Translating MMB Datum?

[patdh1028]

Here's a question: Can you have a datum referenced @ MMB and also allow translation? Then the datum feature simulator would be sized @MMC and position allowed to move relative to existing portions of DRF.

Application: I have two dowel holes that will have dowel pins in them. A planar surface of the mating part will register against both dowel pins, but it only makes tangent contact with the pins not fully enclosed so the pin center-to-center is non-critical. I make one dowel hole Datum B, the other dowel hole C with a generous position tolerance versus B, and all other part features to A, B @ MMB, and finally C @ MMB AND translatable. I don't care (much) about the center-center dist of B and C, but MMB by itself on the C datum will negate the large tolerance on position allowed b/w B and C because the MMB DFS must be at basic. This is correct?

BTW I'm using MMB because it's easier to set up at inspection and the datum shift allowance on a dowel hole is small enough to be negligible in this application.

 

[pmarc]

Answering to your question - yes, it is allowed to specify datum feature at MMB with the translation modifier.

However, based on my understanding of your description, I am not sure the translation modifier is good choice - unless I am not catching something.
Could you provide at least a simplified sketch of the application?

 

[patdh1028]

As yet no responses but I was thinking about the issue and I think I can answer my own question. Paragraph 4.11.6.1 states how to properly determine the size of the MMB of a referenced datum and depending on precedence, will include the location tolerance. I was thinking that MMB pretty much always meant the DFS of the feature at MMC - I see that is not necessarily correct! Actually it's really only correct if said MMB datum is referenced as primary.