Criteria for operator's readings at setup - Seeking help to explain chosen limits.

P

pagnonig

#1
Dear all,

I'm reading a book regarding the SPC control in the Wire & Cables Industry.
I came face to face to an example that I can't fully understand. Here is the example description:

We carried out a capability evaluation of an extrusion machine.
The elongation measured on 15 samples taken from a standard run (without adjustments) has a Sigma=0.12 .

The defined Product Specification for the elongation is 2.45 +/- 0.45 .

Now, we have to give to the machine operator a tool in order to be able to setup the machine correctly.
The criteria is the following:

If, at initial setup, an operator's first reading falls between nominal and +/- 1.44*Sigma, the operator is 68% confident that the process average is within 1.44*sigma of nominal value. When this criterion is met the decision should be DO NOT ADJUST.

If the average of operator's 1st and 2nd readings falls between nominal and +/- 1.02*Sigma, the operator is 68% confident that the process average is within 1.02*sigma of nominal value.Again, DO NOT ADJUST.

The full rule is...

READING >>> Prescibed LIMIT
1 >>> Nominal +/- 1.44*Sigma
avg of 1 and 2 >>> Nominal +/- 1.02*Sigma
avg of 1-3 >>> Nominal +/- .84*Sigma
avg of 1-4 >>> Nominal +/- .72*Sigma
avg of 1-5 >>> Nominal +/- .65*Sigma
avg of 1-6 >>> Nominal +/- .59*Sigma
avg of 1-7 >>> Nominal +/- .55*Sigma
avg of 1-8 >>> Nominal +/- .51*Sigma

My question is:
Which is the logic behind this criteria? Could anybody explain me how this limits have been chosen?

Thank you a lot for your help.
Of course, if you need any more detail please ask.

Regards.
Giuseppe:thanks:
 
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P

Pudge 72

#3
Re: Criteria for operator's readings at setup - Seeking help to explain chosen limits

You are giving the Operator a Tool to set the machine up based on a statistical study that was performed. That is OK - but, you cannot replace machine set-up with process performance parameters. What if the machine set-up is good but your air pressure or another variable presents you with a problem on that particular day - you cannot say that becuse the machine is producing product that you did not expect it to that your set-up master is bad.
What I am trying to say is that while I love statistics as much as the next guy, I think that you are trying to incorporate a method that may prove to be dangerous as well as frustrating. You are setting a benchmark standard that may be influenced by a condition that was not present during the initial study.
I would set it up as a DOE of sorts, develop your Set-Up master under optimum conditions and prove that under that scenario it is at the desired setting. Your readings at Set Up are not going to be all inclusive throughout the run if there are variables that could not be accounted for or measured while developing the parameters of this master.
During your production process, use the dimensional output of the product to determine what settings if necessary need to be adjusted as a result of variables that would not have influenced the set-up master and base your future decisions on that. Use Pre-Control / X-Bar & R ; utilize the different regions of a Pre-Control chart to dictate adjustment instead of having the operator do cumbersome math calculations.
It will work out great - we do this all of the time. By studying expected variation within the process, when it gets to a certain region in the Pre-Control chart, you can determine what and how much to adjust during the run.
 
P

pagnonig

#4
Re: Criteria for operator's readings at setup - Seeking help to explain chosen limits

Dear Pudge,

thank you for the reply.

In any case I'm not trying set uo such a process. I would like to understand where the limits came from because I can't find any analytical description inside the book I'm reading.
The setup rule given by the book's author is meant to minimize the scrap at the very beginning of a production run in an extrusion process, where usually you take dimensional readings (e.g. cable diameter) at the beginning of each produced cable length (e.g. 1000 or 2000 meters).

However, could you please explain something about PRE control chart. Why PRE? You mean it is used only during machine setup?

Thank you.
Giuseppe
 
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