What feedbar torque?

Well, you need to do just a little math. First, what is the leadscrew thread?

8 TPI is normal for imperial threading. How much axial thrust would you expect to need for power turning or threading? 100 linear pounds of force? Assuming ideal power transfer, you need about 2 in-lbs. But, the Acme thread is pretty inefficient, so you would want to set it for 10 In-Lbs, at least.

Jon

The torque limit depends on a number of things, all difficult to guage. First, what are you trying to accomplish with the clutch? It may sound like a stupid question, but if you are trying to protect the gearbox, or other parts of the lathe, you may be able to do this very simply by placing a coupler where your clutch would go and connecting the couler with a necked down pin designed to snap before damage to the lathe occurs. The pins are cheap and easy to replace. If you are trying to prevent damage to the work, then that is another affair.

Charles

Wood>I've got a far East 12x36 gearhead lathe and, following a nasty experience

My initial idea was just to protect the machine (the 'incident' damaged a gear in the apron), but once the idea of a clutch dawned I thought it could also be used to automatically machine up to a stop. The self feed is quite independant of the leadscrew - it uses a seperate feedbar to rotate a worm that drives a wheel that, eventually, drives the saddle pinion or cross slide screw. I think my original question was rather too loose - I should have asked something like 'What is the typical maximum thrust experienced by a lathe tool in a lathe with 12" swing?'. From that I can work out the maximum torque the clutch has to transmit.