MMB - ASME Y14.5 - 1994

[[Cl...]]

I have a part (very similar to the one in the link)

http://www.builditsoftware.com/2016/09/ ... -analysis/

The print calls out a .004 surface profile to Datum's AB&G, (B at MMB) Since the profile characteristic does not allow you to add bonus tolerance, I added a formula to the tolerance block (PCM) getActual("Your feature here").diameter - 0.8908 + 0.001 (ID)

Is this legal? Can I do this? The drawing UOS block references ASME Y14.5 - 1994.

[[De...]]

It depends on the part geometry.

If it is a single edge of the part, so that the shift allowed by the tolerance of the datum would give you a 1:1 ratio of datum shift and movement of the profile then it would be.

If however there is a pattern of features with profile, or multiple features called to the same DRF, or it is a complex profile then the Datum Shift and the datum tolerance would likely not be 1:1 and then your results would be incorrect.

If you have a drawing of your part, best method to see is to draw your actual tolerance zones with the nominal feature, then shift all relevant features as much as the datum shift would allow - then ask yourself, is each feature being influenced the same amount in every direction the datum allows a shift?

Usually the answer is no, that is why in most situations like this the only reliable method is to have hard gage.

[[Cl...]]

All the other (4) profile callouts and (1) position callout have -B- @ MMB. -B- is the ID cone, dead
center of the part. The surface profile in question (27) is the contour (F-G) that runs down the center of
the part.

[[De...]]

I have created a crude example to show what I mean, the bottom left diagram shows the nominal contour and the datum in an oversize condition. The black shows the nominal size of the datum feature. This allows the actual part to shift with regard to the nominal datum feature.

The other diagrams show the results of shifting the part in various directions in relation to the nominal datum location. As you can see the results are not equal. Some features will end up being plus stock and some minus stock even though the datum only allowed shift in one direction.

I would say in your case, that datum shift is not something you can calculate using a formula.

[[Cl...]]

So other than the formula I used, how can I apply MMB to a profile characteristic?

[[Ja...]]

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PC-DMIS or a functional gage. Some people will argue that you can perform a BF alignment in Calypso to accomplish this, but Zeiss has told me that "Unfortunately Calypso can only do MMC on datums that are in the same space axis. The callout is legitimate but Calypso will not be able to calculate it. There is no work around."

[[Cl...]]

Regarding the "Best Fit" alignment option. I was told by Zeiss that this can only be used on a profile if no datum is involved.

[[De...]]

You really can't short of having a hard gage due to the fact that datum shift in one direction will influence different features differently. As you can see above, where datum shift moves one feature to be in a plus stock condition on another feature the same shift causes a minus stock condition.

If there was a way to limit a best fit to the width of the allowed datum shift, then that would be the way to go, but I do not believe this is currently possible.

[[Cl...]]

I don't understand why Zeiss would not have this (MMB) for profiles as an option in Calypso?

[[Ja...]]

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Complicated geometry leads to complicated computations. I'd like to believe, whether good or bad, that Zeiss was more focused on a streamlined UI and elimination of bugs. PC-DMIS can simulate datum shift in complicated situations, but the software crashes when you blink.

I now program without MMB but I leave a comment on the feature that states something along the lines of "MMB not computed. In event of failure, verify part manually."

[[An...]]

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Well,give CALYPSO a "least squares flow".
The geometrical addition of x-translation and y-translation yields a value,that has to be located
within a zone that is defined by the difference of nominal boundary and actual boundary.
Put in an "If Then Else" condition in order to shift the reference point and requalify the profile.

[[Cl...]]

This requires PCM?

[[Aa...]]

I can't speak to how DMIS would handle this, but Kotem's Evolve SmartProfile would handle it correctly without breaking a sweat. It's a matter of computation paradigm. Calypso is built on the idea of sequential computation (exception is made for "best fit bore patterns"). SP is built on simultaneous computation for most situations.

[[Cl...]]

Looked this up (Kotem's Evolve SmartProfile). Looks impressive. Can it interface with CMM software's.

[[Aa...]]

I don't know that I would describe it as "interfacing", but it can import .zmp files.