Aviation Forum / General / Homebuilt / September 2008

"...but I don't have a lathe."

Sound familiar?  The truth is, you probably have any number of things
around your shop that can serve as a lathe.

In a lathe, the work rotates against the cutting tool. ( In a milling
machine, it's the cutting tool that rotates against the work.)  The
main purpose of a lathe is to produce a circular part, which is
achieved by pressing the cutting tool against the work-piece as it
rotates.  Anything that achieves that goal with sufficient accuracy for
the task at hand will allow you to do the job.

The most common thing used to rotate the work is the ubiquitous
quarter-inch drill-motor.  A drill press works even better.  And the
cutting tool may be anything from a hand-held file to an angle-head
grinder fitted with a flapper wheel; even a portable belt sander can be
your 'cutting tool.'

"Tried that," they usually say.  "Didn't work."  Odds are,
they didn't try hard enough :-)

The trick to holding the work-piece in a drill-chuck is to use a
coupling nut.  Put the coupling nut into the chuck and tighten it
evenly, taking the slack out of all three jaws.  (Just put the chuck
key in each hole in turn, taking out any slack.  If you tighten a
Jacob's chuck at only one point there will always be some slack in
the other two positions, usually enough so that whatever is chucked
will not run true.)

The centroid of the work-piece is then drilled to accommodate a bolt
that matches the coupling nut.  The standard 1/4-20 will serve for most
work but coupling nuts come in all sizes.  To keep the work-piece from
spinning on the bolt you may use a lock washer.

When using a hand-held tool as your cutter, be it a file or even a
belt-sander, the trick to producing a true edge is to use your eye to
align the OPPOSITE edge of the work with a known-true reference, such
as the string of a plumb-bob.  Or the blade of a protractor, if
you're creating an angle.  Any deviation is instantly apparent and
just as quickly corrected.

In most drill presses the chuck is attached to the quill with a simple
taper.  A taper has the advantage of providing a virtually error-free
alignment but has the disadvantage of not being able to withstand much
of a side-load.  Even a 3M fiber-wheel (used for dressing aluminum
edges) may cause the chuck to leap off those cheap Chinese drill
presses.  To prevent that from happening when using the drill press as
a lathe, the head of the bolt you use to secure the work-piece to the
coupling nut should be drilled in a cone large enough to accept a small
ball bearing.  Simply trapping the ball between the drill-table and the
bolt will provide enough axial pressure to prevent the chuck from
coming adrift.

- - - - - - - - - - -

There are endless elaborations to this theme but the basic idea is that
you DO have a lathe, if you put a bit of thought into it.  Although
crude in appearance such jury-rigged tools are accurate enough to allow
turning down a piece of 4130 tubing to fit a standard bearing, make
circular flanging dies of any diameter and even create the plug for a
10 x 12 inch spinner for your propeller hub.

-R.S.Hoover