What is the highest index of Cpk?

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    What is the highest logic value of Cpk could reach? If I have a set of measured data which give a Cpk value higher than 400, what does this means?


    Bess Kitan

    Its all depends on the design specification and operating tolerance. In your case it may be due to the big design tolerance and whereas ur process tolerance is very low. It is like having design tolerance in meters and u r operating in mm.
    Hope it is clear now.



    You can also take a look at your gage resolution. Your equipment might not be capable of detecting changes in measurements.



    – You can have an error in the calculation.
    – There can be an error in the specifications (like stating +/-2 when it should have been +/-0.02)
    – As Rams said, you can have a process that has a variation that is smaller than the resolution of the measurements. For example, in a good turning process, using an instrument with a resolution of 0.1mm will tyipically deliver R=0 for most subgroups, because the process variation is in the order of a few cents of mm that the instrument can not detect. Note that, even when you get a R=0, the “actual” range of the parts is not (can never be) zero because there IS a difference between the parts, only that you can not detect it. Having a lot of “fake” R=0 will deliver a “fake” too low Rbar. Do you have a lot of ranges = 0 in your data set?
    – As Bess said, you can have a process that is a lot more “precise” than what is needed. Sometimes, this means that you are wasting money because you could be using a less precise process but that is still precise enough and at a lower cost (less investment, less mantainance, lower cicle time, etc.).
    So, tell us. Which is your case?

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