Hi to all!
First of all i would like to restate that
"there has been no unified formal approach in the
automotive industry on statistical process control"
- AIAG SPC Manual
Cpk - The capability index for a stable process. The estimate of sigma is based on within subgroup variation.
Ppk - The performance index. The estimate of sigma is based on total variation.
For an entirely new process, your process is not stable since you are just evaluating/studying the process. Meaning Cpk should not be calculated and an initial process study should first be conducted.
The purpose of the initial process study is to understand the process variation, not just to achieve a specific index value.
After you understand the process variation, you should Identify, evaluate and, wherever possible, eliminate special causes of variation.
Compute for the Ppk to understand if your current process is capable to meet the customer's requirement, Cpk.
Think of this, if you achieved a Ppk > 1.67 from your initial process study,
what more it would be in a stable process to achieve Cpk > 1.33.
:truce: