The original definition of forging is the "mechanical deformation of metal when heated to a plastic state". However, in recent years it has come to include similar operations done in the cold or room temperature state as well. There are two main types of forgings: open end forging and close end forging. However we also make use of crankshaft forging for a lot of applications Crankshafts can be forged from a steel bar usually through roll forging or cast in ductile steel. Today more and more manufacturers tend to favor the use of forged crankshafts due to their lighter weight, more compact dimensions and better inherent dampening. With forged crankshafts, vanadium micro alloyed steels are mostly used as these steels can be air cooled after reaching high strengths without additional heat treatment, with exception to the surface hardening of the bearing surfaces. The low alloy content also makes the material cheaper than high alloy steels. Carbon steels are also used, but these require additional heat treatment to reach the desired properties. Iron crankshafts are today mostly found in cheaper production engines (such as those found in the ford focus diesel engines) where the loads are lower. Some engines also use cast iron crankshafts for low output versions while the more expensive high output version use forged steel.
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, 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 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, wherein burr is restricted its outflow by an inclined offset surface disposed opposite to the outflow direction of the burr.
