Abstract: A die button flush mounted in the cavity surface of a lower one of two hydroforming dies has a backup plunger received therein that forms a part of this surface when the plunger is in a hydroforming position. A slug ejector received in the lower die also forms a part of this surface and with a tubular part enclosed in the cavity formed by the dies, hydroforming fluid under pressure is supplied to the interior of the part and also to separately act on the plunger. The fluid pressure moves and holds the plunger in its hydroforming position while forming the part to the die cavity surface. The fluid pressure on the plunger is then exhausted allowing the fluid pressure in the part to stretch a portion of the part into the die button while also forcing the plunger to a position that limits this stretching to prevent piercing.

Abstract: A method for cutting an "O" ring retention groove around the cutting edge of a hydropiercing die button. A groove machining tool is given a convex curved end surface the radius of which is slightly less than or, at most, substantially equal to the tightest concave radius that the end surface will have to sweep through as the groove is cut. Consequently, an accurate, slightly curved groove bottom surface is created for the "O" ring, while the machining tool will not bind as it moves through the tightest concave portion of the cutting path.