Simply, they get sharpened pretty much like a twist drill. They cutting is
primarily on the push stroke, so similar clearance angle is appropriate. You
don't want too much clearance, or there isn't support, and the bit will push
through the material, giving a poor hole and jamming up. You need some or the
bit will rub rather than cut. It isn't critical.

I have factory grinds from the 1980's and (roughly) the 1930's, and they are
different, but not much. The 1980's had the grind done as a flat at an angle to
give the appropriate cutting edge (roughly 120 to 130 degrees-- it isn't
consistent) and a bit of clearance -- maybe 10 degrees at the cutting edge, but
less farther back, making the cutting edge on the push a bit ahead of the
trailing edge. The older bits have roughly the same edges, but the bits were
rotated do the clearance is cut like it should be on a twist drill.

I generally sharpen like the 1980's edition, as it is easy and works just as
well. I don't worry about the details, only that there is a bit of a net
clearance angle so the cutting occurs on the push, not the return. No jigs, just
me and my fine wheel on the grinder, or a stone, or whatever. And, for the
smaller ones, magnification. If the cutting edges are at substantially the same
angle and same width, they are fine.


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