Abstract: A method and apparatus for assembling together a rectangular outer tube, a rectangular inner tube and thin flat elongated panels having their side surfaces solidly abutted between the inner wall of the outer tube and the outer wall of the inner tube. The assembly is expanded by applying an internal pressure of at least a thousand psi into contact with a die cavity of rectangular cross-section. In order to minimize the required expansion of the tubes it is desirable that the inner tube conform as closely as possible to the space between the panels prior to expansion. In order to facilitate insertion of the panels, the inner tube is subjected to a controlled collapse in which its sidewalls are curved inwardly and its corners are displaced radially inwardly to provide additional clearance for insertion of the panels. After insertion of the panels the walls of the inner tube are formed outwardly to fit relatively closely within the space defined by the panels.