Abstract: Cubic spline interpolation process of discrete points and linear approximation of the cubic spline curve were analyzed for the CAD/CAM process of roots vacuum pump rotor’s listing points curves.An improved approaching method using the determining step size was presented through researching the methods of approaching of cubic spline curve.This method is simple and it’s approaching accuracy can be controlled.Using a new method to control the single feed amount of the process of planning,which can improve the efficiency of planning.CAD/CAM integrated planning CNC system for roots vacuum pump rotor outline was developed,which has achieved good economic and social benefits in the actual processing.

Key words: roots vacuum pump, rotor, listing points curves, straight line approaching, Computer Aided Design/Computer Aided Manufacturing (CAD/CAM), NC planning, synthetic interpolation theory of curves