John wrote in
rec.crafts.metalworking on Wed, 10 Nov 2010 21:25:39 -0500:

A 120 degree spotting drill makes it easy to calculate the spotting
drill depth if you want to pre chanfer the hole. The chamfer diameter
will be twice the drilling depth of the spotting drill. If you are
drilling and tapping a 5/16 rhreaded hole you would drill half the 5/16
depth plus the added depth for the chamfer.

John

Are you sure that's not 90 degree spotting drills that work that way?
--

Dan H.
northshore MA.