Hole On Perimeter

[lmike63]

How to properly dimension with gd&t holes on a perimeter.
Please see attached jpg.
my concern is the hole on the angle datum -D-.
All holes around the perimeter are related.
Should they be referencing the same datums?

thanks.

 

[pmarc]

lmike63,

The question whether the holes should be referenced to the same datums in the same order of precendence and in the particular sequence really depends on the function of a part.

If, as you are saying all holes are related, I would try with the same datum reference frame for all of them. As long as there is no functional need, the inclined hole (the one nominally perpendicular to D) does not have to be controlled relative to D at all.

Sorry if this is too general answer, but I would have to know at least how the part looks in bigger assembly to be more precise.

 

[lmike63]

Thanks for the quick reply pmarc.

The perimeter of the part that the holes lie on
will be controlled with a profile tolerance.
That being said, I would think datum -D- is not needed
and all holes should reference the same datum
reference frame..........thanks.

The only other real concern would be to where does
that datum -D- hole get dimensioned from?
Is it acceptable to dimension it from the intersection
of B and C? Or maybe from one of the other holes?

thanks again.

 

[KENAT]

I would be tempted to dimension it in the right hand view you have - dimension to center where it breaks through the outside surface.

Unless the 2.25 dimension as is is really critical functionally or something it's not a great dim.

However, it should be driven by function.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

I would keep 2.25 basic dimension as it is, but for sure add basic angle from either B or C datum planes - now it is missing so the orientation of positional tolerance zone for hole D is not fully defined.

Generally speaking, basic dimension are mostly used to define theoretically exact location and/or orientation of tolerance zones of geometrical tolerances. As long as basic dimensions define clear path between datum reference frame (DRF) and toleranced feature, it does not matter how they are presented. They are non-cumulative so there is no concern that one way of basic dimensioning will cause different effect than the other. It can only be an issue that one is more transparent in reading than the other.

If you plan to tolerance perimeter of the part by profile tolerance, you can even dimension the tolerance zone for hole D from any edge of true profile. Just assure proper dimensioning of true profile relative to DRF and everything will be fine.