huangyhg

datums & hole patterns
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
when i have had situations like this, i give the holes separate callouts (your 2nd case) and control datum c relative to datums a and b. they can still carry the same size and position tolerance. we have done this with double dowel pin callouts, (though often the 2nd dowel pin is controlled tighter to the 1st) and when in-line, as yours seem to be, they can be used to establish a b-c axis.
also not sure about your 4 way, 2 way concept. application wise your getting any way within the 0.05 mm tolerance diameter. don't know your application, but if you really want 2 way on datum c, maybe it should be a tight slot??
i've done it similar to what checkerron suggests. if two separate hole callouts with two separate datum callouts, it would be to control the second hole to the first. if this relationship isn't critical, i would call out both holes with the same dimension; however, i would not tie the datum to the dimension, but to the hole(s) separately (similar to your datum c).
believe it if you need it or leave it if you dare.fff"> - robert hunter
jmarkus,
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
i concur with ewh's clarification. i was also thinking of datum c relative to the hole, not the dimension.
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?
checkerron,
i would like to know about small diamond pins too. meanwhile, it looks like slotted holes!
jhg
don't know your application but a hole-slot is always preferable for a functional assembly.
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.

funny, i always dimension them separately but i took a look at asme y14.5m-1994 figure 4-8, it actually has a very similar layout.
kenat, probably the least qualified checker you'll ever meet...
but figure 4-8 uses the full "diameter" of b & c, as opposed to using one as a 4-way and one as a 2-way.
the problem i have with 4-8 is that in this method the tertiary is not positioned with respect to the secondary. that's why i like to call them out separately.
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.

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