"Martin Rost" rostmartin @ hot mail . com writes:

It is torque, not horsepower that cuts the wood.

True. Horsepower lets you cut the wood *faster* though. Imagine a
lathe with a worm drive instead of gearing. The wood would only turn
at a few RPM but it would be pretty much unstoppable.

I can accept that as torque is a measure of rotational force.

Torque is foot-pounds. For a given torque, the closer to the axis you
are, the more linear force is available for cutting.

With a Reeves drive at low rpm you gain torque over high rpm.

True. Torque is foot pounds; you can change torque with gearing.

With electronic variable speed you lose torque at low speeds, this is why
the bigger lathes have 3 hp motors, to make up for the lost torque.
This one I don't buy, but I don't know if it is right, wrong or
partially correct. From posts in rcw, I though with DC motors you lost
torque at low speeds, but AC (3 phase) motors maintained torque at low
speeds.

A given motor can generate UP TO a given horsepower. Speed controls
can only REDUCE the amount of horsepower. An ideal controller/motor
combo would maintain peak horsepower, but vary only the frequency.
Unfortunately, because the coils in such motors are inductors, lower
frequency means less resistance, so you have to reduce power at low
speeds to prevent burnouts. This means less horsepower. I suspect
that at lower frequencies you have to run the motor at a lower
magnetic offset angle, which reduces the torque the magnetic fields
apply to the armature, but I'm just guessing on that one. A really
smart motor/controller combo would detect the amount of torque needed
to maintain a given RPM, and adjust the angle and power accordingly.

The gearing determines the ratio of horsepower to torque, which is why
big lathes still have step pulleys.

Big lathes have bigger motors because you want the motor to overpower
the wood, and still provide usable torque at the largest supported
diameters. Big lathes allow heavier and larger diameter wood, hence
bigger motors.