Stepper spindle drive

Keith,
I'm building a big CNC wood lathe (12" x 120"), and intend to use a
salvaged 1.5 HP AC servo motor to turn the spindle. (The X axis will be
moved by a 4 HP AC servo salvaged from the same IC parts placer machine
over an 80" travel.) By feeding a DC voltage to the input line of the
servo amp, I can make it turn at a constant rate in either direction.

I see no reason why you shouldn't be able to build a hardware pulse
generator to produce a series of step pulses to the axis motor. When
you are doing simple round turning, use a DPDT switch to switch the step
and direction inputs of your stepper driver board to a simple SPDT
switch for your choice of directions on the direction line, and the step
line to your step pulse generator. When you want to do spiral turning,
switch the driver input lines to the output lines of your computer.

One of the list members sent me a schematic for an
acceleration/deceleration circuit that produces a pulse train which
gradually increases in frequency up to a preset limit when started, and
then winds down gradually when shut off. If you have a big workpiece,
this is much safer, and will produce much more reliable results than
trying to accelerate a heavy workpiece instantaneously from a dead stop
to full speed. I can dig up the PDF of the circuit and send you off a
copy if you are interested.

HTH,
Dave