Reducing diameter of straight drill bits

Hold the drill using a 5C collet, or use a 4 jaw chuck and indicate the drill till it runs true. If you use a 3 jaw chuck you may encounter some runout.

Best Regards Tom.

...and you won't need a carbide tool ('though it won't hurt). Drill shanks are nice and soft, and easy to machine. I use HSS tools with excellent results.

-- Jeff R.

HSS drill bits are easy to turn smaller but then they slip more easily in the chuck. If you have 5C collets and a hex block or indexer you could mill three flats to get it into the drill chuck.

jsw

Yes, I have one, and I will do just that, great idea. I did plan on doing that.

Exactly why I need it, to tap 3/4-10 thread.

As the others have said, the shanks are soft. Just don't leave a sharp corner where you turn the thing down, if the drill jams up, it could leave you with a twisted-off shank in the chuck and the drill stuck in the work. That's if you've got a drill that can wrap you up if it jams. Some of the cheapies will just stall out.

Stan

I have two drills like that, that could twist my arms. I prefer using the less powerful drills where possible. It is a very unpleasant sensation. The application is to enlarge a 5/8 nominal hole to become a 21/32 hole, and to later tap it for 3/4-10 thread.

This is actually to move that Bridgeport Interact II.

The application is to enlarge a 5/8 nominal hole to become

Be really careful.. a big starter hole like that will want to grab the drill bit and twist it, with significant force on "you". Any way you can grind a pilot on the end of the drill bit that fits the original hole?

-bill

This is actually to move that Bridgeport Interact II.

What is the material being drilled? Steel is guaranteed to grab and make the drill try to tear your arm off. Cast iron would be easier, if the drill is dressed as for drilling brass.

Why is 5/8" too small?

Is there sufficient remaining meat around the hole if it is enlarged?

Joe Gwinn

Cast iron

The hole is 5/8". I need to tap it for 3/4-10. Thus I need to enlarge the hole to 21/32.

Yes.

I'm sure you have bigger chucks in your stash, couldn't you just make an adapter to mount a larger chuck in the smaller chuck? Shouldn't really cause any trouble with hand-held work other than making it a bit longer.

Ahh. Machine base of the mill, then.

This is an answer to the question "What?", but is not an answer to the question "Why?"

Good iron.

Joe Gwinn

Typically -- you won't even need carbide. The shank of most drills is mild steel, joined to the HSS for the working end. This is so the chuck jaws can dig in a bit to get a better grip.

If you have to clamp it in the lathe chuck on the flutes (the shank is too short), what I would suggest is getting a length of copper

And next time a surplus lot gives you a batch of Silver & Demming drill bits (the reduced shank ones) remember this, and instead of putting them on eBay, save them for your own needs. (I suspect that you have already had and sold more than one set. :-)

Good Luck, DoN.

I actually have quite a few of S&D drills, but not in 21/32.

I regret selling a lot of things... But I am space limited.

I slightly opened up the holes in truck leaf springs that dulled my drill bits by making a half-round piloted step drill out of O-1. I can't find it right now but I think light blue was the right temper, on the third try. It cut reasonably well without any back rake, using a lot of pressure. The truck's owner and I took turns crawling underneath and heaving on the drill.

jsw