We generally use Cpk to identify the process capability of a process and in six sigma projects the Cpk should be greater than 1.33.

 

If Cpk is there to define process capability than calculating Ppk is really important in the project?

I reckon you know about the short/long term difference between Cpk and Ppk. Thus, from a customer perspective, the Ppk is much more important, because it provides the scrap rate in the long run. It describes the actual scrap rate in production, when all variance components (raw material, operators, different machines etc.) are included.