Numerical control device

A numerical control device having a constant surface speed control function for controlling a spindle rotation rate to make a surface speed constant, the constant surface speed control function being adapted to avoid, in a case of cutting a machining target attached to a spindle of a lathe while the spindle continues to rotate at a constant spindle rotation rate, a situation in which the surface speed that is a relative speed between a tool and a contact portion of the machining target decreases as the tool advances toward a center portion of the machining target and a situation in which the surface speed that is the relative speed between the tool and the contact portion of the machining target increases as the tool advances toward an outer periphery of the machining target, the numerical control device comprising:

• a cutting speed setting unit configured to set a minimum cutting speed and a maximum cutting speed with which a range of recommended cutting speeds as relative speeds between the tool and the contact portion of the machining target is designated in advance;
• a surface speed calculating unit configured to calculate the surface speed that is the relative speed between the tool and the contact portion of the machining target, based on a spindle rotation speed having been designated for the spindle to which the machining target is attached and based on a distance from a rotation center of the machining target;
• a comparison unit configured to compare, during the cutting, the surface speed, which has been calculated by the surface speed calculating unit, with the minimum cutting speed and the maximum cutting speed, which have been set by the cutting speed setting unit and with which the range of the recommended cutting speeds is designated;
• and
• an operation control unit configured to control whether to activate the constant surface speed control function based on a result of the comparison made by the comparison unit such that the operation control unit deactivates the constant surface speed control function in a case where the surface speed is within the range of the recommended cutting speeds whereas the operation control unit activates the constant surface speed control function in a case where the surface speed is out of the range of the recommended cutting speeds.