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How not to promote bad practice

How not to promote bad practice

RE: How not to promote bad practice

I agree, misuse of terms like this can create confusion and misinterpretation by those who are not aware of the subtle differences between MMC/LMC bonus tolerance and the concepts of MMB/LMB and datum feature shift. The safest bet is to never conflate the two concepts, and leave out the word bonus whenever discussing MMB/LMB shift.

To your question I believe the use here can be correct if we interpret "the bonus is quite complicated" to refer to analysis of the 0.3 MMC position tolerance applied to the 2x 8mm holes. If we instead consider it, and all other uses, to instead apply to analysis of datum feature shift relative to the |A|B(M)| DRF then I would say it is incorrect. Under certain circumstances it may look like a bonus tolerance and for worst case stack up purposes may be convenient to calculate it as such, but I think utilization of the term bonus in this sense is incorrect.

RE: How not to promote bad practice


I don't see much of a problem here. I would can design an inspection fixture with two pins 7.7mm in diameter. At the datum B feature's MMC and maximum error, the part will be fixtured precisely. At the holes open up and are located more accurately, the part can be wiggled around to make the other tolerances work.

I don't understand the concept of sharing a bonus tolerance.


RE: How not to promote bad practice


I'm not familiar with the specifics but i believe why its more challenging or complex is because i think in terms of CMM programming and software that MMB and MMC are relatively difficult to analyze. Doubly so when those features are instead of a single feature but part of a pattern. Its relatively easy to shift a part so it fits on a physical gauge. I believe its more difficult for a computer to do the same with a fitting routine.

As far as "sharing" the tolerance, i think it just means that the allowable shift is affected by the combined effects of variation of all features in a pattern, instead of a single feature which would presumably be easier to analyze.

RE: How not to promote bad practice

Datum shift is not bonus, CMM doesn't like datums that move, company is trying to sell the product.

What else is new?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: How not to promote bad practice

Datums don't move, but datum features can, relative to datum feature simulators.

The unanswered question is about providing a unique, single, numeric value for MMB/MMC datum feature/feature reference for the purpose of QA/QC reporting. I think that's the huge irritant for the "show me the number" crowd.

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