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  Statistical Techniques and 6 Sigma
  Type chart to use when...

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Author Topic:   Type chart to use when...
Steven Sulkin
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Posts: 75
From:Columbus, Ohio

posted 26 August 1999 07:59 AM     Click Here to See the Profile for Steven Sulkin   Click Here to Email Steven Sulkin     Edit/Delete Message   Reply w/Quote
Trying to determine what type of control chart to apply...

We two CNC Lathes that run small batches. These batches may be of different material and configurations which require setups between runs. There is a lot of variation between runs (due to setups) and very little variation within runs (variation in lathe).

I would appreciate a recommendation. If you successfully implemented SPC in this circumstance an example would also be appreciated.

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Roger Eastin
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Posts: 345
From:Greenville, SC

posted 26 August 1999 11:20 AM     Click Here to See the Profile for Roger Eastin   Click Here to Email Roger Eastin     Edit/Delete Message   Reply w/Quote
We had a similar situation and used some of the "short-run" SPC techniques to help us. It requires, among other things, to know the sigma of each "within set-up" run. By normalizing the "gross sigma" for the process, you can have one set of modified control limits for each NC machine. This technique allows you to focus on the machine and not just the part that you are fabricating. We used this technique years back and it worked. You do lose some resolution on the control limits, but we found that the gains outweighed the loss due to resolution. Anyway, check out some books on this topic in the ASQ Publications catalogue.

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Don Winton
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Posts: 498
From:Tullahoma, TN

posted 26 August 1999 11:33 AM     Click Here to See the Profile for Don Winton   Click Here to Email Don Winton     Edit/Delete Message   Reply w/Quote
Trying to determine what type of control chart to apply

I am not sure I can help here. Control chart applications are very dependent on usage. It sounds as if your chart is varying across setups, but I need more details.

There are some alternatives to the traditional control chart in this type (small run) which may be of help. Under the assumption that each run is different, you may want to look at a pre-control type of technique.

Perhaps I could be more help if additional details were provided. Or, perhaps someone else has had this experience. Personally, I have found control charts for short runs rather, in a word, lacking.


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