Philosophy of Determining CPK for a flatness tolerance.
I have a molded component contains a surface of ~ 50 x 50 mm which has a flatness tolerance of 0.5 mm.
The part is being measured on the CMM and all I get from the CMM report is a value for the flatness between 0 and 0.5 for each part measured.
The dimension is critical so we are expected to calculate CPK.
I can not get my company, the end Customer, or myself to agree upon a satisfactory method to do so.
- My company feels that we should take the value reported by the CMM on 100 parts and calculate the CPK. My concern with this is that the CMM is only giving me a value and this value can be anywhere on the 50 x 50 surface for each given part so I am not sure I am getting a meaningful CPK result (makes me feel like I am taking the average of some averages and trying to prove something with it).
- My Customer feels that we should only be calculating the capability of each part individually - that is looking at the 25 points, or whatever, the CMM is taking on the 50 x 50 surface and calculating CPK for each part. I am not sure how this gives me process capability it will only give me 100 CPK numbers. I also don't see how this gives me process capability.
- My thought is that to show capability I need focus on 4 or 5 points on the 50 x 50 surface and calculate the capability across the 100 pieces at the exact same points generating 4 or 5 CPK numbers.
Any more thought?