this shows unit with cover plate off. The worm gear is pinned to a shaft with the knurled knob on the end under the plate. The shaft can slide axially and there's a sprung ball detent down the bearing hole in the cast housing. Fork on the end of the shaft engages with a tongue on the end of the leadscrew. So sliding the knob in and out engages/disengages both the dog clutch and the worm gear. The worm is driven by the pulley groove on its shaft projecting to the right.

Bit late in the day to measure it and anyway the design is different and I suspect a bit smaller than the C0 version. The belt drive from the mandrel is in my opinion highly unsatisfactory with a short belt with a 90 twist in the loop to accommodate the different axes. To be frank I wouldn't bother with making your own but just arrange a geared DC motor to drive the leadscrew.

I used my Unimat one quite a lot and provided you don't get too greedy with the cut they work OK, neither lathe is designed for heavy cuts anyway.

The Autofeeds for the Sieg mills use gear head motors and a dog clutch arrangement, you would need to work out gear ratio by doing a few tests to see how fast you turn the feed screw both for cutting and rapidly returning the carriage and your variable speed drive to cover that range.

I thought the C0 had an M8 leadscrew not M5

Edited By JasonB on 22/08/2021 07:42:39

I just checked and the U3 leadscrew is 8mm but of course a LH thread.

This would probably be a suitable motor. It is this type available on eBay, though I bought a couple years ago and haven't used them. Size is 36mm dia x 60mm L. Shaft 5mm dia with flat, 8mm long. Speed at 16V supply 340 RPM so would need another reduction stage probably as well as a speed control such as the ubiquitous MFA one. There are lots of other similar motors around. The U3 drive is 0.02mm/rev, so at 500 rpm of the h/s that's 10mm/min. As the l/s pitch is 1mm (I assume) the leadscrew will be doing 10rpm. So driving it direct from the motor you would need 16×10/340 = ~0.5V average which would be right at the low end for the speed control. Hence the desirability of an addition reduction. Motors are available with significantly greater reduction. (Note to Mods: these links are given as examples to show what's available – if you delete them I will be disgusted.)

I think it would be easiest to position the motor at the handwheel end with a modified or new handwheel, possibly with a pin clutch of some sort or a slippable belt. Assuming the C0 uses the same fork and tongue arrangement as the U3 at the headstock end, making a stable connection to the motor could be a bit tricky.

A stepper would be equally good though bigger and more expensive but a modern size 23 one would have plenty of torque even for direct drive. I use a rather old fashioned stepper on my coil winder to drive an M6 leadscrew and there is quite a lot of friction but it works fine. If you used a stepper it's a short step (sorry!) to fitting a digital leadscrew system!

If you did want to use a worm reduction you could perhaps make the worm wheel by free hobbing using a suitably sized tap, with the worm itself being made from a bolt with the same thread. No need for dividing head or gear cutters.

LittleMachineShop have supplied me in New Zealand with spares and upgrade kits for my X2 mill.

Take a look at the Australian Sieg agents website, lots of spare parts pictured there.

I had the same issue, could not find a drive unit outside USA or Australia so decided to make my own from some box section and a few bits from amazom/ebay the worm screw is a 60:1 reduction