Process Capability Index for Mechanical Design

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

RDT

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.