I think the logical reason is that smaller holes
are drilled shallower than large holes and that
large-diameter drills are stronger and can be made
longer than small-diameter drills.
I use a set of screw machine-length drills for
all work that requires accuracy. They are quite
a bit shorter than the normal jobber-length drills.
A small diameter bit has to be short to prevent it from whipping, buckling,
and breaking when used. Larger diameter bits can be longer while retaining
a satisfactory diameter/length ratio. Now large diameter bits *could* be
made shorter. That would lessen the difference in lengths between small
and large bits. But as far as I know, no one makes a set like that.
Yes. Bigger holes also tend to be longer holes. Also the rigidity of a drill
bit is a function of its length/diameter ratio. The smaller the drill the
shorter it needs to be to work without deflection. You can buy "stub" drill
bits which are shorter and more rigid and designed for more accurate on centre
work but the length will still vary with diameter.
Anyway, ain't no thang to work out in advance which is the longest drill bit
you'll need for a job and make sure the machine has clearance for that length
before you start.
Dave Baker - Puma Race Engines (www.pumaracing.co.uk)
email@example.comNoEmails (Dave Baker) wrote in message
Hey, why *buy* stub drills? I have a pretty good set in the 1/8 to 1/2
range. Usually from resharpening full length ones I've broken :-)
Also for holes in the 1/2" to 1" range, the OP could consider 1/2"
shank drill sets. These in my experience vary much less in length than
the big taper shank drills. Before I got my radial arm drill I used to
use these to step-drill holes because it was faster to swap them over
than it was to drive out a TS drill and seat the next one. Now of
course I just drill a pilot hole the correct size, then the full size
hole. Slow speed & power feed make life simple.
You've already gotten quite a few good replies. But there is
one thing which I don't see mentioned, so I'll plunge into that.
First off -- what is the maximum diameter which you actually
In the range up to 1/8", there are a series of drill bits made
with 1/8" shanks, so the small diameter part can be appropriate to the
diameter of the drill.
These also tend to have split points (so they are less likely to
"walk" when starting a hole on a level surface, and are available to
amazingly small diameters.
These are typically solid carbide bits (though the shank may be
steel). They are made and sold for automated machines drilling holes in
printed circuit boards. Since no printed circut board is very thick (I
would be amazed to see one as thick as 1/4", even with multi-layer), the
flutes do not need to be very long, and these can be made a constant
They show up from time to time at flea markets, and usually have
colored collars around the shank, to help pick the right size (with the
right chart to look them up. :-)
But in all likelyhood, since you don't mention your needed hole
diameters, at least some of them are quite a bit bigger than 1/8".
Another point which I did not see mentioned is that each
resharpening removes a length of drill flute proportional to the
diameter, so with the varying lengths, each will usually accept about
the same number of resharpenings before becoming too short (and too
thick in the web) to work well.
I guess that you could make a series of drill bit holders for
the common sizes which would put the tip of each at about the same
distance from the chuck, to get an approximation of what you want.
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