A new flexible CNC bending machine which is based on the penetration bending method (called the MOS bending method), was invented and developed by the author. The machine can bend a circular tube to the required bending radius, using only one penetration die rather than many bending dies for the corresponding bending radii. The distortion of the tube cross section, the bending force and bending limit are experimentally investigated for different die inclination angle and tube materials. Using an inclination angle of 25°, the bending limit is maximized, and the bending force, bending radius and distortion of tube cross section are minimized. The tube material does not affect the bending limit, bending radius or distortion of cross section. Bending radii and bending forces are affected by the tube material. The characteristics of circular tube bending are clearly explained, using the MOS bending methed.