Numerical controller for machine tool

A numerical controller which controls a machine tool with a spindle motor formed from an induction motor, and a feed axis drive motor, the numerical controller comprising:

• magnetic flux content acquisition means which acquires magnetic flux content at the spindle motor;
• storage means which stores a time constant for change in the magnetic flux content at the spindle motor;
• speed command means which accepts a command speed and issues a command instructing a feed speed to the feed axis drive motor;
• elapsed time calculation means which calculates time elapsed from the start of cutting;
• first speed change means which commands the speed command means to change the feed speed of the feed axis drive motor at the start of cutting to an initial speed lower than the command speed, based on the magnetic flux content at the start of cutting acquired by the magnetic flux content acquisition means;
• and
• second speed change means which commands the speed command means to change the feed speed from the initial speed to the command speed continuously or intermittently based on the elapsed time calculated by the elapsed time calculation means and the time constant for change in the magnetic flux content stored in the storage means.