True position - MMC

[[Me...]]

Is it possible to report true position as attached piscture? MMC on datum A which is a surface. How would I create a true position characteristic correctly? Thanks in advance

True position .png

[[Jo...]]

While this is allowed in the standard, modifying a planar primary datum feature at MMB has absolutely no effect on how it's checked. Whenever I see this, I think the designer meant something else, otherwise, just leave it at RMB.

So to answer your question, you can't report it as shown. Ignore the MMB modifier and you'll get the exact same result. The caveat is if something else was intended. You have no way of knowing that though.

[[An...]]

_

[[Ra...]]

Andreas, (without opening the ASME book this early in the morning) by looking at how -A- is drawn, it is a feature of size. However, having the profile of the opposite surface back to -A-, the datum is a plane - not a feature of size. So, how could MMB be applied? Am I seeing this wrong?

[[Aa...]]

Haha!
So the first difference that jumps out to me is that in the original example, datum feature A is just the plane, whereas in Andreas' hypothetical, datum feature A is a width. (The difference is in where the leader line for the DRF terminates.) Calling out a surface profile on one surface of that width back to the width is not necessarily wrong, but it is somewhat bizarre. And like MMC on a primary planar datum, it typically signals that whoever drew it was likely confused. (In this case, it's likely Andreas was just trying to illustrate a point.)

I would imagine that what the designer really intended on the original was to have the width serve as datum feature A. But that's just my conjecture. Then MMC would make sense.

[[To...]]

Please sign in to view this quote.

Really? Andreas? 🤣

[[An...]]

_

[[Aa...]]

😃 It absolutely does. Keep in mind that this datum "shift" doesn't have to be purely translational. It can be rotational as well. So as the diameter of datum feature A moves away from its maximum material boundary size (9.8), the DRF can translate left/right (on the page for your most recent illustration) and rotate clockwise/counterclockwise. So long as the surface of datum feature A doesn't violate that 9.8 limit and the surface of datum feature C doesn't violate the 4.75 limit (where the two limits are perfectly perpendicular and the axes intersect).

[[Aa...]]

Another way to think about it is a single profile on all three surfaces, where the tolerance on the A surface is +0.4/side, and the tolerance on both holes is +0.125/side (with no least material limit).

[[An...]]

_

[[Jo...]]

Regarding the last four posts, are we now talking about Andreas' figure or are we talking about the OP?

[[Aa...]]

I was referring to Andreas' illustration. If the OP's A datum were a width or a diameter, (rather than a plane, which can't be referenced MMB) then the concept would still apply.