Re: Cpk Calculation
My customer wants us to hold 1.33 & 1.67 Cpk on 2 dimensions respectively. We have not made any parts yet and I have to setup our inspection paperwork to reflect a tolerance that will hold the 1.33 & 1.67 Cpk and submit to them. How do I know what tolerance I will need to hold to maintain the 1.33 & 1.67 Cpk respectively? Help!
Gianni, I am not sure if you have already discussed this with your customer, but it is very unlikely (at least in my field in automotive) for a customer to change a product tolerance for the sake of showing a capable process.
The rest of the technical how-to is well explained by the other people in the forum:
1) confirm that you have a capable measurement system for this characteristic
2) run a process trial to understand the variability of this characteristic
3) confirm stability, normality, remove outliers
4) calculate the standard deviation
5) calculate what dimension tolreance you should have if Cpk had to be 1.33 or 1.67
One final comment (hopefully not too disruptive - this is generally quite confusing) is why your customer is asking you to demonstrate Cpk if the process is new. Are you sure it is not Ppk?
Javier