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 Author Topic:   Capability Stuff Marc SmithCheech Wizard Posts: 4119From:West Chester, OH, USARegistered: posted 23 November 1999 12:09 PM                Subject: Inspection Criteria RE1Date: Wed, 15 Sep 1999 09:20:54 -0400 (EDT)From: Greg Gogates iso25@fasor.comTo: iso25@quality.orgDate: Tue, 14 Sep 1999 23:04:58 +0200From: "Frans J.C. Martins" fjcm@icon.co.zaTo: Greg Gogates Subject: Re: Inspection CriteriaDear Group,SNIP> The most common use in the US seems to be if the uncertainty (k=something,> usually 2) is less than some fraction of the tolerance, commonly 25% (10%> traditionally, 25% in Z540, 33% in ISO 10012, ...) then the tolerance is used> as stated. This corresponds to the ISO 14253 standard used with k=0. I admit> that using the expanded uncertainty with k=0 doesn't make sense at first (at> least it didn't to me when Ralph Veale explained to me how k=0 would be a good> idea), but in the language of ISO 14253 it is actually a sensible description> of current US practice.SNIPI believe that the 33% limit is obtained by assuming the upper and lower spec limit to be 3 sigma. If we relate this back to SPC practically:Specification: 100 ± 1the ± 1 is assumed a three sigma value and the 100 as the mean, now.........ZONE A (Stable zone) would be the mean ± 1 sigma thus 100 ± 0.333ZONE B (Warning zone) would be the mean ± 2 sigma thus ± 100 ± 0.667ZONE C (Action zone) would be the mean ± 3 sigma thus 100 ± 1This theory then also traces back to the Process capability indicescalculation which are given by Relative capability or Process capabilityCp = USL - USL / 6 sigma NOTE: to manufacture within specifications the distance between the USL (upper specification limit) and the LSL (lower specification limit) must be less than the width of the base of the normal distribution thus the distance between -3 sigma and + 3 sigma) If Cp is < 1, the process is not capableCp = 1, the process is marginally capableCp > 1, it indicates a potential for higher capabilityandCapability indexCpk = the lesser of: USL - mean / 3 sigma or mean - LSL / 3 sigmaIf Cpk < 1, the process is incapableCpk = 1, still some undetected non-conforming outputCpk > 1 but < 2, still some non-conforming output but with a good chance ofdetectionCpk > 2, total confidence in the process.I trust that this helps and makes some sense.Regards in QualityFrans J.C. MartinsSouth AfricaIP: Logged Don WintonForum Contributor Posts: 498From:Tullahoma, TNRegistered: posted 23 November 1999 09:10 PM                Good Post. I agree.Regards,DonIP: Logged
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