Abstract: To facilitate safe loading of a thermal sizing mandrel in an elongated channel of open rectangular cross section, a corner rod die is first lowered into a fixture, and then a channel is lowered into the fixture to a vertical position surrounding the die. A mandrel is inserted downwardly through the channel open interior to press corner rods of the die outwardly against the channel corners to elastically reform the channel substantially to a specified geometry. Mandrel insertion is facilitated by inducing vibratory energy therein and by forming shoes in the fixture acting against the channel outer sides to square off the mandrel cross section in advance of mandrel insertion.