Tolerance intervals - Asked to provide a tolerance for a manufacturing process

#1
Not sure if this is the right forum, but here goes anyway. I have been asked to provide a tolerance for a manufacturing process. Yes, I know that the tolerance is the VOC, but in this instance the customer has asked for our advice on setting a tolerance, as we know from experience that the tolerance the customer is asking for is not acheivable. The customer is ameniable to changing the tolerance, provided it is not too outrageous. What they mean by that is open for discussion. The process yields a CP value of 1.725 and a standard deviation of 0.0887. The tolerance at the moment is ±.2mm. Now, just taking 3xstd deviation gives me ±.27mm, but I don't think that is the right thing to do. Is there a formula I can use to calculate the tolerance?
 
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#2
DJN said:
Not sure if this is the right forum, but here goes anyway. I have been asked to provide a tolerance for a manufacturing process. Yes, I know that the tolerance is the VOC, but in this instance the customer has asked for our advice on setting a tolerance, as we know from experience that the tolerance the customer is asking for is not acheivable. The customer is ameniable to changing the tolerance, provided it is not too outrageous. What they mean by that is open for discussion. The process yields a CP value of 1.725 and a standard deviation of 0.0887. The tolerance at the moment is ±.2mm. Now, just taking 3xstd deviation gives me ±.27mm, but I don't think that is the right thing to do. Is there a formula I can use to calculate the tolerance?
You have me a little confused here. If the process yields a Cp value of 1.725, then by my rudimentary calculation you assume a specification width of 0.918 or +/- 0.46 not +/- 0.2. The 0.2 specification would yield a Cp value of 0.75.

If you now set the tolerance at +/- 0.27 then you can get a Cp value of 1.0. Are you trying to set a tolerance limit to appease a Cp value, the customer, minimize scrap? I would suggest you look into tolerance intervals in the statistical sense. Then you can find the appropriate sample size and determine meaningful specifications.
 
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