外文翻译平面度测量.doc

ProcedureforperformingflatnessmeasurementFlatnessmeasurementsareperformedtochecktheflatnessofCMMtablesandsurfaceplates.Itdetermineswhetheranysignificantpeaksortroughsexistandquantifiesthem.Iftheseerrorsaresignificantthenremedialwork,suchaslapping,mayberequired.Tounderstandthebasicprinciplesandtechniquesofflatnessmeasurementreferto:Theprinciplesofflatnessmeasurement.Standardmethodsofassessingflatness.PrincipleofflatnessmeasurementTheflatnessmeasurementkitisusedtomeasureflatness.Theangularinterferometerisattachedtotheturningmirrorandtheangularreflectorisattachedontopoftheselectedflatnessbase.Theangularinterferometerisplacedinthepathbetweenthelaserheadandtheangularreflector.Figure1-PrincipleofmeasurementThelaserbeamissplitintotwobythebeam-splitterinsidetheangularinterferometer.Onepartofthebeam(themeasurementbeamA1)passesstraightthroughtheinterferometerandisreflectedbyoneofthetwinreflectorsoftheangularreflectorbackthroughtheinterferometerandintothelaserhead.Theotherbeam(measurementbeamA2)passesthroughtheperiscopepartoftheangularinterferometertothesecondreflectorfromwhereitreturnsthroughtheinterferometerandintothelaserhead.AnangularmeasurementisproducedbycomparingthepathdifferencebetweenthebeamsA1andA2,(i.e.themeasurementisindependentofthedistancebetweenthelaserandtheinterferometer).Theflatnessreadingdisplayedbythesoftwareistheincrementalheightbetweenthefrontandbackfeetoftheflatnessbaseplateonwhichtheangularreflectorisfitted.Thisincrementalheightiscalculatedfromtheangularmeasurementandknowledgeofthedistancebetweenthecentresofthefrontandbackfeetoftheflatnessbaseplate.Thisdistance,termedthefoot-spacing,mustbeenteredintothecalibrationsoftwarebeforemeasurementstarts.Foreachmeasurementline(seeStandardmethodsforassessingflatness),theangularinterferometer(mountedontheflatnessturningmirror)staysstationary,whilethereflector(mountedonaflatnessbase)movesalongthelineinincrementalstepsdefinedbythefoot-spacing.Aflatnessmeasurementiscarriedoutbytakingaseriesofincrementalheightreadingsastheangularreflectorismovedalongthemeasurementpath.Figure2-IncrementalmeasurementsdrawingchangesInFigure2above:PositionIistheinitialpositionatwhichtheinterferometerreadingisnormallydatumed.PositionIIisonefoot-spacingbeyondPositionI.Theinterferometerreadingwillbetheincrementaldistanced1,whichisthedifferenceinheight(withrespecttothedatumline)ofthefrontandbackfeetoftheflatnessbaseplate.PositionIIIisonefoot-spacingbeyondPositionII.Theinterferometerreadingwillbed2,i.e.thedifferenceinheightbetweenthefrontandbackfeetofthebaseplateinitsnewposition.Similarly,thereadingatPositionIVwillbed3,andsoonforallsubsequentpositionsonthemeasurementline.Theactualflatnessofthemeasurementlinewillbethealgebraicsumofthereadingd1,d2etc(plusthereadingatthedatumposition(I)ifithadnotbeenzeroed).Note:Environmentalcompensationisnotrequiredwhentakingflatnessmeasurements,asthedifferenceinpathlengthsbetweenthetwobeamsissosmallthattheerrorduetoenvironmentaleffectsisnegligible.StandardmethodsforassessingflatnessTomeasuretheflatnessofasurface,anumberofmeasurementlinesneedtobetakenoverthesurface.Therearetwostandardmethodsofcarryingoutflatnessmeasurements:MoodymethodGridmethodTheflatnessofasurfacecanbedefinedastheseparationoftwoplaneswhichareparalleltothegeneraltrajectoryofthesurfaceandwhichjustenclosethemeasuredsurfaceasshowninFigure1.Figure1-FlatnessspecificationFlatnessdeviationsforasurfaceplateofaknownsizecanbecomparedwithpermitteddeviationsinnationalstandards.MoodymethodWiththeMoodymethod,measurementisrestrictedtotheeightprescribedlinesasshowninFigure2.Figure2-MoodymapofsurfaceplateTheMoodymethodwasfirstproposedbyJ.C.Moodyin1955andhassubsequentlyachievedwideacceptance.Themethodprovidesarelativelyquickmethodofcalibratingasurfaceplate,withtheresultsbeingpresentedasacontourplotalongtheeightmeasurementlinestested,inaformatacceptableforcertification.Thismethoddoeshaveonedisadvantage-allofthepointsonalloftheeightlinesmustbemeasuredandplotted.Thiscancauseproblemsindefiningafoot-spacingwhichwillmeetthisrequirement,particularlyonslottedtables/surfaceswherethepositionofoneormoreslotsmaycoincidewiththerequiredpositionofoneofthefeetoftheflatnessbase.GridandhalfgridmethodWiththeGridmethod,anynumberoflinesmaybetakenintwoorthogonaldirectionsacrossthesurfaceasshowninFigure3.Figure3-GridmapofsurfaceplateWiththeGridmethod,whilsttheincrementalnatureofthemeasurementtechniquerequiresthatallpointsonagivenlinearemeasured,itisnotnecessarytotakemeasurementsonalllines.Thisallowsthemeasurementlinestobeconfiguredtoavoidobstructions(e.g.slots)ortoprovidegreaterdetailinagivenarea.TheHalf-gridmethodasshowninFigure4isaspecialcaseofthegridmethodwhereanumberofmeasurementlinesaretakeninonedirection(e.g.theX-axis),butonlytheperimeterlinesareusedfortheorthogonaldirection.Figure4-Half