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I need some _very_ basic definitions for Cp and Cpk.

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  • #46859

    Anderson
    Participant

    Hello,
    I am formulating a process capability training program here at work.Β  The goal of this training is to get everyone in the plant familiar with PC / SPC and why it is important.Β Β I can’t get too techinical though.
    What would your most basic / simple definition of Cp and Cpk be?Β  I stated that Cp refers to the potential of a process to meet customer specs… what about Cpk?Β  Any thoughts / comments welcome.
    Β 

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    #155512

    Allthingsidiot O
    Participant

    CpΒ  isΒ  forΒ Dispesion andΒ  CpkΒ  forΒ  Centering (inΒ  brief).

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    #155520

    Jim Shelor
    Participant

    Cpk is the potential of the processΒ to meet the customer specifications at the current mean of the parts being produced by the process.

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    #155523

    GB
    Participant

    Ballistic analogy:
    Cp= “Shotgroup”Β Β  How tight are your shots? (10/10 shots all closely clustered together…may not be centered on the bullseye, but tight cluster…can be “mean-shifted” onto the bullseye)
    Cpk = Ability to hit the target. (10/10 shots hitting the target, but some bullseyes, some at the edges, some in the 8-ring, etc…)

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    #155524

    Trev
    Member
    #155526

    luke skywalker
    Participant

    One explanation that always made sens eto me is this:
    Imagine you’ve just purchased a new truck and are about to park in your garage.
    Cp answers ‘Does the truck fit in the garage?’ (Does our process variation fit within the specifications?)
    Cpk answers ‘Where is the truck located within the garage?” (Is our process centered between the specification limits?)
    Hope that helps!

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    #155553

    Erik L
    Participant

    Mike,
    Cp=”Could” the process fit within the VOC?
    Cpk=”Does” the process fit within the VOC?
    Regards,
    Erik

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    #155554

    Chris Seider
    Participant

    Let me add….
    “on a short term basis without special causes typically found in most processes.”

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    #155556

    Obiwan
    Participant

    “on a short term basis without special causes typically found in most processes.”
    I concur with this, with one exception…you should have removed the special causes BEFORE you attempt to calculate process capability.Β  You should always assure that the process is in control before evaluating for capability.Β  Stability before capability…

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