Abstract: The method of the invention includes obtaining points discretely on a three-dimensional curved surface, by using data specifying the three-dimensional curved surface (CSF), in such a manner that the points line up in both a first direction (BC direction) and second direction (DC direction), obtaining the center (P.sub.c) of a circular arc passing through three consecutive points (P.sub.i,j-1 P.sub.i,j, P.sub.i,j+1) in e.g. the first direction, and computing a normal vector by adopting a direction from the center of the circular arc to the middle point (P.sub.i,j) among the aforementioned three points as the normal line direction of the three-dimensional surface at the middle point.

Abstract: A method of creating NC data for a complex curved surface (10) which has a combination of at least two three-dimensional curved surfaces (11, 12). The NC data creation method includes inputting data specifying each of the curved surfaces (11, 12) and data for specifying cutting path patterns (a pattern 21 of radiating straight lines and a pattern 22 of parallel straight lines) for respective ones of the curved surfaces. Points are obtained discretely from a cutting starting point (P.sub.i) of one curved surface (11) to a boundary point (R.sub.i) with the other curved surface (12) along the radiating-type cutting path pattern (21) set for this curved surface. Points are also obtained discretely on the other curved surface along the cutting path pattern (22) of parallel straight lines specified with respect to the other curved surface (12). A pick-feed is performed points are obtained discretely along the cutting path pattern on each of the curved surfaces in similar fashion.