ProcessCapability Cp Cpk Pp Ppk GlobalTrainingMaterial Creator GlobalMechanicsProcessManagerFunction MechanicsApprover GaryBradley GlobalProcessTeamDocumentID DMT00018 ENVersion Status V 1 0 ApprovedLocation Notes NMP DOCMANR4 PCP PCProcessLibraryDocManChangeHistory IssueDateHandledByComments1 021stDec 01JimChristy S renLundsfrydApprovedforGlobalUseNOTE Allcommentsandimprovementsshouldbeaddressedtothecreatorofthisdocument Contents SectionHeading DescriptionPage1Variation TolerancesandDimensionalControl42Population SampleandNormalDistribution153CpandCpkConcept284UseoftheNMPDataCollectionSpreadsheet445ConfidenceofCpk52 ProcessCapability EvaluatingManufacturingVariation AcknowledgementsBennyMatthiassen NMPCMT Copenhagen Denmark FrankAdler NMPAlliance Dallas USA JoniLaakso NMPR D Salo Finland JimChristy NMPSRC Southwood UK Section1Variation TolerancesandDimensionalControl TwoTypesofProductCharacteristics Variable Acharacteristicmeasuredinphysicalunits e g millimetres volts amps decibelandseconds Inthistrainingwedealwithvariablesonly TheSourcesofProcess SystemVariation Process TwoTypesofProcesses Allprocesseshave Natural random variability duetocommoncauses StableProcess Aprocessinwhichvariationinoutcomesarisesonlyfromcommoncauses UnstableProcess Aprocessinwhichvariationisaresultofbothcommonandspecialcauses Unnaturalvariability duetospecialcauses Shewhart 1931 TheTwoCausesofVariation CommonCauses Causesthatareimplementedintheprocessduetothedesignoftheprocess andaffectalloutcomesoftheprocessIdentifyingthesetypesofcausesrequiresmethodssuchasDesignofExperiment DOE etc SpecialCauses Causesthatarenotpresentintheprocessallthetimeanddonotaffectalloutcomes butarisebecauseofspecificcircumstancesSpecialcausescanbeidentifiedusingStatisticalProcessControl SPC Defect USL LSL nominalvalue Tolerances LSL lowerspecificationlimit 10 7 USL upperspecificationlimit 10 9 Acceptablepart RejectedPart RejectedProduct Nominal10 8 0 1 RejectedPart Atoleranceisaallowedmaximumvariationofadimension MeasurementReport Inmostcaseswemeasureonlyonepartpercavityformeasurementreport ExampleofCapabilityAnalysisData Forsomecriticaldimensionsweneedtomeasuremorethan1partForcapabilitydataweusuallymeasure5pcs2times hour 100pcs butsamplingplanneedstobemadeonthebasisofproductionquantity rundurationandcycletime ProcessCapability Whatisit ProcessCapabilityisameasureoftheinherentcapabilityofamanufacturingprocesstobeabletoconsistentlyproducecomponentsthatmeettherequireddesignspecifications ProcessCapabilityisdesignatedbyCpandCpk ProcessPerformanceisameasureoftheperformanceofaprocesstobeabletoconsistentlyproducecomponentsthatmeettherequireddesignspecifications ProcessPerformanceincludesspecialcausesofvariationnotpresentinProcessCapability ProcessPerformanceisdesignatedPpandPpk WhyMakeProcessCapabilityStudies Aprocesscapabilitystudywouldrevealthatthetoolshouldnotbeaccepted Whenadimensionneedstobekeptproperlywithinspec wemuststudytheprocesscapability butstillthisisnoguaranteefortheactualperformanceoftheprocessasitisonlyaninitialcapabilitystudy TheNokiaProcessVerificationProcess Section2 Population SampleandNormalDistribution TheBellShaped Normal Distribution Symmetricalshapewithapeakinthemiddleoftherangeofthedata Indicatesthattheinputvariables X s totheprocessarerandomlyinfluenced PopulationParameters Populationmean Populationstandarddeviation PopulationversusSample PopulationAnentiregroupofobjectsthathavebeenmadeorwillbemadecontainingacharacteristicofinterest SampleThegroupofobjectsactuallymeasuredinastatisticalstudyAsampleisusuallyasubsetofthepopulationofinterest TheNormalDistribution WhatMeasurementsCanBeUsedtoDescribeaProcessorSystem mean average ordescribesthelocationofthedistribution m ameasureofcentraltendency isthemeanoraverageofallvaluesinthepopulation Whenonlyasampleofthepopulationisbeingdescribed meanismoreproperlydenotedas x bar Themostsimplemeasureofvariabilityistherange Therangeofasampleisdefinedbyasthedifferencebetweenthelargestandthesmallestobservationfromsamplesinasub group e g 5consecutivepartsfromthemanufacturingprocess WhatMeasurementsCanBeUsedtoDescribeProcessvariation sST oftennotatedas orsigma isanothermeasureofdispersionorvariabilityandstandsfor short termstandarddeviation whichmeasuresthevariabilityofaprocessorsystemusing rational sub grouping whereistherangeofsubgroupj Nthenumberofsubgroups andd2 dependsonthenumberNofsubgroupsandthesizenofasubgroup seenextslide WhatMeasurementsCanBeUsedtoDescribeProcessvariation d2 valuesforSST Example WhatMeasurementsCanBeUsedtoDescribeProcessvariation TheDifferenceBetweenSSTandsLT ThedifferencebetweenthestandarddeviationssLTandsSTgivesanindicationofhowmuchbetteronecandowhenusingappropriateproductioncontrol likeStatisticalProcessControl SPC Short termstandarddeviation Long termstandarddeviation ThedifferencebetweensSTandsLT ThedifferencebetweensSTandsLT ThedifferencebetweensLTandsSTisonlyinthewaythatthestandarddeviationiscalculatedsLTisalwaysthesameorlargerthansSTIfsLTequalssST thentheprocesscontrolove