DoN. Nichols wrote:

The servo motors which I have used are rather specialized, and I
don't think that the treadmill motor will be sufficient. The rotor and
the tach generator are just cages of wire, wound to shape, connected to
the commutator, and rigidly attached to the shaft. The magnetic circuit
is a combination of a set of poles inside the cage (which do not move)
and either a permanent magnet field or a DC coil to drive the field.

The reason for this construction is to minimize the rotating
mass, so it can get from one speed to another (including stop) as
quickly as possible.

This is an ironless rotor motor, a very expensive sort used
where extreme acceleration is needed, such as computer tape
drive capstans, which have to go from zero to several thousand
RPM in a millisecond or so. I have an ironless rotor motor with
ceramic shaft on my Bridgeport Z axis, entirely because I pulled
it out of a scrap tape drive. You most certainly do not need
such a motor on a machine tool. A motor with a sufficient
number of windings (slots & commutator segments) will have low
enought torque and velocity ripple to work fine in most cases.
The treadmill motors may be a tiny bit marginal in this case,
but likely will work fine.

Jon