These lathe CNC conversion posts are probably becoming a bit tiresome, but just in case there is someone out there who is interested, I will continue until the job is finished.
The latest was to make and install a spindle speed (and position – thanks David M) sensor. It consists of a disk with a slot cut in the periphery, attached to the main spindle. And an opto-electronic sensor which is connected to its own electronic board, thence to the breakout board and VSD.
So there is one electronic impulse per spindle revolution. That is enough to measure the RPM’s. Essential for cutting threads.
The beauty of this system is that there is no gear selection or changing, and ANY thread pitch can be selected… metric, imperial, BA etc… any odd ball thread that your heart desires.