METHOD OF FORGING A CRANKSHAFT

A method of forging a crank shaft at least including: a first step of forming a pair of fitting grooves in an outer periphery of a crank throw portion of a billet at an axially central portion of the billet, said grooves extending perpendicular to an axis of the crank throw portion as well as to a diametral line thereof; a second step of subjecting said billet to rough forging by using a rough forging mold which comprises upper and lower molds having respective recesses which are adapted to determine the position of the billet in cooperation with the fitting grooves of the crank throw portion and a journal portion, in which step, an outflow of burr is restricted by means of an inclined offset surface disposed opposite to the outflow direction of said burr while the crank throw portion is forced to move towards a crank pin position; and a third step of subjecting said billet to finish forging by using a finish forging mold which comprises upper and lower molds having respective recesses formed corresponding to a target shape of a crank shaft, wherein burr is restricted its outflowing by an inclined offset surface disposed opposite to the outflow direction of the burr. As used for such crank shaft forging method, provided are rough forging mold consisting of upper and lower molds in which raised portions between balance weight forming recesses of upper and lower molds are inclined to together form a cavity which gradually enlarges towards a crank pin forming recess, and which is fittable with fitting grooves of a billet, and inclined offset surfaces are formed on opposing surfaces of both the molds; and a finish forging mold consisting of upper and lower molds in which inclined offset surfaces are formed on opposing surfaces of the molds.