On Jan 16, 3:03*pm, Nick Mueller wrote:
Michael Gray wrote:
which is to be 6.369 tpi

Or almost a 4 mm pitch (3.988 mm)

Nick
--
The lowcost-DRO:
http://www.yadro.de



One way to improve the likelihood of success in machining this worm is
to use an appropriate steel, one formulated for ease of machining.

One of the leaded cold rolled steels such as 12Lxx springs to mind and
would be suitable for working with a bronze worm gear.

For better wear resistance C1144 aka stressproof is also an excellent
choice. This steel is formulated for strength, wear resistance, and
machinability all rolled into one, without subsequent heat treatment.

Fine cuts are absolutely necessary on such deep threads; The coarsest
I have machined myself is 1/8" pitch worm thread (on an Emco-Maier
V10P lathe) which is appreciably deeper than 1/8" acme thread although
the angles are the same. When approaching full depth .001" cuts are
probably required, perhaps less. A dial indicator on the compound
rest would help to size these cuts reliably.

The advice of 2 to 3 passes without increasing the depth of cut to
take out the spring (deflection) is excellent... I do this
automatically on coarse threads (not for every pass) and had forgotten
to mention this. A good cutting oil is necessary.

On a small light lathe, hand-cranked no less, this is going to be a
tedious task... see if you can fit a variable speed gear motor to the
lead screw. A windshield wiper motor with pulse-width modulated speed
control and feed-back tachometer would be the cat's meouw.

You could experiment a little with a commercial speed control such as
those used on wood routers, and try it on a 115VAC brush-type gear
motor. Laboratory equipment suppliers have various geared brush-type
motors and controllers in their catalogues but these are pricey if
new. If all else fails adapt a variable speed hand drill or, as a
last resort, a kitchen food mixer/blender from a second-hand store.

Good luck!

Wolfgang