Patent ID: 7946147

Claim:
A tube expanding method for a damper tube, comprising the steps of: supporting a thrust force of a punch applied to the damper tube by a stopper while retaining one end of the damper tube by the stopper, inserting an other end of the damper tube to a die, inserting the punch to the other end of the damper tube so as to propel, and expand the other end of said damper tube while carrying out an ironing operation; releasing the stopper when a load in an axial direction applied to said damper tube due to an elongation by the ironing operation of the damper tube increases over a critical load which is previously determined to be a smaller load than an expected buckling load in the damper tube; and wherein a whole length of said damper tube is set to a fixed length by cutting and removing one end of said damper tube after expanding said damper tube.