Method for forming crank shaft and method for producing crank shaft

摘要

A steel material having a columnar configuration is subjected to forming by applying finishing processing performed after buster processing and coarse (preliminary) processing to obtain a configuration in which additive metal lump portions are combined with upper and lower portions of a shaft section having a circular cross section. In the following trimming processing, a shaft section is formed to have a configuration in which eliminative metal lump portions are cut and eliminated from right and left sides of the shaft section. In the following sizing processing, plastic flow is allowed to occur from the additive metal lump portions to the eliminative metal lump portions to obtain a shaft section having a circular cross section. When the shaft section is formed by effecting the processing steps as described above, a method for forming crank shafts and a method for producing crank shafts can be obtained, which make it possible to decrease the number of processing steps so that the processing cost is reduced and the processing time is shortened.