As Syp indicated there is probabally a tach feedback on the motor, but most amps now use the torque or current mode of operation, when used in CNC applications, and this has made DC tach's redundant to some extent, that why most DC motors now sold come without. If you use them in this manner, just leave them disconnected.
In this mode the loop is closed via the encoder back to the control.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.