Can a Datum feature reference itself in Y14.5M (GD&T)?

[jmarkus]

If I have a part with two datums, datum B - a hole and datum C - the width of a slot and I want to tolerance the length of the slot can I use a positional tolerance that references A|B|C or can I only use A|B because the slot is C?

Thanks,

 

[ringman]

Some misinformation provided. You state there are only 2 datum features, B and C. Later you mention A. Whis is true?

You could make use of the width C in positioning the length feature.

 

[jmarkus]

Sorry, datum a is the surface that datum B & C are passing through.

 

[ewh]

I agree with ringman. Datum C is the width of the slot, not the slot itself, so you could reference the length of the slot to A, B and C.

 

[jmarkus]

Physically, what would that mean? How could the length of the slot vary, positionally, with respect to the width of the slot?

 

[ewh]

Physically it does seem meaningless. It would appear to control the perpendicularity of the length of the slot relative to the width of the slot.

 

[ewh]

ASME now controls the standard, not ANSI. Try asking this question in the Drafting Standards & GD&T forum, and you may get a better answer.