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  Measurement, Test and Calibration
  CPK on Profile GD&T

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Author Topic:   CPK on Profile GD&T
brossbach
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posted 05 March 2001 11:17 AM           Edit/Delete Message   Reply w/Quote
I hope to make this clear. I need to know on a GD&T perspective. Is it possible to a CPK study on a profile. If it can be done ,can you explain to me or quide me on how to perform this task.

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Paul F. Jackson
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From:Livonia, MI USA
Registered: Feb 2001

posted 08 March 2001 10:30 AM     Click Here to See the Profile for Paul F. Jackson   Click Here to Email Paul F. Jackson     Edit/Delete Message   Reply w/Quote
It is very simple.

Pick a responsible number of points on the feature to examine and compare the displacement of each point from the basic profile +/-. Establish control limits for each point location (or all point locations if you group them into one control study) after a sufficient number of instances have been measured and determine whether their dispersion is "in-control." After it is determined that they are "in-control" figure out what size tolerance zone is required to contain each point (it will be different for every point). That is the profile deviation or consumed tolerance for each point. Process capability will compare that consumed tolerance to the tolerance specified on the drawing.

The rest of this explanation is copied from another thread.

Profile has a target value of zero and may either be constrained (controlling location or orientation and form) or unconstrained (controling form only). One can simply compare the distribution of consumed tolerance to the specified tolerance available to compute the Pp, Cp and compare the mean of the measured data to the Upper Limit to compute the Ppk, Cpk.

Profile tolerances that are not targeted nominally such as a profile that has a tolerance zone specified unequal bilateral or unilateral are a little trickier. To compare the measured data to the specification tolerance for process Capability analysis one must portray the size of the tolerance zone required to contain the worst deviation. Consider a profile tolerance that is depicted unequally bilateral -0.2 ~ +0.6 from its basic profile and is declared a total value of 0.8 in the feature control frame. If the worst deviation (the one nearest or beyond the given limits) was -0.1, one must determine what size tolerance zone could contain that point as it relates to the total tolerance 0.8. I assert that the consumed tolerance must be portrayed as 0.6, which is twice the deviation from the mean of the available tolerance. Likewise if the tolerance was depicted unilaterally 0.0 ~ +0.8 and the worst deviation was -0.2 the deviation would be reported as 1.2. Pp, Cp and Ppk, Cpk assessments for these consumed tolerances would follow the standard models.

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