Reducing a drill bit's diameter

I think I would reserve that sort of project for emergencies. Its 9PM on Friday and you have to drill "exactly" that size hole to fix something before Monday morning when the stores open. I've got two sets of Silver and Deming bits. One is stuby length, and they are handy on the noname mill drill, the regular ones I use on everything else. I destroy enough drill bits as it is. I think I would reserve that sort of mod for when I really needed it.

They won't cut right...I believe. And, a lot of work and grinding dust on your lathe ways.

Not worth the effort. Sell the spares on ebay. When enco or msc has them on sale buy the sizes you need. The smaller sizes are pretty cheap when on sale. The only practicle way to reduce them is with a T&C grinder and an hour of time per drill with the right tooling.

Best Regards Tom.

Why f*ck around with them at all untill you *absolutely need* some specific odd sized drill bit ?

Just relieve the secondary as though it were an end mill; nothing magical about the secondary relief on your garden varity drill other than it's being cheaper to produce.

--in fact, to re-size the OD, I'd probably simply regrind as though it were an endmill, as well..

Id be happy to sell him a KO Lee tool and cutter grinder with all the accessories. Today only for $2200.00

Get it while Im feeling generous!!

Thats a good price, i would jump at it if i already didn't own one myself.

Best Regards Tom.

When I was in the A.F. Boeing had a big mod program on the B-52's and we had a guy in the shop that spent all day sharpening 4 fluted piloted drills. He occasionally ground a reduced diameter drill so certainly it is possible but on the other hand the only commercial drills I've seen without the relief margin on the flutes are long bits used to drill propeller shaft logs in wooden boats so apparently there is a reason :-)

My guess is if you are only drilling, say 1/4" - 1/2" they'd work fairly well.

By the way, contrary what others are saying you can do this on a lathe with a tool post grinder but it is a bitch to get the grinding dust off the ways when you are finished.

I wouldn't. I'd leave them as-is to hog out steel rapidly, leaving the S&Ds sharp for finish cuts.

S&Ds slip in the chuck more easily than full-diameter bits, due to the disadvantage in leverage between the cutting edge and shank diameters. My better set has flats on the shanks, but they don't help when the limited Z height of my Clausing mill forces them to be in a collet. A standard-length S&D in the 14N 1/2" chuck leaves less than an inch of working space above the floor of the mill vise.

Holes over 1/2" can easily be bored to any size in the lathe or mill. Often I need them slightly undersized for a press fit, or oversized for a bearing.

The intermediate sizes you may need are the tap drills

The one I checked vise clearance with is a 33/64" S&D for 9/16-18 and minimum-clearance 1/2" bolt holes.

In practice I've found that I need the less common threads mostly on small lathe parts, where tapered shank bits are more appropriate because they don't give up work envelope to a chuck.

jsw

The old method was always that drill bits did not drill accurate size holes. For an accuracy one drilled a bit undersize and then reamed to size using a machine reamer (0.0005 undersize) or a hand reamer for the absolute correct size.

BAD ADVICE!

End mills have a secondary and a primary relief. Drills have neither, although the margin is relieved to reduce contact area.

Drills are circular ground, and have taper from the cutting end towards the shank, so they don't bind in the hole that is generated. The margin, created by relief to the rear, is intended to pilot the drill so it cuts reasonably straight, and on size. Creating a primary relief as you suggested would be a huge mistake.

Harold

Hobbyists may have to make do with whatever we own. My better S&D bits can cut very close to size if they take only a light cut like a reamer. This requires a slow controlled feed, either the tailstock handwheel or the geared fine feed on the mill, otherwise they dig in.

To preserve their sharpness I mostly use a cheap import wooden-box stub S&D set, which has held up to steel better than I expected. jsw

Well, if you are accomplishing what you set out to do, and apparently you are, then you are "doing it right" :-) I was commenting on the generally accepted practice. But if you are doing it on a lathe and accuracy is required you could always bore the hole to final size.

I read or heard the suggestion that a drill bit cuts closer to size if it doesn't have to remove much metal, and tried it a few times. AFAICT it works pretty well with properly sharpened bits. jsw

Works just fine for enlarging holes. I've done it numerous times, Harold....

If you have a means of circular grinding, then fine, go for it..

--I don't, but I do have an endmill sharpener and when in a pinch, I tend to use it for all sorts of things

No shit.

I've been a machinist for nearly 40 years and have known about circular lands and backtaper for just about the same length of time.

Why exactly do you think they would "bind" if they had a primary side-clearance ?

Is ground circular with a backtaper, and then the margin is narrowed, mostly because that makes it less costly to produce while still substantially reducing friction.

The back taper is to help prevent rubbing and binding in the event the drill runs out due to an off center point; with a primary that's ground similar to an endmill, this is not neccessary, since it will simply cut instead.

Creating a primary relief as I suggested allows a drill to cut "on size".as well...in fact, it allows you to use a drill that would otherwise have cut oversize.

Baloney....you're completely ignoring the part where I asked "why f*ck around with them at all untill you *absolutely need* some specific odd sized drill bit"

And, just because I failed to implicitly mention that I'd only do this in an emergency situation and that I'd probably first drill with a smaller drill drill does not mean I'm suggesting that he should take his "spare drills" and resize them all by grinding a primary on the OD...

Now that that's out of the way, generally speaking, I don't do this with a drill *unless it's over an inch in diameter; instead, I'll just use an actual (expended) end mill instead, since my milling machines are all 40 taper, the holders quite comfortably accept shanks up to that size....so the process will be to drill, pre-bore (with an endmill or modified drill) and then either ream or single-point-bore with a criterion or other similar type boring head.

You are probably correct as drill bits having two cutting edges depend on the cutting edges being identical in order not to wander around in the hole, so the less cutting edge you use the better chance you have of the drill running straight.

Thanks for all the replies!

Accepting the majority opinion, I'm not going to pursue this. Although, in an emergency I might stoop to it.

My original thinking was something like this: I got the 9-piece set cheaply on eBay ($26 for Triumph bits), and could not justify the difference for 18 pieces (1/32's). Filling-in with separate 1/32's would be too expensive ($10 - $20 each for ENCO's cheapest).

But I had recently needed a 1/32 (17/32 for a 5/8 tapped hole?). So, why not take advantage of those redundant bits? I still might, but only on a desperately-needed basis.

Thanks again, Bob

If you have a surface grinder an endmill fixture like this might work:

I found it very tricky and difficult to use.

You can make custom D bits with a lathe, mill and bench grinder.

I bought O-1 drill rod in common shaft / bearing sizes, 1/2", 5/8",

3/4" and 1", and turn it down for other uses that don't need the precision ground finish. jsw

Didn't say it doesn't work. What I hoped to say is drills don't have a primary or a seconary for good reason. They are ciruclar ground to act as a pilot, limiting the drill's ability to cut on the side. When you give up that feature, you invite oversized holes, with the least bit of resistance at the cutting edge allowing the tool to cut oversized. If a guy hopes to achieve a given size, that's hardly the way to go. Much like drilling with an end mill. It works, but you're at the mercy of many things.

Then you should know better than to advise others to grind drills that way, shouldn't you!

And not that it matters, but I've been a machinist since 1957---and I learned about the geometry of drills shortly thereafter. Lets see---that's

55 years. So what? Wrong is wrong---doesn't matter how long.

Didn't suggest they would. What I know they'd do is cut oversized. What the hell good is a drill you can't rely on?

That's total bullshit, and, assuming you have even remotely the experience you claim to have, *you should know it.* Don't think so? Drill a hole with an end mill, and tell he how it holds size. What's the difference?

Harold

The cause of oversized holes is invariably due to unequal lip angle or length (off-center-point), and if a drill is going to cut oversized, it is going to begin doing so at the very top of the hole, at exactly the point where both lips are fully entered into the material, before the margin even enters the cut.

And even after it enters the hole, a circular margin does nothing to correct this because for all practical purposes, the geometry that a circular margin presents to an already oversized hole is essentially an eccentric relief.

Then either you've forgotten much of what you learned, or you never came to properly understand the operation in all those years.

All drills are going to cut oversized to some extent; and so your entire concept of them cutting "on size" is bogus in it's face.

What happens is one or the other flute will ALWAYS excur past the theoretical (drill body) size due to off-center-point and /or unequal-lip-height, and the hole will continue to be drilled at that size until the poiint begins to break out through the bottom whereupon the cutting action is no longer being influenced by point geometry. The circular land has no function here because, as with an end mill, it only has point contact with the wall of an oversized hole (unless it's of the spade type, in which case there will be line contact but only if the drill has no back taper)

Functionally, there *is* little difference, aside from drills having a gullet and point geometry that produces a long, continuous chip which serve to aid their ejection.

FWIW, I frequently drill (shallow) holes with an endmill, too, and they typically come out no more than .005" oversized.