On Monday, July 28, 2014 8:53:50 AM UTC-4, Ed Huntress wrote:
"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



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[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

Gotta love that Popular Science archive.