Abstract: The circular arc spline approximation is given for fitting the plane parametric curves and fitting the plane point set curves after the straight line and the circular arc and the parametric curve and its tangent line are bestowed with direction. This method overcome difficulties of big torsion and of high precision and of huge programming. Especially, there is not obstruction during fitting curves because avoiding to solve nonlinear equations that own possibility of no convergence. The core of a program is described for auto programming the profiles of machine parts in numerical control machining.

Key words: Auto programming Numerical control machining, Circular arc spline, Machine part, Profile