Vibration suppressing device

A vibration suppressing device for suppressing chatter vibration generated in a rotary shaft in a machine tool having the rotary shaft for rotating a tool or a workpiece, the vibration suppressing device comprising:

• a detecting unit configured to detect generation of the chatter vibration;
• an arithmetical unit configured to calculate a stable rotation speed capable of suppressing the chatter vibration based on a value detected by the detecting unit;
• a rotation speed control unit configured to control a rotation speed of the rotary shaft;
• and
• a memory unit configured to store the stable rotation speed calculated by the arithmetical unit in association with a rotation speed range in which the stable rotation speed is included;
• wherein, when a command rotation speed that is rotation speed of the rotary shaft is inputted at time of starting machining operation by rotation of the rotary shaft and, the arithmetical unit determines whether or not the command rotation speed is included in the rotation speed range, reads out the stable rotation speed when the command rotation speed is included in the rotation speed range, outputs the stable rotation speed to the rotation speed control unit instead of the command rotation speed, and starts machining operation at the stable rotation speed.