NC data creation method

An NC data creation method for creating, in an automatic programming apparatus, NC data for cutting a sculptured surface by simultaneous five-axis control comprising the steps of:

• defining a tool nose path on the sculptured surface, wherein the sculptured surface is specified by a lattice point sequence of a number of longitudinal and horizontal meshes and a predetermined planar curve is projected onto said sculptured surface, so that a point sequence of intersections between the projected curve and the meshes defines the tool nose path;
• computing a normal vector at a point Ai on the tool nose path in accordance with the equation
• n=N.sub. + (N.sub.1 -N.sub.2) L.sub.2 /(L.sub.1 +L.sub.2)
• wherein N.sub.1, N.sub.2 represent normal vestors at two lattice points bracketing the point Ai, and L.sub.1, L.sub.2 represent distances from the point Ai to each of the lattice points;
• computing a tangent vector tangent to said tool nose path at said point Ai;
• determining an outer product vector between said normal vector and said tangent vector;
• determining a tool central-axis vector, in a plane containing said normal vector and said outer product vector, in which a direction inclined toward the outer product vector from the normal vector by a designated angle serves as the direction of the tool central axis;
• creating NC data for simultaneous five-axis control using coordinates of said Ai and said tool central-axis vector;
• and
• cutting the sculptured surface along the tool nose path using the NC data.