"This Hole Only" Clocking Datum

[randy64]

Please see attached sketch.

First question is, is it legal to use the "THIS HOLE ONLY" callout to show that the hole at top vert, which is part of a pattern, will be the clocking datum feature.

Assuming that it is legal, the next question is, should the feature control frame for that pattern of holes include datum C, or not (currently, I'm not showing it).

We are using 1994 standard.

Thanks.

 

[CheckerHater]

What makes you think that A will always be selected as primary datum feature for this orientation check?

I see it is right now

Imagine that upper face of the part (the one opposite to A) is assigned as datum feature D

I don’t want to imagine anything.

And size is 13 plus whatever tolerance is assigned to it.

Do you seriously believe that “what if somebody did this drawing completely different from what it is right now” is a valid argument?
Sorry, but I see no reason to continue.

 

[CheckerHater]

Something we both didn't notice in the heat of discussion:

We have a spec that controls perp, par, etc. that does not need to be noted on the detail.

 

[pmarc]

And size is 13 plus whatever tolerance is assigned to it.

Which means that in your opinion title block angular tolerance has no effect on MMB size of datum feature simulator B. Hmmm, that is interesting.

Do you seriously believe that “what if somebody did this drawing completely different from what it is right now” is a valid argument? Sorry, but I see no reason to continue.

Of course I do. It is really not that hard to imagine different dimensioning and tolerancing schemes for this part depending on its functionality. And, as you probably noticed, it is very effficient method to prove that title block angular tolerances result in serious mess in terms of definition of geometrical requirements. If you do not want to prove it is otherwise, it is your choice.

 

[randy64]

pmarc, you said, "If the hole at top vert was rotated 10 deg CW, the two patterns would still be held together - but this time not through basic 37 deg, but through basic 27 deg. No basic angular dimension from top vert to the first pattern would be needed."

That makes sense to me. On my example, as a practical matter, would it make most sense to daisy-chain the clocking dimensions on the inside hole pattern. I say this because without datum C, I couldn't use the "XX degrees to C" notation.

Thanks again to all for your input.

 

[fsincox]

If you can understand how to "clock" a single pattern of holes, with out them all falling in the same location, then you are there!
Frank

 

[pmarc]

randy64,
Yes, I think "daisy-chain the clocking dimensions on the inside hole pattern" is right choice.

 

[Tarator]

Is there a special reason why you want use only 1 of the holes as Datum C? Since you use C(M), you could easily use all the holes as Datum C. Or you could drop Datum C, and control all the holes with Position as a pattern, where the SIM REQ rule would automatically apply.