Numerical control apparatus and numerical control method

A numerical control apparatus which controls movement of a drive axis of a machine in accordance with a machining program, the numerical control apparatus comprising:

• a curvature radius calculating portion which calculates a first radius which is a radius of a passage arc passing through three points of a start point, an object point, and an end point which are continuously present among a plurality of moving points of the drive axis recorded in the machining program as a curvature radius at the object point;
• a point sequence evaluating portion which calculates a chord error which is an approximation error between the passage arc and a front line segment connecting the object point and the start point or a rear line segment connecting the object point and the end point and compares the calculated chord error and a previously defined permissible error;
• a curvature radius correcting portion which calculates a second radius which is a radius of an arc in which a chord error with the front line segment and the rear line segment is the permissible error or less when the calculated chord error exceeds the permissible error and corrects a curvature radius at the object point to the second radius;
• and
• a speed calculating portion which calculates a permissible passing speed of the drive axis at the object point based on the curvature radius at the object point.