A question about spindle sync

I have a spindle set up as an A axis and would like to cut threads. The motor is a nema23 stepper with 3600 pulses per rev. My question is can I feed the astep into the encoder.0.phase-A and use a hall effect sensor for encoder.0.phase-Z?
The max spindle rpm would be about 500 which equates to 1.8khz. I could use a binary counter to scale the 1.8khz down if necessary.

It should be possible, but using a stepper as a spindle is not preferred for anything over 200 RPM.
Stepper motor have a lot of torque at low speeds, but very little at high speeds, and 500 rpm is high for them.
Have a look at the torque chart, the torque line drops like a rock beyond 200 rpm.
The config for it should be set in velocity mode and have encoder feedback to Linuxcnc, so pretty easy to set up.
Do give it a try and report back how it works, as i never did try that.

You probably don't actually need an encoder counter, it should be possible to do something clever with the stepgen position feedback.

This probably needs a custom HAL component that emulates index-enable behaviour while resetting an offset inside the component.

input pin -> stepgen.N.position-fb
io pin -> index-enable
output pin -> spindle-position
internal-data -> offset modulo full-turns.