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What should be action taken on "process" and "product" when the Cpk <1.00?

Discussion in 'SPC - Statistical Process Control' started by Pongsakorn, Sep 20, 2019.

  1. Pongsakorn

    Pongsakorn Member

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    When Cpk is less than 1.00, it means that the process is incapable, please comment if the action to be taken as follows are proper
    - In term of Process
    Shutdown the process that is Incapable until Cpk is improved to be greater than 1.00.
    - In term of Product
    To do 100% inspection/test on product until Cpk improved to be greater than 1.00.
    (for this case, if the test is "Destructive", it will not be practical)
     
  2. Golfman25

    Golfman25 Well-Known Member

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    I suppose you need to figure out why the process is not capable. Was it never capable to begin with? Or was it capable and something changed?
     
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  3. Bev D

    Bev D Moderator Staff Member

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    Well if you shut the process down, you don't need to inspect the product.
    If you are in development some Customers will require that you become 'capable' before launch. If you are capable and an assignable cause occurs (out of control process) it is in your best interest to determine the cause and correct it quickly. But an out of control process doesn't always go so out of control that it creates defects.

    100% inspection is never required just because you are creating defects. 100% inspection is based on the severity of the defect. Remember, even a capable stable process can go out of control and produce defects at a unpredictable rate. SPC and acceptance sampling are used for this...

    Also remember that a Cpk of 1 means that the 6 sigma process spread matches the tolerance spread and even if you have perfectly Normal distribution it is unlikely that you are actually making too many defects; the idea that there are parts beyond the mean +3SD is a theoretical hedge against the theoretical infinite tail.
     
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  4. Pongsakorn

    Pongsakorn Member

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    Dear Bev, this is from customer defining that process Cpk less than 1.00 is Incapable Process and I guess that customer will need to shut down that machine giving Cpk less than 1.00 and use other machine instead.
    Bev, I will be so happy if my customer is knowledgeable and reasonable like you are.
     
  5. Gejmet

    Gejmet Member

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    Dear Pongsakorn,
    If the process you have described in stable (many people read off and quote capability indexes when the process is not) then for spread the process is doing the best it can. To do better would require a major change to the process. if the cpk = cp (process stable) then location-wise the process is doing the best it can because its on target.

    If the process is not on target it may be, depending upon the process, relatively easy to reset. Once Cp = Cpk for a stable process is achieved, if the index is still not good enough for your customer then a major change is necessary.

    Get the process stable and on target, this is the best that you can economically do. In other words get the best out of your current process if you are not already.

    Switching production to another machine sets off the whole validation process again.
     
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  6. Bev D

    Bev D Moderator Staff Member

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    I would clarify that not all stable processes will require a 'major' change to improve their capability. sometimes it's easy quick and cheap. sometimes it's difficult time consuming and expensive.
    the opposite is also true: some out of control conditions are easy, fast and cheap to correct and some are difficult, slow and expensive to correct.
     
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  7. Gejmet

    Gejmet Member

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    I think its maybe a play on the word "major". The important thing to say is that a stable process which is on target is displaying that it only contains random variation and therefore if within this state its not meeting capability it will always be more difficult and usually more costly in my experience to change the process so that there is less random variation whilst keeping it on target.

    Most folks have enough issues controlling special cause variation and getting the process stable let alone with capabilities that dont meet some target. Of course some folk adjust processes when only random variation is in existence therefore just increasing variation which is clearly a wastefull activity and this is my essential point.
     

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