Rigid tapping is similar to threading on a lathe, the way it is normally achieved is with an encoder on the final spindle, not the motor.
A closed loop system where the servo position are fed back and controlled by a single trajectory planner is usually capable of electronic gearing, that is where in the case of a mill, the Z axis is electronically geared off of the spindle encoder, therefore the system does not require a spindle that has servo capability.
The Z servo tracks the spindle encoder.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.