Possible to program spindle as A axis and use as lathe?

[bebob1]

I havent gotten a lathe CAM yet so I am unfamiliar with how it works.

If I want to create threads, can turning lathe CAM output gcode in terms of controlling a stepper based spindle? Or do they operate on RPM in order to time the threading cuts correctly?

[doorknob]

I have not done it, but my understanding is that typically a slotted disk connected to the spindle (or similar method) is used with an opto-interrupter to generate an index pulse (one pulse per revolution of the spindle) which is fed back into the controller software, which uses that pulse to determine spindle motion and rpm for synchronization.

See http://www.machsupport.com/docs/Mach3_Threading.pdf for more details.

That said, I believe that some CNC lathe setups use a servo motor to drive the spindle (for precise control of spindle positioning), and so with such a setup the spindle could be treated as another axis, using signals from the encoder to synchronize the threading operation. Conceptually, you could use a stepper motor for the spindle, but stepper torque falls off as speed is increased, and so whether or not that would work for you would depend on whether you could obtain adequate performance from the stepper.

[bebob1]

So does the gcode tell the lathe to wait for the trigger signal to move always? So the CAM side has to know to do this already?

So does the CAM have another style of output for threading where it is based on spindle rotational position instead? Or is lathe thread gcode generally the same, except that the spindle rotational position if it was a servo or stepper, is just offloaded to the CNC motion control software side to automatically know when the trigger is?

I cant fit an encoder because I cant add connections to my parport since its a commercial driver, and I dont have a breakoutboard to utilize unused pins.

[Al_The_Man]

In a commercial system there is no need to make the spindle an axis, just an encoder on the spindle and the Z axis gears itself off the spindle encoder, so if the spindle speed varies slightly the Z changes accordingly.
With Mach and systems that do not close the loop in the control, gearing is not usually possible so what is done is a one-per-rev pulse on the spindle indicates the rpm and the Z axis moves according to this one/rev pulse.
The CAM system usually just calls a Threading routine and does not know the method used.
BTW, and encoder on the spindle makes it a C axis rather than A.
Al.