Abstract: A process to form a continuous fin helically around the exterior of a cylindrical tube by rotation of a primary forming roller and rotation of a contiguous spindle roller. The process includes advancing and inserting the fin in a gap between the primary forming roller and the spindle roller. The gap between the primary forming roller and the spindle roller is closed and the fin is tightly gripped therebetween. The tube is axially moved for a first, chosen length, while, at the same time, the tube is rotated at a first speed, the primary forming roller is rotated at a first speed and the spindle roller is rotated at a first speed. Thereafter, the tube is axially moved for a second, chosen length, while, at the same time, the tube is rotated at a second speed, the primary forming roller is rotated at a second speed, and the spindle roller is rotated at a second speed.

Abstract: A pneumatic mandrel for use in machining and fusing plastic pistons and cylinders has a forward end wall with a port therethrough in pneumatic communication with a vacuum pump via a pneumatic tube extending longitudinally through the mandrel. A cylinder may be snugly mounted on the mandrel and may also be fixed against rearward axial movement by a stop member means the rearward end of the mandrel. The cylinder overhangs the forward end wall of the mandrel by substantially the length of the piston so that the end walls of the piston and cylinder will be aligned when the piston head is drawn by vacuum into firm abutment with the forward end wall of the mandrel. Preferably, a sink about the forward end wall port forms a peripheral landing about the end wall to assure firmness in the abutment. Relief ports are also provided through the mandrel for release of the vacuum to facilitate removal of the cylinder and piston from the mandrel.