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  Statistical Techniques and 6 Sigma
  "Pre-control" / Sampling

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Author Topic:   "Pre-control" / Sampling
Don Watt
Forum Contributor

Posts: 70
From:Notts,United Kingdom
Registered: Mar 2000

posted 15 September 2000 03:52 AM     Click Here to See the Profile for Don Watt   Click Here to Email Don Watt     Edit/Delete Message   Reply w/Quote
I've just inherited a new department and in order to try to start to move forward, I'm interested in trying to apply pre-control charting to guide in-process inspection / adjustment decisions.

I think it would suit an application where an operator is running a bank of 5 machines.
I'm told that we tried SPC ("didn't work - batch sizes too small / "Quality" depends on how good the set-up is")There also seems to be a mindset of if it's in tolerance it must be OK (reduction in variation is not viewed as continuous improvement - unfortunately)

Current inspection ssems to be a "finger in the air job" i.e. no data to support when to inspect / when to adjust.

To get to the point, does anybody know of a sampling plan to use a starting point, rather than trying to invent my own.


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Rick Goodson
Forum Wizard

Posts: 102
From:Wuakesha, Wisconsin, USA
Registered: Aug 2000

posted 15 September 2000 09:19 AM     Click Here to See the Profile for Rick Goodson   Click Here to Email Rick Goodson     Edit/Delete Message   Reply w/Quote

There are pre-control plans available but I would caution you that they all start with a caveat that the process first has to be in statistical control before you use the pre-control techniques. If you have not been able to implement SPC successfully it will be difficult to move to pre-control. A Cpk greater than 2.0 is not unusual as a minimum pre-control requirement.

Most plans are based on three zones. The green zone is 50% of tolerance and centered around the nominal dimension, the yellow zone adds another 25%, the balance is the red zone. There is a text, 'Modern Methods for Control and Involvement', ISBN 0-471-87696-X, with a brief discription of the technique or you might try papers by Dorian Shainin (1965) or N. R. Brown (1966). I also highly recommend you look at 'Statistical Quality Control' by Grant and Leavenworth, ISBN 0-07-024162-7. The authors present a number of reason why you should not use pre-control and in fact go so far as to state "The best advice we can give is to recommend almost any technique discussed in this book in preference to precontrol. The pattern of variation is a characteristic of the process and should never be confused with design tolerances."

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Don Watt
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Posts: 70
From:Notts,United Kingdom
Registered: Mar 2000

posted 15 September 2000 03:31 PM     Click Here to See the Profile for Don Watt   Click Here to Email Don Watt     Edit/Delete Message   Reply w/Quote
Hi Rick,
Many thanks for the information, I'll certainly be having a look at the Grant & Leavenworth book.

I was under the impression that pre-control may be ok with Cp between 1 & 1.33 (the limited studies that I've done so far indicate that we have this basic capability)
If Cp of 2 or greater is required, I need to think again :-<

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Jan Ketelaar
Lurker (<10 Posts)

Posts: 1
From:Ipoh / Malaysia
Registered: Oct 2000

posted 10 October 2000 11:42 AM     Click Here to See the Profile for Jan Ketelaar     Edit/Delete Message   Reply w/Quote
Hi Don,

I know what you are looking at. I also had the beauty of walking into a new department and it is still difficult to teach an old dog new tricks. I do work with a cp of 1-1.33 and it works okay. Soon I will start to introduce a system simular as the 50% / 25% zone. Only, I will use 1, 2 and 3 sigma. 2 Sigma being a yellow zone. If I have not been killed by then I will post the results....

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