Calculating Cpk for Geometric Tolerances at MMC or LMC (#166)
(In accordance with Y14.5M-1994 standard)
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The question of calculating Cpk on geometric tolerances modified at MMC and LMC continues to come up. Last month’s Tip showed how to inspect features of size where the geometric tolerance is modified at MMC or LMC without having to find the axis or center plane. The geometric tolerance in these cases is a single limit control. Once that is realized, it is easy to calculate a Cpk index on such geometric tolerances. In this case, polar measurements are made to assure that the surface of the hole does not violate the inner boundary. The inner boundary is 20 - 0.1 - 0.6 = Ø19.3. The surface of the hole may not be closer to the basic location than R9.65 mm which is the radius of the inner boundary. Recording the distribution of the closest point relative to the inner boundary of corresponding holes on multiple parts, provides the data necessary to calculate Cpk of the process for the position tolerance of each hole. Any $15 calculator with statistical functions may be used to calculate the average of the data, a standard deviation and the resulting Cpk.
