It would be an interesting and challenging construction, one to learn something from. I would have a go at it, especially as the intended lathe is cheap enough to invest in the idea, even if it fails.

Good luck

Brian

Andrew – what is the problem that you are trying to solve?

The Emco Compact-8 that I have is my "go-to" lathe. I do have a larger English "1124" lathe, that rarely gets used, the Compact-8 does everything well, and the "issues" it has are easily bypassed.

Threading – I simply spin the chuck by hand, if I am making a short thread. I've learned to keep a leather glove handy for times like this!

I do have a second Compact-8 with a VFD on it, which was a bolt-on change as the motor was a standard frame; I'll admit that this one was going to be CNC'd, but real work as got in the way, and other than having fun with the spindle motor, it has sat for a year, so no experience turning with it under VFD control.

Edited By John Alexander Stewart on 27/11/2017 12:00:40

If you just want a lower speed for screw cutting why not just fit a 3-phase motor and vfd?

I had a 920 lathe for a couple of years as a second lathe and found it to be a good lathe except for the speed being a bit high for screw cutting. I had to sell one of my lathes to make room for a cnc mill and chose to keep my UK made Atlas clone despite it being rather more worn but because of the back gear and the number of accesories I had made to fit it. I have since fitted a vfd and now rarely use the back gear.

Russell

There are a lot of these about so a generally applicable solution would be nice rather than a specific one aligned to one motor. Also you want to be able to get the high speed settings still as you are bound to suddenly need it if it isn't available.
A back gear also delivers more torque for cuts on larger dia but as you say you want it for screwcutting that isn't a problem, and belts can still deliver reasonable grip.
Have you considered the possibility of getting another intermediate pulley into the 'space' at the top rear? (with modified cover)

Use an internal threading tool that cuts on the far side of the hole and run in reverse starting at the bottom of the whole so the tool moves out of the hole at whatever speed you fancy

If you just want to slow the machine for an activity like screw cutting you would be OK. If you need to slow the machine to turn large diameters then the increased torque from gearing down could be useful.

Mike

For a variable, low speed, why not make a Mandrel handle?

The limit on availability of torque is your strength and the length of the handle.

Howard