Its usually done one of two ways, if the spindle is driven by a regular non servo spindle drive, when using it in a C axis fashion, the spindle is de-activated and a C axis servo motor is enabled to position the chuck.
The other method requires a spindle motor and drive system that is capable of servo control.
The problem with the second method is that it requires an encoder on the final spindle shaft and if there is a multi-change gear box between the motor and the chuck, then this can cause problems in positioning.
If there is no gearbox (direct drive), then the latter method can be used.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.