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Theprinciplesofdesignareofcourseuniversal.Thesametheoryorequationsmaybeappliedtoaverysmallpart,asinaninstrument,ortoalargerbutsimilarpartusedinapieceofheavyequipment.Innocase,however,shouldmathematicalcalculationsbelookeduponasabsoluteandfinal.Theyareallsubjecttotheaccuracyofthevariousassumptionswhichmustnecessarilybemadeinengineeringwork.Sometimesonlyaportionofthetotalnumberofpartsinamachinearedesignedonthebasisofanalyticcalculations.Theformandsizeoftheremainingpartsarethenusuallydeterminedbypracticalconsiderations.Ontheotherhand,ifthemachineisveryexpensive,orifweightisafactor,asinairplanes,designcomputationsmaythenbemadeforalmostalltheparts.
Thepurposeofthedesigncalculationisofcoursetoattempttopredictthestressordeformationinthepartinorderthatinmaysafelycarrytheloadswhichwillbeimposeduponit,andthatitmaylastfortheexpectedlifeofthemachine.Allcalculationsare,ofcourse,dependentonthephysicalpropertiesoftheconstructionmaterialsasdeterminedbylaboratorytests.Arationalmethodofdesignattemptstotaketheresultsofrelativelysimpleandfundamentaltestsandapplythemtoallthecomplicatedandinvolvedsituationsencounteredinpresent-daymachinery.
Inaddictin,ithasbeenamplyprovedthatsuchdetailsassurfacecondition,manufacturingtolerances,andheattreatmenthaveamarkedeffectonthestrengthandusefullifeofamachinepart.Thedesignanddraftingdepartmentsmustspecifycompletelyallsuchparticulars,andthusexercisethenecessaryclosecontroloverthefinishedproduct.
Traininginrapidandaccuratenumericalworkisinvaluabletothedesigner.Thedesignershouldkeepanaccuratenotebook,asitisfrequentlynecessaryforhimtorefertoworkwhichhehasdoneinthepast.Asketch,carefullydrawntoscale,isalsoanecessity,andprovidesaconvenientplaceforputtingdownaportionofthedatausedinconnectionwiththeproblem.Itgoeswithoutsayingthatalldata,assumptions,equations,andcalculationsshouldbewrittendowninfullinordertobeintelligiblewhenreferredtoatalaterdate.
InfluenceofMachineToolonMachiningAccuracy
Amachinetoolhasthefollowingfunctionsinthemachiningprocess:
Clampingofthetoolandworkpieceandestablishingtheircorrectrelativeposition.
Supplyingtheworkingmotions(primaryandauxiliarymotion) andmaintainingthecorrectrelativepositionoftheworkpieceandtoolduringthemachiningoperation.
Supplyingtheenergyforcuttingorerosionwork.
Machiningaccuracyisevaluatedbyconsideringthedifferencebetweenthedesired(nominal)dimensionsandpropertiesandthoseofthefinishedproduct.Thesmallerarethemachiningerrors,thegreateristheaccuracy.
Themachinetoolinfluencesmachiningerrorsby:
Errorsinworkpiecepositioninginrelationtothetool.Thismaybecausedbyerrorsintheproductionandassemblyofmachinetoolpartsandassembliesandbyerrorsinthepositioningandclampingoftheworkpieceandtoolrelativetothemachinetool.Itiscalledinshortthegeometricinfluenceonmachiningaccuracy;
Errorsintheworkingmotionsofthetoolandworkpieceduringthemachiningprocess,orkinematicinfluenceonmachiningaccuracy;
Errorsinthedisplacementofmachinetoolpartsandassemblies,workpieceandtoolasaresultofforcesappearinginthemachiningprocess,calleddynamicinfluenceonmachiningaccuracy.
Errorsinthedisplacementofmachinetoolpartsandassemblies,workpieceandtoolasaresultofthermalexpansioncausebyheatgeneratedinthemachiningprocessandasaresultoffrictioninthedrivingmechanisms.Thisiscalledthermalinfluenceonmachiningaccuracy.
Geometricerrorsonamachinetoolshouldwhereverpossiblebemeasureddirectly.Theirevaluationdependsontheabsolutemagnitudeoftheerrorandonthesenseofdeviationfromnominaldimensions.Errorswhosesenseindicatesthat,asthewearsetsin,theywilldecrease,areconsideredtobelessharmful.
Theimportanceofgeometricerrorsfortotalmachiningaccuracyisrelativelygreat.Thiscanbedemonstratedonthebasisofastatisticalcomparisonoferrorsmeasuredonfinishedproductswitherrorscalculatedfromgeometricdeviationsonly.Somestatisticalreferencedenotestheshareofgeometricerrorsintotalmachiningerrorsvarieswithinarangeof50%~75%.Thusaconclusioncanbedrawnthatgeometricerrorsinmachinetoolsshouldbereducedasfaraspossible,butstillwithinlimitsofaneconomicallyjustifiedaccuracy.
Inthecaseofmachinetoolswithcomplexworkingmotions,thekinematicaccuracyofthemachinetoolisofgreatimportance.Thiscanbeobservedwhenmachininggears,threadsandothercurvedsurfaces.
Thekinematicaccuracyofamachinetoolresultsfromtheaccuracyofcouplingofitsinternalkinematicchains.
Akinematicerrorisdefinedbydeviationsofthefinalandinitiallinkofakinematicchainfromnominalpositionsordisplacements.
Kinematicerrorsmaybecalculatedormeasuredbymeansofspecialinstrumentscalledkinematicmeters.
Checkingofkinematicerrorshasnotyetbecomeawidespreadpracticeandtherarenostandardsestablishingtheirpermissiblevalues,justastherearenostandardsconcerninggeometricerrors.Oneofthemainreasonswhythemeasurementofkinematicaccuracyhasnotspreadisthatrelativelycomplicatedandexpensiveequipmentisnecessary,togetherwithtime-consumingoperations.
Thenoisecontrolproblemcanbebrokendownintothreesegmentsincludingthedesignconcept,thedesignfixandtheacousticfix.Thelattertowfixeswillprobablyoverlapinmostcasesandinteractwitheachotherwhilethedesignconceptwillsettheconstraintsfortheoverallproblem.Thedefinitionoftheproblemisusuallygeneralinmatureandarisesmostoftenfromacomplaintofsomeoneorsomegroupsegmentofthepopulation.
Identifyingthesourcesdoesnotusuallyrequiresoundmeasurementsatthisjuncturealthoughthosemeasurementswillberequiredpriortoattemptingthoseproceduresindicatedinthe“DesignFix”column.
Ofgreatimportanceinthedesignconceptstageisthecorrectidentificationofthepopulationaffectedbythesource,thatis,thereceiveandthepathofpropagationtothem.Havingidentifiedthepeopleaffectedbythenoisesource,itisnecessarytoexaminetheacceptablenoiselimitationstobeimposeduponoracceptedbythoseindividuals.Aretheselimitsthosewhichareestablishedorimposedbyexistingcriteriaeitherlegalorotherwiseoraretherepeculiarconditionswhichmakesthisproblemunique,forexample,isthereaconditionedpopulationinagenerallyhighnoiseenvironmentsuchasthatwhichexistsinmajorurbancenters?
Priortoadesignfixandaspartofthedesignconcept,therestraintstothedesignmustbeexamined.Aretherelegallimitations?
IstheproblemsuchthatthemaximumnoiseexposureforestablishedtimeperiodshasbeenpromulgatedbysuchlawsastheFederalOccupationalSafetyandHealthAct?
IsthemaximumsourcelevelspecifiedbyafederalagencysuchastheaircraftnoiseemissionlimitsestablishedbytheFederalAviation?
Theoperationandmaintenancerequirementsmustbecriticallyreviewed.Constraintsimposedbyrequiredaccesstointeriorparts,say,tosafetyswitchesforeitheroperationormaintenance,mustbeviewedatthedesignstage,andparticularlyifthedesigninvolvestheattemptednoisereductionlfanexistingpieceofmachinery.Consultationwithexperiencedoperatorsatthisstagecansavemanypotentialredesignrequirementsafterthedesignisfixed.
Wemustaddressthesocialaspectslftheproblem.Theestheticslfthefinaldesignshouldbeconsideredhere.Willthefinaldesignenhancethevisualparametersoftheproblem?
Willtheresultlftheacousticfixbeamoreappealingproduct?
Giantnoisecontroldevicesareoftenfacedbyengineersinvolvedinthedesignofpowerplantfacilitiestobelocatedclosetotheircustomersininhabitedareas.Theymustdecide,forexamplebetweentheforceddraftcoolingtowerwithitslargeareabutlowheightprofileandtheinherentlyquieternaturaldraftcoolingsystemwhichrequiresastructureusuallymorethan50000fthigh.Thesocialimplicationsof“nosolution"
shouldbeexaminedasagainstatotalofpartialreductionlfthesourcenoise.Tosomeextenttheno-solutionconceptwillhaveabearingontheeconomicconstraintsimposedonthedesignconcept.
Veryearlyintheapproachtoasolutiontoaproblemofcontrollinganoisesource,theeconomicconstraintsmustbespelledout.Noisecontroldevicescanoftenbeveryexpensive.Lowfrequencyattenuationismorethanthetreatmentofhighfrequencysounds.Thiscostmustbeweighedagainstbenefitstobeobtained.Noisecontroldollarsareoftenconsiderednonproductivedollarsatfirstglancebycontrastwith,forinstance,heatrecoverydeviceswhichnotonlysaveenergybutpayforthemselvesinfuelcosts.However,whenthesedollarsareweighedagainstpotentialsavingsincompensatoryawardsforhearinglossorasthepriceofagoodpublicrelationshipwithneighbors,theytakeonalessfrighteningaspect.Whenacompanywithaseverenoiseproblemwhichishazardoustotheiremployeesisfacedwitha$500perdiemfineorathreat