CNC Spindle synchronization

[Eatonbrayer]

I have had to replace the servo motor on my small CNC lathe with a 1.5hp 3 phase induction motor, driven from single phase, through a VFD. I use the machine with Mach3 and everything is working fine. Including the r.p.m. encoder. Unfortunately with this "economy" replacement I have lost spindle synchronisation.
My question is, can there be any possible way to synchronise the spindle using this motor, for screwcutting. - If so can I use the simple "hall effect switch" and magnet on the spindle method.
Your advice appreciated.

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[skrubol]

It's got an encoder on it? Or is it just a tach? If you can run 1:1 timing pulleys you should be able to use the index pulse on the motor.
If not, any way to get a reliable index pulse should work as long as the motor doesn't bog when you enter a cut.

[Eatonbrayer]

It has an encoder and rpm is being recorded fairly accurately. When I set up the "input" to active low. I get an intermittent signal in the diagnostics. The green indicator pulses anything from every 5 secs to 25 secs without any apparent regularity. As I understand it, this should pulse once per revolution? Regardless of speed. e.g. 250 thro 1000 tried.