You do need fairly tight control of the spindle in tapping applications and a servo can be used, but it also depends on what system you are using.
Some systems do not require servo type control, as long as rpm can be controlled fairly accurately through a digital tach/encoder on the motor itself and an encoder on the final spindle.
The Z axis will then synchronize to the spindle rpm through the final encoder.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.