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.
There might be a small discontinuity where you flipped the screw, but
that could be cut off later.
Free men own guns, slaves don't
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.
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.
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| > Seems to me that if you used a die you would likely get good
| > but if you pulled or pushed on the stock while it was passing through
| > die you would gain or lose a thread or more over the length of the
| > Again, it's a tolerance stackup, but over a long item it adds up to be a
| > whole bunch, especially on a lead screw.
| Great information. Question though, is that an SAE "whole bunch", or
| a metric "whole bunch"?
Neither. It's an "Oh, s**t!" whole bunch.
Die cutting would not be very accurate for a long thread. The die will want
to wander and the concentricity will be bad. I missed the part about cutting
a lead screw in the OP. Precision lead screws are usually ground. Although
some ball screws and lead screws today are cut out of hardened blanks on
thread whirling machines. http://tinyurl.com/5bn5d
Non precision lead screws (those not used in a machine tool) are for the
most part thread rolled. Long threads like those on all-thread rod are
through-feed rolled on a double roll type thread rolling machine.
You can easily make a long machine screw to the accuracy required by cutting
the threads in sections on a manual lathe. The problem with cutting a lead
screw on a lathe is that all of the accumulated error in the lathes drive
train, plus deflection, run out, slide wear, and thread form errors in your
tool all stack up and prevent you from making a lead screw that's as good as
the one in the lathe.
That is what the optical comparator was invented to do, check thread form
and thread lead.
Modern manufacturers of ball screws and precision lead screws use special
electronic gaging machines that check the lead, run out, and accumulated
A die won't give a precision lead to the part. Because this is
a leadscrew for the lathe, that's not the best way to make
Leadscrews not only have to have the correct thread geometry,
but they are effectively length standards in and of themselves.
A die-cut leadscrew would work, but it would impart all the
lead error into whatever parts were then fabricated when
using it on the lathe.
please reply to:
Screw threads have been fabricated for a long time, using manual
methods like chasing. The real trick to to create the metrology
needed to check them, and refine them. Basically you are talking
jo blocks and maudsley.
please reply to:
I would guess (note the word "guess") that the better way would be to turn a
couple of threads onto the end of the stock and then stick it into a
precision nut which is the work holder for a grinding machine and grind the
thread. The grinding wheel is always going to be a constant distance from
the nut so just apply constant pressure to continuously turn the stock and
work it into the nut and a screw of unlimited length could be done
accurately. Maybe I can make an attachment for my lathe!
I have absolutely no idea if this is going to work, but it is just an
idea to save on 2nd, 3rd, etc, realignments (which probably saves
nothing if you are making it for your own lathe).
1) Make first bit of threaded screw.
2) make threaded tube to match screw, tube extends through head stock,
3) make "lock nut" (or 2?)
4) mount threaded tube into jaws,
5) load lock nut onto thread part of screw
6) load threaded bit of screw into tube
7) lock threaded screw into threaded tube
8) align in jaws and tail stock
9) realign tool.
10) cut next section
11) unlock lock nut(s)
12) screw threaded rod through threaded tube and relock
13) got to 10.
Theorectically you could make it as long as you like, but you need
steadies on the long bit,
 each turn just advances the rod the size of the pitch.
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