**Re: Capable, but not stable question**
I have a control chart for a customer attached with Data and supporting CpK.

I have a control chart that is out of control, so technically the Cpk value is not valid because you have to be stable before you can be capable. Any thoughts on this. I think it is beating a dead horse to try and be stable with a high cpk.

FYI: the **process is grinding an OD on a CNC Grinder.**

First of all,

**your process is perfectly capable**. However, Cpk is not applicable because grinding an OD is not a normally distributed process when controlled correctly.

Second, you are

**not out of control** and it

*is ***stable**. - but you need to prove it. First, this is the wrong chart for precision grinding. You should be using the

X hi/lo-R Chart. Your control limits should be LCL: 2.99895 UCL: 2.99985. You have no points out of control.

The variation you see is either gage error (

insufficient GR&R) or measurement error (from roundness),

*not process variation*. You do need at least another decimal place of resolution to detect that. You have no process variation in 30 consecutive parts on a CNC grinder (unless you have the wrong wheel material, or a great big hard part in a tiny wheel). What you need is to take every 10th part from your 300 pc run and look for the

positive slope from the wheel wear. If you see that, you have the classic

*continuous uniform distribution *that you should see if your grinding process is in control. Capability for that distribution is: (USL-LSL)/(UCL-LCL) or 1.33. Don't mess with that normal distribution stuff - it has

*nothing to do with your process* unless it

*is *out of control!

To

*maintain *that control, you need to follow the directions for the X hi/lo-R charting methodology.

Precision grinding is the

*gold standard *for the continuous uniform distribution.