If this is a form of 3 phase induction motor type spindle, for both rigid tapping and spindle orient you need an encoder on the final spindle shaft.
For a top of the line Spindle drive, Mitsubishi would be my choice, or a cheaper alternative is a high quality VFD that has a PG, pulse generator on the motor or a encoder equivalent.
Does the present motor have a PG or encoder on at present?
I see you mention a Tach, would this be a DC motor? If so you could use a AMC 100A40(AC) or B100A40(AC).
The (AC) gives you built in power supply.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.