Hi all,

Some years ago I saw a control chart that was measuring the hole size on a part. The upper and lower control limits were not straight horizontal lines, but rather on a slant to accomodate the punch wear. I cannot for the life of me remember what this chart was called or even how the limits were calculated. It was a perfect application for metal stamping, being that we always want to start out with the punch near the upper spec so as to allow for wear and get as much out of it as possible before changing it.

Has anyone else ever seen this type of chart? And if you're familiar with it, could you please send me the info on how to calculate the control limits?

Thanks!
Jonell

This may help.

http://www.spcpress.com/ink_pdfs/Selden.pdf :drunk:

This may help.

http://www.spcpress.com/ink_pdfs/Selden.pdf :drunk:

There is a statistical difficulty here though. Rather than just estimating the average line and the control limits from the data, you are also estimating the slope of the line. This leads to the proper control limits for a linear regression to be curved. They should be close to the center line at the center of the data, and spread out away from the center line as you move to the edges. This is due to the errors in estimating the slope.

If I were doing this, I would run a separate control chart of the tool wear (the first derivative of the data, for you calculus types). As long as I had a stable long term (through several changeouts of the tool) figure for the tool wear, then I might feel comfortable with applying that to the dimensional chart itself.

Hi all,

Some years ago I saw a control chart that was measuring the hole size on a part...
Has anyone else ever seen this type of chart? And if you're familiar with it, could you please send me the info on how to calculate the control limits?


Donald Wheeler wrote an article entitled "can I have sloping limits" for quality magazine back in 1999. I have attached a copy of it.

There was also another good article: Sarkar, Ashok, and Pal, Surajit, “Process Control and Evaluation in the Presence of Systematic Assignable Cause”, Quality Engineering, Volume 10, Number 2, 1997, pp. 383-388
but I don't have an electronic copy of it.

I haven't personally investigated the problems cited by Steve P but I have used these approaches for years in machining operations (stamping, milling, lathes, broaches, etc.) and they work...one of the most value-add application of on-line control charts in my experience...