Patent ID: 7464571

Claim:
A hydroforming method for forming a slot with an aspect ratio of 3 or more in a metal tube blank comprising the steps of: holding a metal tube blank in a pair of molds, with at least one die port formed in one of the molds, the die port enabling a piercing punch to move slidably therein and being configured with an opening with the aspect ratio of 3 or more, the piercing punch being configured to have a concave segment in a length-wise direction on the top face thereof, sliding and moving said piercing punch to a position so that the top face thereof becomes flush with a cavity surface of the pair of molds; bulge-forming in a manner of following a contour of the cavity of said pair of molds and that of the top face of said piercing punch by applying a pressure to a working fluid supplied to an inside of said metal tube blank; and retreating said piercing punch to pierce the metal tube blank and form said slot.