Secondary datum MMB, tertiary RMB

[Burunduk]

I don’t want to hijack jimbid20’s thread too much, so I’m posting this here.

I’d like to get everyone’s opinion:

Looking at this modified figure from the standard, do you see any issues with B@MMB as secondary and C@RMB as tertiary?

I think there might be a problem because you might not be able to use the full shift from the difference between the OB’s RAME and the 40.2 diameter of datum simulator B.

I also don't see how this datum scheme can mimic any functional application.

 

[3DDave]

"I think there might be a problem because you might not be able to use the full shift "

Not sure how you conclude there might be a problem. There is no requirement that every datum feature reference must always be able to use the full datum shift. I realize the standard doesn't bother to spend much time on cases where that doesn't happen, because that's not the story they wish to tell. Since the above always allows for the assembly of this part into a hard gauge, the stand in for the actual mating part, there seems to be no problem.

In any case, this is not a suitable stand in for the other problem, which is why you are avoiding the example I gave to look at.

Had you paid attention you would have noted that "Figure 7-35 Planar Datum Feature Constraining a Rotational Degree of Freedom: Secondary Datum Feature at BSC" was probably more applicable because the tolerance would probably not carry over, nor be defined, in a suitable manner from the casting drawing to the machining drawing.

Since the other problem fails to show the actual condition of "F" it wasn't for me to say which was the more appropriate.

 

[Burunduk]

3DDave,
In cases like this, the secondary or tertiary planar datum simulator, referenced at RMB and having a location relationship to the preceding datum, must 'progress,' as the standard says, from the MMB location toward the LMB until it makes 'maximum possible contact' with the datum feature.

Imagine that once the part in my example is placed on simulated datum A, and inside the fixed-size bore simulating datum B@MMB, there’s a fixture component with a planar face simulating datum C pushed against the flat. It doesn’t stop when making initial contact or after a slight push, but is forced all the way until contact at C is as full as possible.

This process will fully immobilize the part and won’t provide any extra allowance usually associated with the MMB modifier. It will be as stringent as everything @RMB, but unlike RMB, it will force acceptance to occur only at one extreme condition of the part relative to the fixture, where it’s pushed off-center from the B simulator in the direction opposing C.

Something similar will occur in the example you referred to, where the clocking face is a horizontal portion of a cutout, but it would work by the same principle.

 

[3DDave]

As long as there is a location for the part such that datum feature B fits in the virtual size gage while oriented by datum feature A, then the inspection/use of datum feature C only requires that it make contact within the limits given by the profile tolerance. If the profile was only acceptable with the cylinder shifted to the right, then that's a valid place for the part to be. If it is then moved anywhere else to accept the holes, then that is locked down for location and the part turned/oriented to match the tertiary plane.

What is not there is a requirement to apply infinite force, or any force, that would cause the part to be relocated or trapped by the movement of a one-sided control. The gauge for that feature could be magnetized and allowed to slide right to left, the magnet ensuring maximum contact, while allowing the part to slide with it as well as sliding in the plane of the gauge feature, or use double-sided tape for non-magnetic items.

 

[Burunduk]

Ok, that does make sense. Thank you 3DDave.