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What should be Maximum Cpk value?

Discussion in 'Capability - Process, Machine, Gage …' started by Pongsakorn, Dec 27, 2016.

  1. Pongsakorn

    Pongsakorn Member

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    Anyone has any idea on how to determine the maximum Cpk to trigger for spec limit review and adjustment.
    This is the comment form customer audit that besides minimum Cpk value, the maximum Cpk value should be determined also since the customer observed that the Cpk of some process is high (2 digit) and the spec limit should be adjusted.

    Thanks for the advice.
     
  2. Bev D

    Bev D Moderator Staff Member

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    The ONLY reason to change a specification limit is that the specification limit doesn't accurately correlate with failure. Specifications are about what makes the product work as intended. Cpk is about how the process performs in relationship to the specification. If your Customer doesn't understand this - they are wrong. Pure and simple.
     
    Miner and Pongsakorn like this.
  3. Pongsakorn

    Pongsakorn Member

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    Hello Bev, thanks for your comment. I think that customer is trying to encourage for continuous improvement by tightening the spec limit to be narrower since Cpk for the current spec limit is very high. Please help to comment on this point since I have to provide the proper response to customer. I believe that the customer may insist to see the maximum limit for Cpk value.
     
  4. Golfman25

    Golfman25 Well-Known Member

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    It's stupid. You have a very capable process. I am sure your resources are better served continually improving something worthwhile.
     
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  5. Pongsakorn

    Pongsakorn Member

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    Hello Golfman25, I agree with you but sometime it is difficult to convince customer who has had the objective in mind. :(
    However, I will try.
     
  6. Andy Nichols

    Andy Nichols Moderator Staff Member

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    Tell them that any efforts your organization does to achieve this will be reflected in a price increase - that will halt their ideas...
     
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  7. Bev D

    Bev D Moderator Staff Member

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    Here's the physics answer: Tightening the specification doesn't improve the process. Reducing the actual variation of actual parts improves the variation of the process. You already have a very capable process: the actual variation is small compared to the specifications. Tightening the limits will only result in throwing parts away that might exceed the tighter limits at some point in the future (out of control conditions do occur from time to time). If the original specifications are set properly then tightening them only increases cost but not quality. Andy is correct: tell them that any tightening will result in a price increase.

    This is the kind of stupidity that gives the quality profession a bad name...
     
  8. Pongsakorn

    Pongsakorn Member

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    Hello Bev, your recommended answer sounds reasonable, I will give provide the response to the customer using your worthwhile guidance.
    Thank you so much.
     
    Atul Khandekar likes this.
  9. ncwalker

    ncwalker Well-Known Member

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    Yes. I agree with the brethren on this one.

    And will add, if you have a really high Cpk, one course of action is to move that process to a less capable machine and use the "really good" machine on a more difficult part. Possible cost savings for you. :)
     
  10. Bev D

    Bev D Moderator Staff Member

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    Unless it's not the machine. It could be the mold, the material, the feature, the fixturing, the program if CNC, the machine parameters, the tooling, etc...or all of these
     
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  11. tony s

    tony s Well-Known Member

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    Why would you set a limit for capability? If your process produces products that are very well within the specification limits then that should be maintained. If your customer wants tighter specification limits than the necessary, then you'll be exerting more effort and costs unnecessarily.
     

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