Mandrel loading method and apparatus in a thermal sizing-annealing process

To facilitate safe loading of a thermal sizing mandrel (14) in an elongated channel (15) of open rectangular cross section, a corner rod die (22) is first lowered into a fixture, and then a channel (15) is lowered into the fixture to a vertical position surrounding the die (22). A mandrel (14) is inserted downwardly through the channel open interior to press corner rods (22) of the die outwardly against the channel corners to elastically reform the channel (15) substantially to a specified geometry. Mandrel insertion is facilitated by inducing vibratory energy therein and by forming shoes (18) in the fixture acting against the channel outer sides to square off the mandrel cross section in advance of mandrel (14) insertion.