Combining high Cpk (3, 4 or 5) with a set of XR charts

[margherita]

I don't know how to I can combine high Cpk (3, 4 or 5) with a set of XR charts.
Too often the process uses (control limits) only a little part of the tollerance. It is in control but sometime the operator has to adjust it because there is an out of control signal or because the process changes, for any reason, its position.
Too often it is difficult to understand this change because it is too small.
Can anybody suggest me a more powerful SPC tool?
Thanks

 

[Laura M]

I've come across this in machining appliations and went to individuals and moving range charts (I-MR). Often times within subgroup variation was so small that it drove "artificially" narrow control limits. With R-bar being next to zero, the slightest mean shift makes the next subgroup go out when you are still well within tolerance range. Don't know if this is what you are experiencing or not, but my first suggestion would be to make sure you have rational subgroups.

Laura

 

[Laura M]

Have you tried this? Did it help? Anyone else out there have any other ideas? Just thought I'd bring this one to the top of the pile again to see if there are other ideas out there.

 

[Marc]

We need Don on this one.

 

[Don Winton]

Well, I am back. My apologies for the delay.

I don't know how to I can combine high CPK (3, 4 or 5) with a set of XR charts.

I assume we are speaking of Xbar-R charts. What follows assumes so.

Too often the process uses (control limits) only a little part of the tolerance.

If this is the case, you would probably want to evaluate whether a control chart would add value at all. Too often, we tend to see the chart as a required item when in actuality it is not. Determine if the chart would add value then determine if the chart is useful. From the brief synopsis above, it sorta sounds you have a solution looking for a problem.

It is in control but sometime the operator has to adjust it because there is an out of control signal or because the process changes, for any reason, its position.

Sounds like you may want to chart the inputs to the process rather than the outputs. Just a thought.

Can anybody suggest me a more powerful SPC tool?

There may be several. For example, you could use a variation on Laura M’s suggestion. You could chart the process capability rather than the process using the X-mR chart, you could use Xbar charts on the inputs rather than the outputs or ignore charts altogether and concentrate on process capability as a metric of your process. The choice is up to you. Be certain a chart is your best choice.

Now, a Rant.

It is a mistake to try to push an SPC apparatus where it neither adds value nor contributes to your productivity. Haphazardly using SPC usually causes more harm than good. This should be foremost on your mind.

End Rant.

Regards,
Don