i am retro fitting my old lathe with stepper motors driver and breakout board all from china (work grate on my mill )  what do need to synchronize the spindle to  the feed to cut a thread sum  thing must tell the feed when to engage when the spindle is in right place

You will need a sensor which will provide an index signal to Mach.
Mach uses the index signal to control the threading and also rpm readout.

Have a read of the Mach3 Lathe Manual for how it's done. Also have read of Threading on the Lathe- Mach3 Turn writeup.

RICH

thanks rich i will read it