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I’m within the specification, why should I do Control Chart ?

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  • April 8, 2022 at 5:58 am #256917

    AlexandreRICHARD
    Participant

    Hello,

    With my specification chart, I can see that everything is within the specification.

    But with my control chart, I can see that my process is out of control.

    Why should I put energy and time on those variabilities if I’m within the customer specifications ?

    What will be the benefits ? Why should I care about variability (if my goal is to be within the spec) ?

    Thank you

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    April 13, 2022 at 8:02 pm #256982

    Chris Seider
    ★ 10 Years ★
    Participant

    If you have zero defects, then you wouldn’t need it. It would be hard to imagine zero defects over a long period of time if you show a lack of control for the short amount of time captured in your chart.

    If you had a Cpk and Ppk of 2+, then go elsewhere first.

    Please be aware that specification limits aren’t used for control chart limits–they are calculated based on the process performance.

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    April 24, 2022 at 12:24 pm #257094

    Ken Feldman
    ★ 10 Years ★
    Participant

    As an analogy….once you reach your desired weight after a diet, do you stop weighing yourself? No. You keep monitoring your weight to be sure it doesn’t change. The same would be true if you reach your desired blood pressure because you are taking drugs to help you. You would still periodically take your blood pressure to make sure nothing changes.

    That is why you continue to monitor your process with a control chart even if your process shows you are currently within your customer specs. You may not collect data on as frequently, but you would still monitor to be sure nothing changes.

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