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Re: What Is The Highest Index Of Cpk?

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  • #30819

    Hong
    Participant

    I think my case is closer to what Ram said. Actually it is a plastic injection molding process. CMM is used in measuring this dimension. The dimension is 456.53, Tolerance is +/- 1.57. The mean captured for 8 samples is 456.602. Std Dev =0.001, whereas Range =0.004.
    Actually I just not sure / convinced that the process can be so precise controlled as this just a case part which produce under a normal environment (non air-conditioned) in an Asia country.
    So, is that means there’s don’t have a definite index for highest Cpk should reach?
    Pls advice, thanks.
     
     

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    #80824

    Gabriel
    Participant

    Hong:
    As said before, there is no limit for Cp/Cpk values. Imagine a turning process that has a spread (6 sigmas range) of 0.05. Give me a tolerance of 0.05 and you’ll get Cp=1, give me a tolerance of 50 and you’ll get Cp=1000, with the same process.
    However, there are some strange things in what you tell. For Cp/Cpk, sigma is calculated from “within subgroup” variation, and that means that you need several subgroups of several parts each (for example, 20 subgroups of 5 parts). Also, stability is a prerequisite for Cp/Cpk, which are meaningless numbers if the process is not stable. That means that you must plot those several subgroups in a control chart and find no out-of-control signal, and that those subgroups should be taken in a time range large enough to let all the possible sources of variation to act on the process. I wonder, how did you do all that with only 8 parts?
    By the way, I do not know your process, but a sigma of 0.001 in a characteristic that is 456 is only a 0.0004% of variation over the nominal, which looks too small for a plastic injection.

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    #80833

    Carl H
    Participant

    I agree with Gabriel on all of his points.  The centering and small variation (SD) is amazingly low…
    If you do calcualte Cp or Cpks, be careful in quoting them directly when only 8 measurements used.  In your case, the Cpk might be estimated at ~499 with a lower 95% confidence at only 280.  If you come to find the real SD is 0.100 not 0.001, the Cpk estimate would be ~5.0 and 2.8 respecivly.
    Regards,
    Carl

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    #80837

    Gabriel
    Participant

    Yes, be carefull… a Cpk=499 might be Ok, but 280…
    (just a joke)

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