DEFRGBIT0086480EUROPEAN PATENT APPLICATIONA319840321EP8310139919830214212008219820215JP1984032119841219830824198334 6B 21D 53/86 AenMethod for the manufacture of front fork of single unit type for bicycles from center butted ferrous tubular materialfrProcédé de fabrication de fourches de bicyclette d'une seule pièce à partir d'une ébauche tubulaire de fer refoulé au milieudeVerfahren zum Herstellen von Fahrradgabeln aus einem Stück aus ein Mittelteil gestauchten EisenrohrKimura, IsaoKimura, IsaoDEFRGBIT19840321198412

A front fork blank of the single unit type is manufactured from a center butted tube, the wall thickness of its middle portion exceeding that of both end portions of the tube, as a starting material. The center butted tube is produced by a known machine including a mandrel and a die. The center butted tube is bent at two positions with the right angle to form a tubular fork. The tubular fork is set in a sectional die which has a concave recess corresponding to a protuberant socket, and subjected to the known oil hydraulic bulge-forming process in order to bulge-form the socket at the middle of the top of the tubular fork. The socket is opened to receive a stem (Fig. 13).