Effect of feed pitch on formability in tube expansion using ball spin forming

Abstract. This study proposes a simple and localized forming method using a train of four balls in a tube-hoop direction that expands tube diameter at arbitrary axial positions. The balls travel in the tube direction while rotating around the tube axis. Compared with conventional methods, the expansion mechanism is compact as only one input-rotational drive forms taper and parallel sections. This paper particularly focuses on the effect of feed pitch for the parallel section forming and the ball gap to the cylinder on dimensional precision. An outer cylinder was used for securing the outer diameter as the inner surface of the cylinder can suppress the overshoot of the tube diameter. The experimental results verified the effect of the cylinder. When the ball gap to the cylinder was small enough to push the tube wall to the cylinder-inner surface, the tube diameter was stable at the target value regardless of the feed pitch for the parallel section forming. The tube thickness t decreased to 0.5 mm with increase of the clearance between the balls and the cylinder-inner surface just as designed. However, the thickness could not be less than 0.5 mm.