Method for forming crank shaft and method for producing crank shaft

A method for producing crank shafts, comprising the steps of:

• cutting a steel material having a columnar configuration to a predetermined length;
• performing a first forging on said steel material to obtain a first intermediate formed product, said first forging comprising the sub-steps of (a) performing buster and coarse processing on said steel material and (b) performing finishing processing on said steel material, wherein said finishing processing comprises forming a shaft section of said crank shaft such that a cross sectional configuration thereof comprises a substantially circular shape of a predetermined diameter with a matching allowance, and additive metal lump portion and a burr extending in a widthwise direction;
• performing a stamping process on said first intermediate formed product, said stamping process comprising removing a portion of said shaft section made up of said burr and an eliminative metal lump, thereby obtaining a second intermediate formed product;
• performing a second forging on said second intermediate formed product, said second forging comprising causing a plastic flow to occur from said additive metal lump portion to a space created by removal the eliminative metal lump in said stamping process, thereby causing said shaft section to develop a circular cross section having said predetermined diameter and obtaining a third intermediate formed product;
• and
• polishing at least said shaft section of said third intermediate formed product to remove said machining allowance, thereby obtaining said crank shaft in which said shaft section thereof has a circular cross section.