"Ed Huntress" wrote in message
...
... There is a way to drill a hole like this straight, without a
real gun drill. Start the hole with a regular drill bit, drill it
maybe 1" deep, and follow up with a home-made single-lip drill (a
D-bit). Unlike a real gun drill, you'll have to peck it to get the
chips out, and it's God-awful slow. But it works.

--
Ed Huntress

In my limited experience D bits are tricky to sharpen without a
surface grinder and fixturing, to keep them cutting at the center and
not binding at the full-sized OD behind the cutting edge as it wears
smaller. They don't cut at all if the center point is high and can't
tolerate it being too low because the thin wire left in the middle
becomes an obstruction.

I use D bits to ream tapered holes in fluid nozzles for my
experiments, not for deep hole drilling since grease passages in axles
don't need to be straight. They can cut brass and aluminum if simply
ground half-round from unhardened drill rod without normal edge relief
clearances and forced into the pilot hole with the tailstock
handwheel.

-jsw