Feed axis device of a machine tool

A feed axis device of a machine tool, comprising:

• a driving motor having an output shaft;
• a speed reducer comprising first and second epicyclic reducers each having a sun gear, a planetary carrier holding a plurality of planetary gears, and an outer ring defining inner teeth, the first and second epicyclic reducers being combined with each other so that,
• the sun gear of the first reducer and the sun gear of the second reducer are connected to the output shaft of the driving motor;
• the planetary carrier of the first reducer and the planetary carrier of the second reducer are connected to each other to provide an output part of the speed reducer, and
• one of the outer rings of the first and second reducers is fixed;
• a sensor configured to detect chattering vibration generated when machining a workpiece;
• and
• an actuator for variably applying rotational torque, based on a machining condition or amplitude of chattering vibration, to the other of the outer rings of the first and second reducers in order to change contacting pressure between the teeth of the speed reducer engaging with each other.