Safety Regions in Process Capability Plots

摘要

Aprocessisusuallydefinedtobecapableiftheprocesscapabilityindexexceedsastatedthresholdvalue,e.g.,Cpm4/3.Thisinequalitycanbeexpressedgraphicallyasaregionintheplanedefinedbytheprocessparameters(μ,σ).Intheobtainedplotspecialregionscanbeplottedtotestforprocesscapability.Theseregionsaresimilartoconfidenceregionsfor(μ,σ).ThisideaofusingregionsinprocesscapabilityplotstoassessthecapabilityisdevelopedfurtherforthecapabilityindexCpm.Anewcircularregionisconstructedthatcanbeused,inasimplegraphicalway,todrawconclusionsaboutthecapabilityoftheprocessatagivensignificancelevel.Usingcircularregionsseveralcharacteristicswithdifferentspecificationlimitsanddifferentsamplesizescanbemonitoredinthesameplot.Undertheassumptionofnormalitythesuggestedmethodisinvestigatedwithrespecttopoweraswellascomparedtootherexistinggraphicalmethodsfordrawinginferenceaboutprocesscapability.