Profile Tolerance and CPK



Does profile have upper and lower tolerance, example +/-0.05 mm? If one called out profile 0.1 mm, what is the spec for profile to be calculate in CPK study?


Ron Rompen

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The answer isn't as simple as yes or no. It depends (in part) on how the tolerance is called out. For example, most tolerances are bilateral - you can vary (in the case you presented) up to 0.05mm to either side of nominal. However, there are also unilateral tolerances (you can vary up to 0.10mm but only in one direction), and unequal bilateral (you can vary more in one direction than in the other; example +0.07/-0.03).
Calcualting Cp/Cpk in a profile can be difficult at the best of times. I have used Johnson curves in the past, which assume that a value below zero cannot be reached (I can't explain the math, but I know there are those here who can). Other than that, you can try calculating it as a ?0.05 tolerance and see how that works.


If the tolerance deviation is not specified in the call out then you should follow the design guidelines (usually displayed at the drawing stamp/legend based on the number of decimals) and consider it equally disposed (bilateral).

The drawing should specify "all outside" or "all inside" when unilateral tolerances are desired.
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