In article ,
Nick Hull wrote:
What I would think of doing is cut the thread from the headstock end
toward the tailstock end, using no tailstock but supporting the long
unthreaded part with a center rest and other outboard expedients. I
would start by cutting a doot of thread normally and flipping the screw
over and screwing it into a special 'nut' in the lathe jaws.

I would make a long tight fitting nut and slit one side and
semi-permanently (glue?) it to the chuck so it could not move. Then
screw in the just threaded part and clamp down and cut threads away from
the chuck. When you need to move the screw just loosen the chuck jaws
and screw in the section you just threaded.

Hmm ... how about soft jaws on the chuck, bored to the right
radius, and single-point threaded? Your initial thread on the workpiece
(cut before you mount and prepare the soft jaws) would have to be long
enough so you could then spend the time to properly align the tool with
the threads (perhaps by disengaging the tumbler gears until you get
things in sync), but once that is done, your remaining problem is
starting the thread without a lead-in groove or a shoulder. If you have
a lever setup to feed the tool into engagement and back out, you could
do pretty well, as long as you don't engage too far back after several
thread passes have already been made.

There might be a small discontinuity where you flipped the screw, but
that could be cut off later.

If there is a discontinuity there, there will be one each time
you shift the workpiece. It is better to tune things to eliminate the
discontinuity from the start.

But certainly an interesting approach.

Enjoy,
DoN.
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