Datums & Hole Patterns
Datums & Hole Patterns
(OP)
If I have two holes the same diameter, and I want hole 1 to be the B datum (4-way) and hole 2 to be the C-datum (2-way), is there a difference between having one size & position tolerance called out off of the hole 1 with a prefix 2X, and the datum B leader coming off that size c/o (see attachment)
and
both hole 1 and hole 2 each having their own callouts (but with the same size and pos tol values)?
Thanks,
Jeff
and
both hole 1 and hole 2 each having their own callouts (but with the same size and pos tol values)?
Thanks,
Jeff





RE: Datums & Hole Patterns
RE: Datums & Hole Patterns
RE: Datums & Hole Patterns
Believe it if you need it or leave it if you dare. - Robert Hunter
RE: Datums & Hole Patterns
I assume you will call up datums as |A|B|C|.
You can apply your datum symbol directly to your second hole. I understand why you are doing it the way you are, but it is the tertiary datum. By definition, all it can control is rotation.
Whatever goes through hole_C should be diamond shaped so that there is contact only with the sides. Otherwise, you should slot the hole.
JHG
RE: Datums & Hole Patterns
DRAWOH brings up an interesting point too. I really like to use a diamond pin in a 2 pin tertiary datum situation, but most of the commercially available ones I see start at 6mm (.250") diameter.
Many of my applications are smaller than this.
Has anyone seem purchased diamond pins smaller than a .250
dia or 6mm?
RE: Datums & Hole Patterns
I would like to know about small diamond pins too. Meanwhile, it looks like slotted holes!
JHG
RE: Datums & Hole Patterns
I get the felling that you a) are trying to put your own style on the way you call this out b) don't fully understand datum and the order of precedence c)that you are trying to apply a midplane datum to a cylindrical feature.
In any case, just put a FOS callout on each hole and apply a datum to each. As long as neither is not primary,then reference one with respect to the other you have than achieved what I believe you are looking for which is locking the remaining 3 DOF, x and y translation are locked with one hole and last is the z-rotation with the other. This should be clear to all who have a basic GD&T understanding.
An item of note is that if your pattern of two holes is located in a manner in which you can't tell them apart, since they are equal in size, then you are opening yourself to some more confusion not to mention R&R issues.
Advice, use a slot and apply the tertiary datum to the slot FOS. It doesn't have to be much of one, and be done with it, your manufacturing/assembly will thank you in the end.
RE: Datums & Hole Patterns
KENAT, probably the least qualified checker you'll ever meet...
RE: Datums & Hole Patterns
RE: Datums & Hole Patterns
This maybe contradicting myself but if a hole-slot can't be used than, the question is if one hole is functionally more important than the other, if not, then a pattern datum should probably be considered, instead. If this is what is chosen then create a BASIC dim scheme from one of the holes which will become DRF origin. This will lock the three remaining DOF's which I believe is the OP's objective.
RE: Datums & Hole Patterns
This is my point, above. In Figure 4-8, datum_B is secondary and datum_C is tertiary, as per the various positon tolerance callups. The secondary datum controls X and Y. The tertiary datum controls rotation. There is no need to show which part of datum_C's surface works.
Figure 4-8 makes perfect sense to me.
JHG