Definition Cpk: Definition and Computation

U

unueco

#11
Thanks again to all who have responded; you've given me a lot to ponder! Now the next level of challenge: how to pitch my concerns to upper management.

Maybe I'll just send them the url to this thread anonymously and let them think it was their idea! :lol:
 
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S

Sturmkind

#12
Processes like stamping and injection moulding would lend themselves to small lot sampling plans because if the setup parameters are correct for each run then the variability is known and the largest difference from lot-to-lot is base material (resin or steel). If the charts exhibit a state of control without runs, trends, grouping or stratification then the overall Cpk should be quite valid. A reference to ANSI/ASQC Z1.9-1993 (the old MIL-414) Sampling...for Inspection by Variables would support this contention I believe.

Both processes (stamping and moulding) would surprise me if the Cpk is very good since for features of size the moulders tend to make the details 'steel safe' and stampers tend to run at hole high limit to take advantage of punch/button wear and any available MMC postional bonus. Consequently, review of the X-bar R chart for control would be even more important than the tyranny of a series of Cpk numbers that follow a Chi-square distribution anyway (reference dissertations by the late Robert Dovich regarding mulitple sampling and Cpk values 'The Pitfalls of Acceptance Sampling').
 

bobdoering

Stop X-bar/R Madness!!
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#13
Consequently, review of the X-bar R chart for control would be even more important ...[/I]
I agree that control charting would be better, but not X bar-R. "...since... stampers tend to run at hole high limit to take advantage of punch/button wear" is exactly why X-bar-R is the worst choice.
 
S

Sturmkind

#14
Bob,

Upon reflection, I agree with you that the X-bar R chart is an inappropriate tool for a tool wear situation. I recalled that when tracking 2 critical dowel hole locations that were done on a multiple station drilling/reaming machine that I used a run chart successfully.

The run chart derived from CMM inspection drove 2 activities:
1) The QC department dictated sampling frequencies since we knew that as tools wore that frequency needed to be increased to stop the process before a bad part was produced and yet minimize needless tool changes and downtime.
2) Having tracked both hole positions on the run chart over time we could accurately predict whether a drill, reamer, bushing, or all 3 needed to be changed which minimized machine downtime.

We never bothered to calculate Cpk as we knew that we were drifting across a tolerance range and were using virtually the entire tolerance through the course of a run. As has been noted in Bill's post that violates the first rules of Cpk as a metric as the data is not identically, independently & normally distributed, and is not in a state of statistical control (trend). However, we were able to predict and steer the process.

Thanks for the very good catch!:agree1:
 
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bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#15
We never bothered to calculate Cpk as we knew that we were drifting across a tolerance range and were using virtually the entire tolerance through the course of a run. As has been noted in Bill's post that violates the first rules of Cpk as a metric as the data is not identically, independently & normally distributed, and is not in a state of statistical control (trend). However, we were able to predict and steer the process.
Actually, you are in control, but to a non-normal distribution - which is the point of the X hi/lo-R charting methodology. It allows you to predict and steer the process, too. In fact, I bet what you did was very similar to components of the X hi/lo-R charting.
 
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