DATUM TARGETS

[aniiben]

ASME Y14.5-2009
4.24.12 Secondary Datum Axis

For MMB, the centering method used to establish the datum axis has a set of three equally spaced features set at a fixed radial distance based on the MMB.

Regarding fig 4-53:

Question: Should I understand that if B had been callout at MMB in the positional callout (and maybe in the profile callout) then a perpendicularity on Ø.170 ± 0.1 outside diameter to datum feature A is needed?

Otherwise how to calculate the MMB requested on the above paragraph, if the orientation relationship between B and A is missing?

But, on the other hand, as far as I can see, in general when datum targets are used no such a relationship (between primary and / or secondary/ tertiary) is needed.

Therefore, no perpendicularity needed if B is RMB and perpendicularity is required if B is MMB? Is my understanding correct?

 

[pmarc]

aniiben,
This isn't really a question about datum targets. Regardless if datum feature B is referenced RMB or MMB and regardless if secondary datum axis is derived from datum targets or from the entire feature, it shall have a perpendicularity control relative to A, othwerwise the feature won't be fully defined.

Besides, if you take a look at the RMB definition (1.3.49) it says that: "regardless of material boundary (RMB) indicates that a datum feature simulator progresses from MMB toward LMB...". This means that even in RMB case there has to be a MMB from which the progression of datum feature simulator should start.

 

[greenimi]

Glad I am not the only one confused about this subject.

I see no examples in the standard where if a datum target is used the relationship with the higher precedence datums is shown. Did I miss the applicable pictures or figures? I know that Y14.8 has in the non mandatory appendix B an example B-1 where perpendicularity to datum A target is shown. Why not in the main document?


P.S. I also have seen “almost” the same question (conceptually) posted on linkedin on AGI Group.
Must be something in the air (water) around those datum targets