Free-form curve interpolation method and apparatus

A free-form curve interpolation method, comprising the steps of:

• (1) designing a workpiece by representing a free-from curve with a NURBS curve using a CAD system;
• (2) creating a NC program by a CAM system with a format capable of instructing the interpolation of said NURBS curve to a CNC;
• (3) inputting said created NC programs to said CNC, whereupon said CNC executes the steps of:
• (3-1) recognizing a NURBS curve interpolation start command;
• (3-2) calculating a distance to move during interpolation period, on the basis of a speed (feed rate) command of said NC program;
• (3-3) calculating a change in a parameter for moving by the distance calculated in said step (3-2) from a current position specified by a value of said parameter used as a function expressing said NURBS curve;
• (3-4) calculating a position specified by substituting a parameter obtained by adding the parameter value calculated in said step (3-3) to the calculated change of parameter, for said function expressing said NURBS curve, as a point to be interpolated next, so as to obtain the next position on said NURBS curve separating from said current position along said NURBS curve by the distance calculated in said step (3-2);
• and
• (3-5) calculating a movement amount of each axis from said current position to said next position, and outputting interpolation pulses corresponding to said calculated movement amount to each axis driving control circuit, thereby executing an interpolation on a path along said NURBS curve.

A free-form curve interpolation system for a numerical control unit which controls axis movement of various machine tools, comprising:

• preprocessing calculation unit for reading NC programs including an interpolation command of a NURBS curve expressed by a function using a specific parameter as a variable, calculating a movement amount per unit time on the basis of a feed rate command in the interpolation command of the NURBS curve and outputting a movement command;
• parameter change calculation unit for calculating a change in said parameter brought on when moving by said movement amount;
• and
• movement amount calculation unit for obtaining a coordinate value to be interpolated next by substituting a value which is obtained by adding the value of said parameter change to a current value of said parameter, for a function expressing said NURBS curve, so as to calculate a movement amount of each axis corresponding to said coordinate values.