Calculating Cpk
for Geometric Tolerances at MMC or LMC
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 R9.65mm 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.
