Best way to counter-bore parts

Just wanted people's opinion on this... I need to create a flat spot
on 1/4" aluminum rounds for both 4-40 screw holes and 6-32 screw
holes. The flat spot needs to accommodate the screw head size, but
also the nut size, which is a little larger. I guess this would mean
the counter-bore would have to cut down about 1/8". Most of the bits
I've seen don't seem to counter-bore a wide enough spot. Is there a
bit where I could get, say a #36 hole with a counter-bore a little
wider than 1/4"? This would be used in a press, not a mill. Hundreds
of these have to be done, so it be great if it could be done with one
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You can have step drills custom ground that will do it in one shot.
A counterbore with interchangeable pilot (see McMaster p.2422) will work if you can live with two operations: drill and spotface.
Reply to
Ned Simmons

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