The question's too open-ended for accuracy but experimenting with a set of bathroom scales I find I can press down 20kg without exerting myself unduly. That's 200 Newtons rather than Neil's 100N but we're very much in the same ball-park.

The diameter of the hand-wheel gives a mechanical advantage, say 4:1. Neil's 100N becomes 400 and my 200N becomes 800.

The gearing between hand-wheel and rack provides a further mechanical advantage, on my machine 10:1. That puts us in the range 1000N to 8000N

As I weigh 90kg (oh dear), that gives the maximum force I could apply to the handle by using it to lift myself off the ground, say 900N x 4 x 10 = 36000N

A lathe fitted with power-feeds (big motor with much gearing down) would apply much larger forces – enough to break the machine. That's why they're fitted with shear pins.

Quite an interesting question to ask of a lathe is: 'what will break first if I overload it?' Ideally it will be something quick and cheap to replace. Mini-lathe owners sometimes replace the plastic gears with metal. Not convinced that's a good idea: apart from the increased mechanical shock a stalled motor might fry the electronics.

On my Chinese lathe, the first thing to break is the shear pins. (Don't ask how I know.)

What breaks first on your lathe when it's abused by an angry gorilla?

Apologies in advance if my maths is wrong. I offer the answer partly in expectation that some kind person will correct me! Again…

Dave

Edited By SillyOldDuffer on 05/03/2018 11:50:29

Ratio of effective diameters will provide the mechanical gain. It will not be more than that.

I'm not sure that the force that can be applied by pushing or pulling is relevant. Rotating a handwheel uses a different combination of muscles.

I don't have a Myford but when I put a spring balance between the tailstock and the toolpost on my Atlas (similar design to Myford but a bit bigger) I could get up to 40 kgf. I am only 56 kg and in my 70s so you can probably double that for a bigger, fit young man.

Russell.