Configuring step/dir Lathe spindle for threading

Jonathan wrote:

I've just connected a 12Nm stepper motor to my micro lathe spindle in the hope that I can use it for threading, see here:

This seems like a slightly odd thing to do. Would it not have been simpler to have attached an encoder and threaded in the normal way?

My question is how can I do threading using G33/G63?

Onterestingly, a stepper motor can be used as an encoder, as they produce quadrature pulses. But that is probably not any help as such, but does suggest one way to proceed.

For spindle synchronised moves you need to connect up the motion.spindle-revs and motion.spindle-index-enable pins.
The former would be trivial (stepgen.3.position-fb woudl be ideal) but the index-enable pin is much more difficult. That requires a once-per rev signal, then when it goes high simultaneously zeros the encoder position and takes the enable pin low to indicate that is has indexed.

One way of doing this would be to use the sim-encoder HAL function, combined with the encoder HAL function. (though that does seem a little eccentric)
www.linuxcnc.org/docs/html/man/man9/sim_encoder.9.html
You would need to connect the sim-encoder.0.speed pin to the same HAL signal as the stepgen velocity input. You can then connect the A/B/Z phases from the simulated encoder to the encoder counter, and it will act like a real encoder as far as the rest of the system is concerned.
It would be more sensible to do it with a custom component which simply emulated the index-enable pin behaviour, but that would involve writing a custom component. (it might already have been done)