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  Statistical Techniques and 6 Sigma
  More GD&T

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Author Topic:   More GD&T
Tim
Lurker (<10 Posts)

Posts: 7
From:Cochran, GA, USA
Registered: Jul 1999

posted 12 August 1999 11:21 AM     Click Here to See the Profile for Tim   Click Here to Email Tim     Edit/Delete Message   Reply w/Quote
Ok guys. This one really baffled me. I was in a meeting with our vendor when the program manager said. We can't compute Cpk using GD&T. (I have since convinced most of them that you actually can, thanks to everyone's input.) He then said but we can compute Cp. I have read some previous posts comparing Cp with Cpk. But I can't for the life of me figure out why you can't (or at least they say you can't) compute Cpk but can compute Cp. Are there any differences that would keep you from making the math work.

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Batman
Forum Contributor

Posts: 111
From:Kane, PA 16735
Registered:

posted 12 August 1999 03:02 PM     Click Here to See the Profile for Batman   Click Here to Email Batman     Edit/Delete Message   Reply w/Quote
Hi Tim!
GD&T is a common set of rules for creating a part drawing with form, fit, and function in mind. Every part has some limits above and below which you cannot go. These limits are called tolerances.

Cpk (and Cp) is a statistical calculation from [usually a sample of] the actual produced parts comparing that data TO the tolerance on the drawing.

In this order:
Create drawing with tolerances, etc.
Make parts to drawing
Analyze (measure) parts - compute average and sigma
Compare this calculation to the tolerance on the drawing.

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Don Winton
Forum Contributor

Posts: 498
From:Tullahoma, TN
Registered:

posted 13 August 1999 09:23 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
quote:
Are there any differences that would keep you from making the math work?

No there are not. Both equations use mean and standard deviation, along with constants (3, 6, USL, LSL, etc.) Since you can determine mean and standard deviation, if you can compute Cp on a variable, you can compute Cpk on the same variable.

Regards,
Don

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