"Jim Wilkins" wrote in message ...

"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

================================================== =======

[Ed]

There are several configurations of drill bits and reamers that are called
"D-bits." It sounds like you're making the kind with a center point. They're
a b**ch to make, as you suggest. I've had much better luck with the simpler
kind that's flat on the end.

I learned to make these from the old English MAP hobbyist books, which
describe several types. This article from a 1967 issue of Popular Science
describes D-bits pretty well:

http://tinyurl.com/qdcnuoa

--
Ed Huntress