A tube is placed in a hydraulic pre

A tube is placed in a hydraulic press or bellows forming machine. Circular external die rings of suitable contour are placed outside the tube at longitudinal intervals approximately equal to the developed length of the completed convolutions. The tube is filled with a medium such as water and pressurized until circumferential yielding occurs. This forming operation continues with a simultaneous circumferential yielding and controlled longitudinal shortening of the tube until the proper configuration is obtained. fudividual or multiple convolutions may be formed by this method. Depending on the bellows configuration, several partial-forming steps with intermediate heat treatment may be required. Reinforced bellows may be formed by utilizing external reinforcing rings that act as part of the forming dies. After completion, when the dies are removed, the rings remain as an integral part of the bellows.