I just got a set of reduced shank bits, 9/16 to 1" by 1/16ths. I
previously had a few bits in these sizes, which are now redundant. I
was thinking of reducing their diameters by 1/32 to give me some of the
intermediate sizes. E.g., reduce the "spare" 9/16 by 1/32 to give me a
But this would eliminate the margin that provides relief. Would this be
a huge problem? I could re-create the relief, but that would be so much
more work. Anybody ever done this?
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
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.
Why fuck 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..
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.
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
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
Is ground circular with a backtaper, and then the margin is narrowed, mostly
because that makes it less costly to produce while still substantially
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
> Creating a primary relief as you suggested would be a huge mistake.
Baloney....you're completely ignoring the part where I asked "why fuck
around with them at all untill you *absolutely need* some specific odd sized
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
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,
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?
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
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.
On Fri, 12 Jul 2013 21:36:48 -0400, Bob Engelhardt
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
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.
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.
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.
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.
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.
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.
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