PURPOSE: To automatically prepare data about a specified contour of inner blank part even when an intricate shape is to be machined by eliminating unmachined part making reference to a specified tool diameter when preparing data in the caption.

CONSTITUTION: When a straight line A is drawn by using a specified tool, intersections (a, b, c, d) are obtained. These intersections are filed in the memory of a computer. Since all the intersections are included in a contour, a locus of a tool center as illustrated in the figure is obtained by connecting segment data of (aWb), data along the contour of (bWc), and the segment data of (cWd). Next, when a straight line B is drawn, intersections (e, h) are included in the contour, while intersections (f, g) are included in an island-shaped contour. By connecting the segment data of (eWf) and data along the island-shaped contour of (gWh), a locus of a tool center is obtained as shown in the figure. In this instance, an inner blank part data providing a round along the island contour can be prepared.