How to use AC motor with axis?

Nigel Pearson wrote:

1) Have people tried closed loop feedback seperate from the axis motor shaft?
i.e. measuring the table position, which is 2 or 3 mechanical moving parts away from the motor pulses
(reduction gear, axis screw float/backlash, table gib slop+rotation)

Yes, but usually with tighter coupling than you are suggesting, and generally with velocity-control servos.

But, depending on what you want, it might work. How close do you want to get to the commanded position?
Does the motor move at constant speed, or is it software-variable?

2) How hard is it for LinuxCNC to keep up with TTL scale (quadrature) signals?

That all depends on the sample rate of the interface, the resolution of the scales and the speed of motion.
With the parallel port you are likely to hit the limit if you have fine scales and/or try to move fast. With the Mesa / Pico / General Mechatronics interfaces the sample rate is several Mhz so you are pretty unlikely to hit the limits.

3) If that is possible, any suggestions for interface boards to do it?

What do you have now? I would be tempted to experiment with the parport if that is what you have.

Thanks for your reply, Andy.

I only have slight experience with parallel port setup.

These motors could have variable speed if I work out how to drive them that way. Of course, like most non-steppers, they are low torque at low speed, so there is some tuning issues there.

As a follow up, I opened up one of these drives this week: 

I thought the forward/reverse lever was just a switch, but on this model, they are also a lever that engages the motor shaft with the small bevel gear.
(i.e. the clutch, so that you can manually turn the mill axis handle without driving the power feed motor as well.)

This means effectively the forward & reverse is electrical, but the on/off is both mechanical and electrical.

Not easy to interface to - would need to mechanically lock the direction lever in one direction, and then override all the electronics to get fwd/rev.
Possible, but not worth the time. Removing/selling the power drives, and adding steppers, will be much faster/easier

If you wanted to retain hand control, you could consider adding an electric clutch that was activated released with 24 V power. I seriously considered this using some old clutches salvaged out of a photocopier..