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CpK determination

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  • #51257

    Lost it!
    Participant

    I’ve lost it.
    Maybe somebody can help me out.
    First of all I’m not a star in statistics, so maybe I’m not making sense at all.
    In the calculation of CpK. What standard deviation do I use? Do I use the standard deviation from the measurement system alone (determined by repeatedly measuring a standard) or do I use the total standard deviation that includes the variability of the property I am measuring plus the deviation caused by the measurement itself?
     
    Thanks

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    #177385

    Found it
    Participant

    Neither. Use the estimate of the standard deviation from Rbar/d2 obtained from the control chart.

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    #177387

    Ron
    Member

    First the formula for Cpk assumes that you know the actual process Cp (otherwise known as Pp) if you don’t have long term data take a sample of thriy to 100 pieces and measure them. Calculate the standard deviation of this large sample size.
    Once you have establish the proces capability you can calculate your capability ratio.
    If you use only a samll sample you are risking accuracy and inference erros.
     

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    #177388

    mcintosh
    Participant

    Found it is correct.  Ron is not.  See the three part explanation that starts with this one:  http://www.spcforexcel.com/ezine/oct2004/oct_2004.htm

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    #177393

    Lost it!
    Participant

    But which variation?
    I have two.
    1) variation originating from the measurement system
    2) variation in the object I’m measuring
    (and ofcourse total variation)

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    #177396

    Szentannai
    Member

    Hi,
    in principle you would want to use the process variation (variability of the product) and this variability will unfortunately include the variability of the measurement system. How you estimate the product variability is another story – but it will always include the measurement noise as well.
    Regards
    Sandor

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    #177400

    Ron
    Member

    first do a gage R&R if your measurement system is okay you always measure the part to part variation of the process.

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    #177405

    Derya
    Participant

    Hi;
    Before calculatin cpk, first of all you must make sure that %contribution of mesaurement system is <%1. Ýf not, you must improve it and then calculate cpk.
    If measurement system is ok, then you must use pooled standard deviation.
    Derya

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    #177411

    Lost it!
    Participant

    Thanks you all!!!
     
    I was on the right track afterall. I was already in the process of evaluating my measurement system in a Gage R&R but somehow got completely confused in all the statistics. It cleared up after a good night sleep and your input.
     
    Thanks!!!

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    #177414

    Mikel
    Member

    There is no requirement to have % contribution < 1%.Where do you come up with this number?

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    #177418

    Ron
    Member

    Wrong!
     

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    #177433

    Derya
    Participant

    Hi Stan,
    According to you, what is the requirement? and is it possible to mesaure process capability rightly without an acceptable measurement system.
    Thanks for your opinion

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    #177434

    Craig
    Participant

    I think Stan’s issue was with 1%.  The statement should have said less than 10% is preferred. (R&R/Tolerance, or P/T ratio)
    Before you release a measurement system to a production area, you should do a 5 part MSA. (Repeatability, reproducibility, linearity, bias, and stability).
    When you estimate cpk, the measurement variation (which as been deemed to be low I hope) is included in the estimate of your standard deviation.

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    #177438

    Derya
    Participant

    % contribution ( % PD, or variance of MS/variance of process)

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