Tool radius compensation method for numerically controlled apparatus

A tool radius compensation method for a numerically controlled apparatus in which a tool, having a radius, and a workpiece having first and second surfaces, are moved relative to each other by a triaxial drive to enable the tool to three-dimensionally cut the workpiece, positional coordinates of the center of the tool, which is offset at a distance equal to the radius of the tool in a direction normal to a surface to be cut, being computed from commanded positional information to effect the workpiece cutting, said method comprising the steps of:

• (a) determining a unit offset vector on a plane defined by vectors normal to the first and second surfaces to be cut at a three-dimensional corner point of the workpiece based on the commanded positional information;
• (b) determining the coordinates of the position of the tool center from corrected components along respective X, Y and Z axes of the unit offset vector, the tool radius, and the position of the three-dimensional corner point;
• and
• (c) moving the tool center along a smooth curve contiguous to the path of movement of the tool center before and after the three-dimensional corner point has been cut.