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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 Active 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.
     
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  3. Pongsakorn

    Pongsakorn Active 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 Active 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 Active 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.
     
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  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. :)
     
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  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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  12. charanjit singh

    charanjit singh Member

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    Does the your customer rep have some idea about process capability? Does he/she know the difference between Cp and Cpk and how you calculate the latter? May be you have to educate the person a little. And if he/she is already knowledgeable is simply trying to push you to keep reducing sigma as far as his/her imagination goes, you should highlight how the cost is going to increase exponentially beyond a point.
     
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  13. Andy Nichols

    Andy Nichols Moderator Staff Member

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    Good luck with that! My experience of customer auditors is that they often have no clue and are not open to the idea that they might actually learn something by keeping quiet for once and hearing what the supplier has to say. As much as people rail on CB auditors, there's no oversight of customer auditors and therein lies a potentially huge problem...
     
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  14. ChR1sT

    ChR1sT New Member

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    Cpk problem.jpg

    @Bev D
    In the electronics test industry, we very often set Test Limits based on empirical data. You may well say "that's crazy, there MUST be a spec!". Well consider this; a unit functioning window for a certain parameter is 2.1 (LSL) and the upper point in the window is 3.1 (USL) though the measurement results across N:>18,000 discrete units is as follows:
    • Mean: 2.349
    • Median: 2.349
    • Std_Dev: 0.007381
    • Cpk: 11.25
    • The LSL is 33.751 Std_Dev from the Mean
    • The USL is 101.741 Std_Dev from the Mean
    • Using Kolmogorov-Smirnov the data is Normal (p>0.05)

    This sounds great to most QA savvy folk, however, when looking at the data I saw that there is a single outlier at 3.075. This outlier is 98.327 Std_Dev from the Mean!
    Now despite the fact that the system can operate at this abnormal level, one must ask "why is there such an outrageous result?" and is it actually good to ship. Noting that electronic components can shift rapidly over time in early and late life the classic "Bathtub Curve".

    To bring the point home I ask folk; if this was a parameter of a pace-maker, "would you want this unit embedded in you?".

    Clearly something (special cause) is "wrong" with this unit and no judicious person would want it.

    You may then say that the initial LSL and USL are incorrect, however these are the actual limits (window) for acceptable "operation".

    See the attached image.

    Your thoughts?
     
  15. Bev D

    Bev D Moderator Staff Member

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    I’m not really sure what you’re asking…. Are you concerned a bout how the ‘specifications’ are set? Or are you concerned about the extreme value? We really don’t have enough information to determine if the extreme value is OK or not. Physics is impervious to opinion, ego and stupidity.

    I would with the minimal information we have at least strongly suggest either:
    (1) Investigate why the extreme value occurred and if it is real does it matter?
    (2) Investigate whether it matters.

    THEN we can discuss the larger issues….
     
  16. ChR1sT

    ChR1sT New Member

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    @Bev D Thanks for responding :)

    Here is the question: "If I was to offer you a pace-maker and you knew that one of the electrical characteristics was a clear outlier though at the moment functionally "In spec", though undeniably abnormal would you want this unit implanted?"

    A feature of electronics parts is that they can initially be "in spec" though "on their way out"! In other words "being in-spec is not enough, the parts must be normal as well. You may suggest to bring in the spec, however from batch to batch and certainly from vendor to vendor the Means can move about significantly.

    Something I've noticed is that formal training on Stats and Quality tends to be based on mechanical/physical objects, whereas in electronics manufacturing (IC's, PCBAs etc) a different approach is needed. Here is an example:

    regards
    Chris
     
    Last edited by a moderator: Aug 25, 2023
  17. Andy Nichols

    Andy Nichols Moderator Staff Member

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    Its looking like a sales pitch, Bev…
     
  18. Miner

    Miner Moderator Staff Member

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    This could also be an artifact of some of the practices of electronic component manufacturing that are not seen in mechanical manufacturing. I speak of the practice of sorting components into different levels of precision. You do not get a normal distribution. If you order a resistor with precision tolerance, you get a slice from the middle of the distribution. If you order a resistor with the standard tolerance, you get what started as a normal distribution, but with a slice missing from the middle. Not knowing the detailed practices, it is possible that the upper portion of this distribution is separated from the lower portion into different lots. What you show might have been the result of one component from one lot mixed with the components from the other lot. Yes, there is a lot of speculation here, but enough basis to launch an investigation. If it turns out to be the cause, then it would explain how the outlier occurred and why it would be low risk.
     
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  19. Bev D

    Bev D Moderator Staff Member

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    My response hasn’t changed. And although I am an electronic engineer with a major and considerable experience in semiconductor manufacturing and design as well as substantial experience in medical device design and manufacturing I can say that you have not provided enough information to make an informed decision regarding your exact question. We will not be baited…

    I will say that Stats training in general doesn't’ really go beyond enumerative theoretical homogenous situations. There is nothing special about electronics in that regard. Electronic devices tend to have a lot of non-homogeneity (as does any batch processing and other disciplines.). “Normality” is a concept that has no magical properties.

    I suggest you read Deming’s “On Probability as a Basis for Action” before you make any more “statistical” conclusions…It’s FREE - just ‘Google it’ and down load the PDF and read it. Then read it again. Then come back here…
     
  20. Bev D

    Bev D Moderator Staff Member

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    In addition to Miner’s response I have an example of how all engineers must be knowledgeable about variation and the realities of the components used in their designs.
    I was the Quality Director for a division of a large electronics designs and manufacturing organization. An engineer used a small ‘prototype’ device to help design his circuit. He set the design space according to the distribution of the samples he got early in the design phase. He didn’t use the data sheet specification. Of course you know the rest of the story. When we began production the boards didn’t work. The new batch had shifted (safely within the data sheet specifications but with only a small amount of overlap with the original samples. A simple case of non-homogeneity