Vibration suppressing method and vibration suppressing device for machine tool

A vibration suppressing device for suppressing chatter vibration of a machine tool during machining of a workpiece, the machine tool being provided with a rotary shaft for rotating a tool or the workpiece, assuming that a decimal part of a value obtained by the following formula:

• 60×Chatter frequency/(Number of Tool flutes×Rotation speed of Rotary shaft)
• is given as phase information, the vibration suppressing device comprising:
• a determination unit configured to compute a modal parameter of the tool or the workpiece while the tool is attached to the rotary shaft;
• a first calculation unit configured to calculate a relation between chatter frequency and the phase information of the tool flutes as an approximation formula based on the modal parameter and machining conditions;
• a vibration detecting unit configured to detect time-domain vibration caused by the chatter vibration;
• a second calculation unit configured to calculate a chatter frequency and a frequency-domain characteristic value of the chatter frequency based on the vibration detected by the vibration detecting unit;
• and
• a third calculation unit configured to obtain a chatter frequency corresponding to the phase information that has been selected as a target using the approximation formula if the characteristic value calculated by the second calculation unit exceeds a predetermined threshold value and thereafter to calculate an optimum rotation speed using the obtained chatter frequency and the number of tool flutes.