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Process Capability Index – Cpk

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  • #44795

    Ron Ellison
    Member

    Hi Guys
    Calculating a single Cpk value for an upper or lower specification is generally straight forward – apply the formula
    When the process has more than one specification  element, say two or three, the object has a height, a diameter and weight specification each with a upper and lower acceptable value.
    Is it possible to calculate one Cpk value for the product or are they expressed individually, this is – a height capability, a weight capability and a diameter capability.
    What is the formula for combining Cpk values
    Appreciate any help on this issue
    Ron

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    #144338

    marie
    Participant

    Cpk per characteristic should be calculated.  If this is a machining process where multiple characteristics are generated on a single part, each chara. should be studied & capability determined according to standard statistical methods. ( even if on a mutliple station machine) Any chara., not in control, is adjusted and re-studied. 
    Cpk of differing characteristics are not combined to get an overall Cpk.  The process is no better than the worst Cpk of the like status (importance of) characteristics.
    apples & oranges
     

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    #144378

    boettler
    Member

    Need to have a Cpk for each because each is considered a defect opportunity.

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    #144379

    FL
    Participant

    Cpk is the index measure your process capability to achieve certain target. Therefore, when you perform a Cpk estimation using Minitab, you only need to key in the following
    1. USL
    2. LSL
    3. Target
    4. Historical mean (optional)
    5. Known std dev (optional)Various specification integrated in the process will affect capability to achieve the “Out Come” You need to perform a DOE to optimize the process parameter setting, then only you collect data for Cpk stduy.

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    #144405

    Ron
    Member

    Thanks all for your comments – much appreciated

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    #144638

    k.bhadrayya
    Participant

    Dear Ron Ellison
    If I can get your mail ID , I can attach my document where in the defects are measured by mesurement and count of defects for various caharacteristisc and calculated the overall Cpk value of the product. This will be self expalinatory. How to account  the individual caharacteristic Cpk values to obtain an overall value of a product.
    Thanks
    K.Bhadrayya
     
     

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    #144743

    Ron
    Member

    Hi K
     
    If you can post a fax number I can reach you on or fax your email to me on 61 7 47470445
     
    THANKS

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    #144777

    Mikel
    Member

    Wrong!
    This is as dumb and Praveen’s conjured sigma score.

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    #144782

    Eric Maass
    Participant

    In terms of its effect on composite yield, the composite Cpk could be approximated by min(Cpk1, Cpk2, …, Cpkn)
    Skip Creveling’s company, PDSS, came up with a Capability Growth Index for Cp and for Cpk; if I recall correctly, the CGI for Cpk is something like:
    CGI = 100% * Sum from i to n of  { Min(Cpki, 1.5)/1.5 } / n   where n is the number of critical parameters.
    So, CGI ranges from 0 to 100%, and is clamped by 1.5 at the upper end (and presumably clamped by 0 at the lower end).

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    #148179

    ssstudent
    Member

    Hi Ron,
    Cpk whenever calculated has to be done seperately for each specification element. It is the measure of capability of process to produce part within specfication w.r.t that particular element.
    Ans these cannot be combined together.
     

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