Position Bonus/MMC, Cpk, and Inspection Results, Nominal Geometry 4

[ptruitt]

I am interested in using statistical tolerancing on upcoming projects, but I am not sure what the Cpk will be based on if the 'size' nominal is not the same as the 'form' nominal if I use bonus tolerance. Is there a formula that averages the two or weighs them? Is it correct to say that 'form' is the area between the Virtual and Resultant conditions?

Since more and more software is using the CAD model for machining and inspection, should nominal CAD geometry be established at the nominal form geometry or at the nominal size geometry when using the MMC modifier on holes and similar features such as slots?

As others have said on another thread, 'position' information is often not reported on inspection reports. But if it is, is there a yes/no indication that the 'size' measured was allowed given the measured 'position'?

I prefer using profile call-outs whenever possible to keep my nominal size and form the same, but some of my suppliers are not comfortable with 'profile', so I need to figure out how MMC works with statistical tolerancing.


Peter Truitt
Minnesota

 

[pmarc]

Paul,

I would disagree with your interpretation of RAME. Here's what Y14.5-2009 is saying on related actual mating envelope: "a similar perfect feature counterpart expanded within an internal feature(s) or contracted about external feature(s) while constrained either in orientation or location or both to the applicable datum(s).

Taking example of from your presentation RAME is perfectly oriented relative to datum A and perfectly located from datums B and C.

 

[PaulJackson]

pmarc,

The operative words here are either, or, or... I am using orientation only in describing the "oriented actual mating envelope" to describe the size that I would use in the variable limit tolerance analysis.

 

[ptruitt]

I don't like abbreviations, but a Related Actual Mating Envelope would probably need extra stamps at the post office, so I hereby bless RAME.

It seems as though some of you folks or an ASME technical group should consider writing ONE publication that is unified where possible but carefully explains the disagreements with workarounds or guidance that might significantly mitigate problems applying and measuring bonus tolerance. (It sounds as if the disagreements are minor or non-existent once the words have been completely beat into submission.) It might also recommend best practices for organizations using bonus tolerance based on their production volumes or level of regulation/safety.

The 'best practices' for using bonus tolerance in my organization is my concern: I want to use the proper hieroglyphics on my drawings and be able to explain exactly what they mean. Most folks would agree that if it can't be measured, I might as well leave the design to everyone's imagination. The general concept of bonus tolerance is clear, but the application of the concept through the organization and into the supply chain is not. I do have some experience with measurement uncertainty and measurement bias and that makes me concerned. Based on what I have heard so far, knowing my skills and the skills of the suppliers I work with, knowing the production volumes we see, and knowing our quality budget, I do not see an appropriate use of bonus tolerance in my organization. I will, at least, urge my colleagues to ask our suppliers to demonstrate their method of calculating bonus tolerance to measure their competence, if they want to use bonus tolerance.

I would also be interested to hear an answer to a related question that I had earlier: I have been looking into software that compares the CAD model to point clouds and reports how well a part conforms to the design. (SmartProfile, GeoMagic, PolyWorks, VeriSurf) I am guessing that they would, somehow, quantify HOW WELL a feature with bonus tolerance met the spec'. It would be interesting to see if all those software packages evaluate the feature the same way. Does anyone have experience with these programs?


Peter Truitt
Minnesota

 

[Belanger]

Peter, while I sympathize with the dilemma, it sounds like you're letting the inspection method (and tracking thereof) define the product requirements, not the function.

You stated, "I do not see an appropriate use of bonus tolerance in my organization. I will, at least, urge my colleagues to ask our suppliers to demonstrate their method of calculating bonus tolerance to measure their competence, if they want to use bonus tolerance."

The theory of bonus tolerance allows for more usable parts and fewer rejects, if it is something like a clearance assembly. I would hesitate to scuttle those benefits simply because the statistical monitoring is difficult!

I'm not saying that I have a solution or could explain it better than the above posts. But golly, don't abandon the concept of MMC unless there is a real-world physical reason to do so.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[ptruitt]

I don't design parts that can't eject from the mold. I don't see that designing parts that won't be measured properly (given my supply chain) as being much different. (I have not done the audits, so I may be off-base and worrying about a problem that does not exist or is not as significant as I expect.) This topic is helping me know what to look for if I am asked to do the audits. So part of what I am doing is playing the devil's advocate and suggesting clarity amongst the experts which may, some day, result is more competency at smaller companies. I am also trying to compare the benefits of bonus tolerance to the following:

No doubt I am getting into another sticky-wicket, but composite profile callouts for holes/shafts make sense to me for most situations where I might otherwise use bonus tolerance: Assuming equal-bilateral tolerances for size and form, the means of the inner and outer boundaries of form and size coincide, making it possible to stack the histogram for both on top of each other with the same mean. The CAD model can represent the nominal geometry at the mean, unlike bonus toleranced features, which seem to have a fuzzy nominal geometry. Nominal CAD geometry with coinciding size and form would increase the chances for successful and lean CAM and better Cpk, I would think. Stack-ups are definitely simpler and less error-prone. The form allows much of the same shift that bonus tolerance does, allowing more shift within the form as the size increases. One can reasonably argue that a design might allow more tolerance using bonus tolerance, but I am thinking that that depends on the specific design or specific manufacturing methods. When the nominal size does not fall at the mean of the virtual and resultant boundaries, the amount of available tolerance is not as effectively used. (Like the difference between square and round tolerance zones.) Profile form boundaries can easily maintain wall thickness. Plug gages can still be used to measure holes and a single-axis measuring stick to go from the plug gage to the edge of the part can glean the form, if nothing better fits the quality budget. I think that I would have the same measurement uncertainty and bias as I would have if I used bonus tolerance without statistically and painstakingly maintaining the physical size / virtual axis relationship and a staff of GD&T lawyers. Nevertheless, I see the suppliers looked stunned when I bring this up. (Not unlike a few years ago when we saw our first GD&T call-outs.) So far, I just back off and give them size and position. The stuff I am presently working on actually seldom benefits from bonus tolerance because of all of the press fits and similar design situations.

Thanks, everyone for this very valuable discussion.


Peter Truitt
Minnesota