PURPOSE:To obtain NC data for curved surface working where an arbitrary curved surface is used for designation of a cut area in the depth direction by designating the cut boundary in the depth direction to output working point to the cut boundary. CONSTITUTION:Data to specify a three-dimensional curved surface 102 which gives the cut boundary in the depth direction of a three-dimensional curved surface 101 is inputted together with data, which specifies the three-dimensional curved surface 101, to define the work curved surface 101 and a boundary curved surface 102 with sets of plural dot string courses. A tool offset dot string B(i, j) corresponding to a dot string A(i, j) on the work curved surface 101 and a dot string Q(i, j) of the front end of the tool are obtained, and the dot string of the front end of the tool is projected on an X-Y reference plane to obtain a projection dot string B'(i, j), and the projection dot string is projected on the boundary curved surface 102 to obtain a projection dot string C(i, j) on the boundary curved surface. it is checked whether a segment Q(i, j-1)Q(i, j) connecting dot strings of the front end of the tool and a segment C(i, j-1)C(i, j) connecting projection dot strings on the boundary curved surface intersect each other or not, and a tool offset segment to an intersection point P(i, j) is outputted if they intersect each other, thus generating NC data.
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