In article .com,
wrote:
I need a source for 1/2-8 LH Precision Acme Lead Screw for the cross
slide of my Sheldon Lathe. The specs are listed below for the screw. I
have a .125" direct reading dial. I think it is a single start screw
but am not sure.

Hmm ... by "direct reading", do you mean that the cross-slide
moves 0.125" for a full turn, or that it moves 0.0645" per turn,
resulting in a removal of 0.125" diameter from the workpiece?

If the dial is 0.125" for one turn, it should be single start.
you would have a full turn adding up to 0.250" or 0.500" for a
double-start 8TPI, depending on which measurement system you are dealing
with.

MSC has double start 1/2-8 LH, but what is the difference in single and
double start?

A double start has *two* thread grooves. In this case, they
would each have a pitch of 4 TPI, and one would run half-way between the
turns of the other. As a result, it moves twice as far for a given
rotation.

*And* -- your dial would have to be 0.250"

Here are the specs on the screw from someone I know who worked for
Sheldon:

The thread is an American Standard 29 degree Acme GP thread, 1/2"-8
left-hand. The screws made by Sheldon had a 5 degree 25' helix &
measured
.550" to .552" over 3.072" wires. They were made of Stress-proof 1/2"
ground & polished stock.

Hmm -- wasn't that "three 0.072" wires", not "3.072" wires"?

Can you still get the leadscrew from Sheldon? It is likely to
have parts which are larger in diameter than the thread (let alone than
the thread root), which would have to be turned to act as bearings, and
to mount the gear for the power cross-feed, so starting with a bare
threaded shaft would mean that you would have to join it to other pieces
of metal to complete the leadscrew assembly. I know that my cross-feed
leadscrew goes to above the 0.500" diameter at several points past the
thread.

Any suggestions greatly appreciated.

Hopefully, someone else will know where to get the pre-threaded
stock. I'm afraid that I don't.

Of course, you could try turning a new leadscrew shaft yourself.
They've told you the material and the starting size, and how to measure
that you have the thread depth correct. You'll have to take out the old
leadscrew and make a lot of careful measurements and a good drawing
before putting it back in while you turn the new leadscrew.

You'll need a steady rest to support the rod as you turn the
threads, or the threads will be too tight in the middle of the screw's
length, thanks to it bowing while you turn.

Enjoy,
DoN.
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