What if Cpk is 0.89

Your  process  is  not  capable and  not  centered 

Mohsin,
 
You cannot tell the reason for the low capability of your process by looking only at Cpk.  You need to examine both capability indices (Cp and Cpk) and both performance indices (Pp and Ppk) to determine the problem.
 
Cp = What is my process capable of doing assuming the process is centered and all process variation is common cause variation?
 
Cpk = What is my process capable of doing using the actual process mean and assuming all process variation is common cause variation?
 
Pp = What is my process capable of doing assuming the process is centered and using actual total process variation?
 
Ppk = What is my process actually doing using the actual process mean and the actual total process variation?
 
To determine the reason for a low process capability or a low process performance, the following comparisons are used;
 

If Cp >> Cpk – a mean centering problem exists.
 

If Cp >> Pp – a total variation problem exists.
 

If Cpk > Ppk – a total variation problem exists.
 

If Cp >> Ppk – both a total variation and a centering may exist.
 
If any of these indices are <1, your process is not capable of meeting your customer’s requirements.  Any of the indices being <1 will cause Ppk to be <1.
 
I hope this helps.
 
Jim Shelor

All 4 indices are calculated using the same set of data.  The data can be sub-grouped to calculate all four, or it can be individual to calculate all four.
You cannot use some indices calculated from sub-grouped data and other indices calculated from individual data for any single analysis of all four indices.
I am not sure what you are driving at with your question.
Jim Shelor

hello moshin,
if your Cpk is at 0.89, then definitely your process is not capable. what you could do is to check again your process based on the following :
1. shift of your distribution – is it shift going to higher side of the specs or lower.
2.  check for chronic factors involving your distribution.  factors that leads to defects.
3.  repeatability is one of the major contributions for non capability of the process.  your product may be actually within specs but your equipments of process maybe erratic and sometimes gives misleading results. Use GRR.
4.  go back to your 5m’s-1e and identify contributions leading to rejections.
 
hope these would help
 
regards.