Hi,
You don't mention what type of
cnc controller you have and whether or not the
cnc has the capability to do rigid tapping. The spindle drive amplifier must also be capable of providing full torque to near zero motor speed (should be a full vector type ac drive).
If your system has the capabilities, it is also best to use a quadrature encoder attached directly in some way, to the output shaft of the spindle.
If you don't have these capabilities, then you could be throwing away good money.
Good luck,
Ed Mader
President - EMR, Inc.