Abstract: A numerical control unit for a non-circular workpiece fabricating machine automatically smoothes and interpolates lift data containing errors so as to obtain new substantially error-free lift data, and allows the fabricating machine to fabricate a non-circular workpiece with a smooth rotation of a main spindle and with less vibrations of a wheel spindle stock. The numerical control unit converts lift data whose pitch is unequal into lift data whose pitch is equal. Errors in the equal pitch lift data are extended by a second finite difference calculation, are repeatedly smoothed and interpolated until they meet an application criterion. Then, the lift data are subject to inverse calculation to obtain new lift data. The new lift data are used to obtain X/C-axis data taking a diameter of the wheel spindle stock and period of time per rotation of the workpiece into consideration. Thereafter, the workpiece is fabricated on the basis of the X/C-axis data.