasurementstarts.ForeachmeasurementlineseeStandardmethodsforassessingflatness,theangularinterferometermountedontheflatnessturningmirrorstaysstationary,whilethereflectormountedonaflatnessbasemovesalongthelineinincrementalstepsdefinedbythefootspacing.Aflatnessmeasurementiscarriedoutbytakingaseriesofincrementalheightreadingsastheangularreflectorismovedalongthemeasurementpath.Figure2IncrementalmeasurementsdrawingchangesInFigure2abovePositionIistheinitialpositionatwhichtheinterferometerreadingisnormallydatumed.PositionIIisonefootspacingbeyondPositionI.Theinterferometerreadingwillbetheincrementaldistanced1,whichisthedifferenceinheightwithrespecttothedatumlineofthefrontandbackfeetoftheflatnessbaseplate.PositionIIIisonefootspacingbeyondPositionII.Theinterferometerreadingwillbed2,i.e.thedifferenceinheightbetweenthefrontandbackfeetofthebaseplateinitsnewposition.Similarly,thereadingatPositionIVwillbed3,andsoonforallsubsequentpositionsonthemeasurementline.Theactualflatnessofthemeasurementlinewillbethealgebraicsumofthereadingd1,d2etcplusthereadingatthedatumpositionIifithadnotbeenzeroed.NoteEnvironmentalcompensationisnotrequiredwhentakingflatnessmeasurements,asthedifferenceinpathlengthsbetweenthetwobeamsissosmallthattheerrorduetoenvironmentaleffectsisnegligible.StandardmethodsforassessingflatnessTomeasuretheflatnessofasurface,anumberofmeasurementlinesneedtobetakenoverthesurface.TherearetwostandardmethodsofcarryingoutflatnessmeasurementsMoodymethodGridmethodTheflatnessofasurfacecanbedefinedastheseparationoftwoplaneswhichareparalleltothegeneraltrajectoryofthesurfaceandwhichjustenclosethemeasuredsurfaceasshowninFigure1.Figure1FlatnessspecificationFlatnessdeviationsforasurfaceplateofaknownsizecanbecomparedwithpermitteddeviationsinnationalstandards.MoodymethodWiththeMoodymethod,measurementisrestrictedtotheeightprescribedlinesasshowninFigure2.Figure2MoodymapofsurfaceplateTheMoodymethodwasfirstproposedbyJ.C.Moodyin1955andhassubsequentlyachievedwideacceptance.Themethodprovidesarelativelyquickmethodofcalibratingasurfaceplate,withtheresultsbeingpresentedasacontourplotalongtheeightmeasurementlinestested,inaformatacceptableforcertification.Thismethoddoeshaveonedisadvantageallofthepointsonalloftheeightlinesmustbemeasuredandplotted.Thiscancauseproblemsindefiningafootspacingwhichwillmeetthisrequirement,particularlyonslottedtables/surfaceswherethepositionofoneormoreslotsmaycoincidewiththerequiredpositionofoneofthefeetoftheflatnessbase.GridandhalfgridmethodWiththeGridmethod,anynumberoflinesmaybetakenintwoorthogonaldirectionsacrossthesurfaceasshowninFigure3.Figure3GridmapofsurfaceplateWiththeGridmethod,whilsttheincrementalnatureofthemeasurementtechniquerequiresthatallpointsonagivenlinearemeasured,itisnotnecessarytotakemeasurementsonalllines.Thisallowsthemeasurementlinestobeconfiguredtoavoidobstructionse.g.slotsortoprovidegreaterdetailinagivenarea.TheHalfgridmethodasshowninFigure4isaspecialcaseofthegridmethodwhereanumberofmeasurementlinesaretakeninonedirectione.g.theXaxis,butonlytheperimeterlinesareusedfortheorthogonaldirection.Figure4HalfgridmapofsurfaceplateAdisadvantagewithbothgridmethodsisthattheyrequireareferenceplanetobedefined.AnalternativetechniquesuchastheMoodymethodisneededtodefinethisreferenceplane.ToperformflatnessmeasurementsusingtheMoodymethod,carryoutthefollowingsteps

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