Cpk vs. "Bad" Parts - Unreasonable Diameter Tolerance and a 1.67 Cpk

bobdoering

Stop X-bar/R Madness!!
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#21
A "good" Cpk result doesn't mean there won't be nonconforming parts. It's supposed to help predict the number of nonconforming parts produced by a stable, normally-distributed process. As we know, there are serious problems with using Cpk for predictions (or using it at all).
It is based on a sample - the smaller the sample, the more of a stretch the whole concept is.

We also do not have enough data to determine if the characteristic exhibits a stable, normally-distributed process. If there is random springback issues, then using the total variation equation to determine what contributing variations are affecting the population may be necessary to figure out the true capability. Dumping data into an equation is never enough.

So, we do not have a lot of evidence to support the use of Cpk as a viable estimate of capability, which can also be part of the problem.

We don't know at this point if what the customer found is what the customer asked for in terms of Cpk.
I agree, that was the basis of my measurement caveat.

But, I was trying to give some basis to the issue of Cpk versus bad parts in the population. I also agree that it is not a precise measure of capability by any means.
 
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falconer65

#22
True has a good point.

Many a part has been judged "good" or "bad" by a CMM due to the variables that can be set on the CMM. Everything from how many points, at what depth, datums, evaluation method, etc.

I would think the air gage would be the standard to set the CMM to. I would measure a part with the air gage and get the CMM to agree. The CMM can even be programmed to measure at the agreed upon locations like the air gage.


On the agreed upon measurement method and tolerances, it is tough, but someone(sales?) may need to remind the customer.

Erik
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#23
Bob does have several good points; another thought is that is the negative but inevitable consequence of "statistical tolerancing". (with the added complication of different gaging methods).

A high Cpk to a looser 'tolerance' range than the specification limits of the individual parts can work if the resulting distribution is Normal and the looser limits are properly set such that the probability of getting a stackup interference is very low. Even so, there will be times when teh mating parts are stacked up in the wrogn direction.

The OP says the Customer found a few parts to be 'tight' (a fit problem) and then they measured them but they were 'marginal' meaning that the OP found them to be 'in spec' when measured.

It is probably time to either swallow and do what the Customer asks, or to have the difficult technical conversation with the technical folks regardign measurement systems and bias and real tolerance requriements. But be ready - the tolerance may in fact be real.

I had a group of engineers come to me a year and a half ago to discuss the use of statistical tolerancing with some of our suppliers because their design tolerances were too tight for the material & manufacturing process selected. My advice at the time was to change our design to either have a looser tolerance or a material/process that was capable of holding the required tolerance. I did explain to them that one of the assumptions of statistical tolerancing is that the manufacturing process is homogenous and Normal. If it isn't you will get clusters of product at the min/max levels increasing your chance of a stackup collision. You can guess which path they chose. You can probably guess what they are doing now - yep, redesigning the product to be more robust...
 
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