pmarc, dingy2,
Your objection to the analysis procedure is… that it does not confirm that the maximum material boundary is not violated when only a sampling of the feature’s surface is used to generate data about its size and location… specifically disregarding its possible orientation deviation. Did I get that right?
So what!
Say that I have to demonstrate that my processes are capable to 1.67 Cpk before I am able to sell any product. If I use attribute gages I would have to demonstrate that less than 1 part in a million defective.
I could say that, as I was producing those parts that I am hopefully confident that my process is…
1.0 capable if it had no defects in 740.8 parts
1.33 capable if - no defects in 31574.39 pieces
1.67 capable if - no defects in 3488555.79 pieces
Of course I would probably need another instance of a defect at a similar interval to establish some marginal confidence that my prediction is valid.
My point is demonstrating predictive conformance with attribute gages is counterproductive and very expensive in terms of product required, gage purchase, gage maintenance, and scrap if defects are detected with moderate sample frequencies.
Let me say that…
All inspections are estimations and all inspections make assumptions to some extent… even the hard attribute gages aside (I refuse to call them functional until I verify that the tolerance modifiers were indeed specified according to the feature’s function).
With hard gages there are all sorts of problems that I have witnessed but a few that come to mind are details that enter the features before datum surfaces are contacted, incomplete pattern engagement, non-unified datum mobility allowance among feature patterns and most common worn gages.
With layout or dedicated bench variables inspection the assumptions are numerous and to the untrained, inexperienced inspector their application is seldom noticed. A trained inspector can examine the process, the fixturing, the sequence, the tooling method, and all other details, identify its strengths and vulnerabilities and adjust his assumptions to aggressively scrutinize vulnerabilities and abbreviate scrutiny of its strengths. He is aware of jaw squeeze, closely related features accomplished in separate operations, etc. To most others, engineer’s, designer’s, and the like the assumptions they employ when they inspect are transparent… ignorance is bliss.
With coordinate measuring machines in the hands of a programmer who is inexperienced and has never done layout inspection the assumptions are legion! Dangerous I might add. Does anyone question form when reading the size on a CMM inspection report? Only those aware of how many points were gathered, how may sample planes chosen, what algorithms were used… not to mention what should and should not be aggressively scrutinized or abbreviated according to the process vulnerabilities. Once again in the hands of an inexperienced or untrained inspector like engineer’s, designer’s, and the like… ignorance is bliss.
So all of these inspection methods have their weaknesses including the hard attribute gages… If I was checking a bore and I suspected that it was particularly vulnerable to an orientation deviation according to the observed process and I wanted always to use its “related actual mating envelope” in my variable limit position tolerance capability calculations… and I had not yet programmed that routine in my software… I just might move reference to one end of hole rotate orthogonal to the hole’s specified orientation, collect three eight point circles top, middle, and bottom, then use all 24 point’s X and Y values stripping off the Z (depth) values to figure a 2D maximum inscribed circle size and location to use in my capability equation.
The point is that it is not the tool that is the problem it is the unmerited assumptions used in the inspection process, the lack of attention to sources of process variation, and the lack of understanding of the tools available that make for poor process prediction and subsequent optimization.
You go ahead and use your attribute gages… I want to know what variation I have so I can either ignore it or fix it so that I can ignore it or continuously manage it for my customer’s benefit.
Paul
BTW… It is better to use both the related actual mating envelope size and its position separately in the continuous data capability equation rather than combining them in the resultant condition and comparing them to the virtual condition as is suggested in the Tec-Ease example. They are independent sources of variation and should be analyzed as such.
