When is a process considered to be capable?

[DJN]

Capable Process

When is a process considered to be capable? is it when the CP and CPk values are greater or equal to 1.67? or is it when ± 3 sigma is greater than ±tolerance? Or,is it when there are no points out of the control limits?


David

 

[D.Scott]

DJN - A process is always capable. The question is capable of what?

To start, the process has to be determined stable. That would mean normally distributed with no out of control conditions.

The Cpk would then indicate how many sets of 3 sigma were between the center of your data and the closest specification limit (1.0 Cpk would be equal to one set of 3 sigma).

The further away your mean (average) is from the closest specification limit the higher your capability.

If a customer required 1.33 Cpk, you would be required to be 4 sigma from the limit - 1.67 would be 5 sigma - and 2.0 would be 6 sigma (2 sets of 3).

Now you can ask, "is the process capable of meeting the customer requirement?" - Review of the Cpk would then tell you.

Hope this is what you needed.

Dave

 

[DJN]

Thanks Dave, just what I wanted.


David