Index Home About Blog
From: frisbie@flying-disk.com (Alan Frisbie)
Newsgroups: rec.crafts.metalworking,rec.woodworking
Subject: Re: Bore 5/8" hole 36" long thru slab???
Date: 14 Jan 96 12:36:28 PST

In article <1996Jan7.212202.735@flying-disk.com>, 
frisbie@flying-disk.com (Alan Frisbie) writes:

> A friend of mine had a similar problem when he was trying to
> duplicate some antique musical instruments.   He tried every
> high-tech drill and technique imaginable.   He was all ready 
> to have them laser-drilled when he found the solution: a
> 100-year old ship auger.   It worked like a charm.

I was wrong about the name of the tool -- it was not a
_ship_ auger, but a _shell_ auger.   My friend gave me
an extensive education on deep-hole drilling in wood
which I cannot hope to reproduce here, but I will try to
give you the basics.   I apologize for continuing this
woodworking thread in rec.crafts.metalworking also, but
it *does* require metalworking to make this bit.

A technology closely related to your application is
"gun drilling", even though drilling gun barrels is only
a small fraction of the applications today.   It simply
refers to making very deep holes in small diameters.

It uses straight-flute drills with high-pressure coolant
fed right down to the tip.   This also serves to eject
the chips, since there is no spiral to help them along.

The shape of the drill is simply a rod with a "V" groove
down its length, with suitable relief ground on the end.
They are similar in appearance to "die drills", which can
be seen in the MSC and similar catalogs.

Since the cutting action of these drills is non-symmetric,
it requires a bushing or pilot hole to stabilize it.   
These drills are not cheap, with $100 being typical for
a 3/8" x 36" bit (3-5 weeks delivery time).   A major
supplier, who has lots of literature available, is:

   Eldorado
   336 Boston Post Road
   Milfort, CT 06460

   203-878-1711 (voice)
   203-878-6156 (FAX)

If you decide to use their bits, be sure to tell them that
you will be using it for wood.   They will then grind a
special contour on the end to make it work better with wood.
They recommend a feed rate of 0.001" per revolution in wood.

The bad news is that gun drills are not really suitable for
use in wood.   Wood requires more of a "shearing" (LOTS of rake)
action than metal.   The shell auger provides this, but at
the expense of being very difficult to grind.   A lot of hand work
is necessary.   I will attempt to describe it here, but will
FAX a drawing to anyone who requests it.   The basic shape is
similar to a gun drill.

Start with a round drill rod.   Using a grinding wheel slightly
narrower than the rod, dress the wheel to a half-circle on
its rim.   Use this wheel to grind a groove in the rod from
*near* (but not *at*) the tip to several inches back from
the tip.   The exact length of the groove is a tradeoff
between strength and convenience in chip removal (see below).   
The depth of the groove should be such that you wind up with
approximately half the rod remaining, and a sharp edge on either
side.

The very end of the rod is ground to have a very fine
point exactly in the center.   The remainder of the end is
left flat.   This point is similar to brad point drill bits,
and serves to help center the bit.   It is also optional,
and its usefulness depends on the type of wood, phase of
the moon, etc.   :-)

The actual cutting edge is ground on the very end of the rod,
and it extends from the center to one edge (the "leading" edge)
of the rod.   It is ground to the same form as a Forstners
(flat bottom) bit.   The chip clearance area should be extended
back to the lengthwise groove previously cut.   The sharp edge
of the lengthwise groove should now be extended all the way to
the end with *careful* hand grinding.   If you grind too far,
scrap it and start over.   :-(

Hardening the bit is an issue with no clear solution.   It
is a tradeoff between having a longer-lasting cutting edge
and possible warping.   The "pros" have the right equipment
to minimize warping, and will grind it after hardening.
If you only have a few holes to drill, it is probably best
to not harden it.

The action of this bit is to first shear the wood fibers with
the end of the bit, which provides a path for the drill.   The
sides of the drill then finish the job by trimming the fibers
on the side of the hole.   Obviously, the density and direction
of the wood grain will have a big effect on how smooth and
accurate your finished holes are.

In use, you will need to withdraw the bit often to clear
the chips.   If you fail to do so, they will pack up and
cause no end of grief.   Sometimes, depending on the wood,
it may help to rotate it in reverse occasionally to allow
the "trailing" cutting edge to clean up any fibers that
were missed by the "leading" edge.

Drilling deep, straight holes in wood is a real art.   My
friend spent many months experimenting with different drill
shapes before he finally got it right.   Even then, it
still doesn't always work right.   Every failure was bandsawed
in half to try and determine why it failed.   This was what
eventually led to success -- most of the time.

--  Alan E. Frisbie               Frisbie@Flying-Disk.Com
--  Flying Disk Systems, Inc.
--  4759 Round Top Drive          (213) 256-2575 (voice)
--  Los Angeles, CA 90065         (213) 258-3585 (FAX)

 
Index Home About Blog