Process Capability Index for Mechanical Design

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    Cp = .7 and Cpk = .4 for a particular feature on the part. How can I convince the designer to change tolerances on the part?



    This is a good question.

    But, first – was the manufacturer requested to adjust his process?

    If yes, were those results achieved after the adjustment?

    Were products rejected from use due to parts manufactured out-of-tolerance?

    Also, is the measurement taken critical to product’s quality?

    If the answer to both of the above is “Yes” then you may request the designer to revise tolerances.

    Ensure with designer that changing the tolerances won’t affect FFFS of the product.

    I would suggest to present all data collected and to show the strict tolerances create to company too much overhead to different groups (QC, MRB, Purchasing, etc.) that could be resolved by the change.


    Good Luck




    Robert Tipton

    Hello –

    I would like to provide some additional insight.

    1.) Assure that the Gage is Acceptable for the measurements.

    1.1) Check Increments and Resolution, Gage Resolution should be less than 10% Resolution, I recommend 5%.

    Example: +/- .010 = Total Tolerance of .020.   Increments to be less than .001 = 5% Resolution.

    1.2) Gage Accuracy, Check a Known Value at Low End and High End of Specification.  This assures the Gage is Accurate at both values.

    1.3) Gage R. & R. , ideally less than 10% of the Part Tolerance, or less than 20%, never more than 30%.

    Variable Data – Long Method 10 Pieces, 3 Operators, measured 3 times, is the preferred method.

    2.) Short Term Capability Studies, begin with 15-30 pieces, depending upon cost

    2.1) Optimize process and complete a new Capability Study, Comparing the PPk Values.

    2.2) do Design of Experiments to Improve Process, then New Capability Studies.

    2.3) After diligently trying to improve the process, than ask for relaxed tolerance.

    Hope this help




    Mike Carnell

    @sanchanmodi Why would your first response be to widen the specification? What made you ask that?



    Looking at a Cp and Cpk values that low, I would first look at the specification limits to see if they are realistic for the process involved.  If they are, you have to reduce variation in the process to achieve an acceptable Cp value.  Once that is accomplished, if the process is not centered, you may want to ask for a tolerance shift to center the process.  Most times manufacturers are reluctant to open tolerance ranges, but are somewhat receptive in shifting the tolerance up or down.

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