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MSA(MeasurementSystemAnalysis)isamethodofanalysisofmeasurementtools
andmeasurementsystemscapabilityandstabilityusedinqualityengineering.
SPC(StatisticalProcessControl)isamethodorientedtowardsstatisticalproc-
esscontroldedicatedtogatheringandpresentingdatawhichdescribevariationof
processconditionsandcharacteristics.Itisacurrent,doneinrealtime(on-line),
processcheckusedtodetectiftheprocessgetsoutofcontroland,inconsequence,
toimproveprocessqualityconstantly.ThegoalofSPCuseistoreducevariation
oninputandoutputand,nexttoremovecausesofvariations.InSPC,itistested
whatthenaturalvariationis-thatiswhatthenaturaldistributionofmeasure-
mentsis-oftheproductionprocessinprogress,andwhatitscapabilitytofulfill
therequirementsgivenbyspecificationsis.Asforquality,itisnotjustenoughto
keeptheprocessintolerance.Thereisatendencyforconstantimprovementby
minimizationofvariationandcentralizationofprocessmeanvalues.Inthiscase,
aparabolic“Taguchilossfunction”isacceptedasamodel.AccordingtoTaguchi,
themoretheproductcharacteristicdiffersfromthegiventargetvalue(depending
onproductpurposeandoptimalvalue,whichistheclient’sexpectation),thebigger
thelossis.So,itisveryimportanttosettheprocesstothetargetvalue,minimize
thevariationandthiswaylimitthelosstotheminimum.Theplotofcostscaused
bydetectionofdefectsatcertainphasesofatechnologicalprocessshowshowim-
portantpropersupervisionoftheprocessis(Fig.1.5).Lackofdetectionofadefect
inphasencausesapproximately10timesbiggercoststoremoveitinphasen+1.
Figure1.5.Costsofdefectsdetectedinsubsequentproductionstages(owncasestudy)
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