What do those "dn"-s mean? How do I sketch that?
Looks like depth
Hmm... I have never seen that. Maybe J. Mather can clarify if that is a proper callout.
From the picture it appears that you have 3 holes drilled through at .25" and then drilled with a .375" drilled down .1". Based on that I would say dn stands for "down" from the top surface. But, that is all speculation based on that picture.
If my speculation is correct, to make the two overlapping holes, you'd just create two features, a hole .25" thru and then a hole .375" blind .1" down from the surface.
I agree it's not the proper callout and it looks like "down", or "depth".
Standard is to use symbols now rather than English abbreviations (or words).
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