Machine Tool

A machine tool for performing gear cutting on a workpiece with a hob, comprising:

• a first holding member for holding the hob;
• a second holding member for holding the workpiece;
• first drive means having a first drive motor for rotating the hob held by the first holding member about its axis;
• second drive means having a second drive motor for rotating the workpiece held by the second holding member about its axis;
• first detecting means transmitting a first detection signal when detecting that the rotational position of the first drive motor is at its zero position;
• second detecting means transmitting a second detection signal when detecting that the rotational position of the second drive motor is at its zero position;
• and
• control means for controlling the operations of the drive motors,
• either the rotational speeds of the first drive motor and the hob or the rotational speeds of the second drive motor and the workpiece being set at rotational speeds different from each other, wherein
• the control means is configured to, when re-rotating the drive motors being stopped to restart machining the workpiece, position the rotational positions of the drive motors, the hob and the workpiece by rotating the drive motors in advance, recognizing, on the basis of the speed ratio of the rotational speed of the first drive motor to the rotational speed of the hob and the speed ratio of the rotational speed of the second drive motor to the rotational speed of the workpiece and the detection signals transmitted from the first and second detecting means, detection signals transmitted when the rotational positions of the drive motors and the rotational positions of the hob and the workpiece correspond to their respective zero positions and stopping the drive motors so that the rotational positions of the drive motors, the hob and the workpiece correspond to their respective zero positions, and then rotate the drive motors and restart machining the workpiece.