Yes,
I'm surprised that your vendor is surprised as, no doubt, acceptable Cpk
limits would have been clearly defined in your purchasing
specification......
Lee parker wrote:
> Ryan
>
> Put Cpk into your search engine and you should get many references.
>
> I imagine the reason for this low Cpk was that the standard deviation was
> large even though the average was with in range. This indicates a poorly
> controlled process that has an unacceptable likelihood of producing
> unacceptable product, which is the purpose of the measurement.
>
> Best regards
>
> Lee
>
> J. Lee Parker, Ph.D.
> JLP Consultants LLC
> 804 779 3389
>
>
> ----- Original Message -----
> From: "Ryan Grant" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Wednesday, March 28, 2007 11:39 AM
> Subject: [TN] Explanation of Cpk (Ppk)
>
>
> Hi all,
>
> I have a supplier that is wondering why we rejected a shipment for
> finding a critical dimension as having a Cpk of 0.65 (Ok, technically,
> we are calculating Ppk). None of the 45 measurements were actually out
> of spec (although one measurement just missed exceeding the upper limit
> by 0.002mm). Now they want to know why we are rejecting for low Cpk.
> Sigh...
>
> Rather than go through the tedium of carefully explaining what a
> distribution is, what it means, how to calculate it and what the
> calculation implies, etc., does anyone know of some websites I can point
> them to that can explain this?
>
> Any help is appreciated.
>
> Thanks,
> Ryan Grant
>
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--
Eric Christison Msc
Mechanical Engineer
Home Personal, Communication Sector
Imaging Division
STMicroelectronics (R&D) Ltd
33 Pinkhill
Edinburgh EH12 7BF
United Kingdom
Tel: +44 (0)131 336 6165
Fax: + 44 (0)131 336 6001
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