To radially enlarge an intermediate portion of a shaft, the shaft is held by a pair of holders with a gap between the pair of holders in an axial direction of the shaft, compression force is applied in the axial direction to the intermediate portion arranged between the pair of holders, and alternating load is applied in a direction intersecting the axial direction to the intermediate portion to enlarge the intermediate portion. When enlarging the intermediate portion, a temperature of the intermediate portion is set to be above a blue brittleness temperature range of the shaft, and a temperature of the holders is set to be below a tempering temperature range of the holders.

A roll bending machine includes a roller unit and a tension device. The roller unit rolls and bends a strip member at a rolling rate varying in a width-wise direction of the strip member. The tension device applies tension to the strip member after being rolled by the roller unit in a rolling direction of the strip member, thereby reducing compressive stress which usually result in formation of wrinkles on the strip member. This minimizes the probability of formation of wrinkles on the strip member.

A fixed die and a punch, which moves toward and away from the die, are provided. A series of bulges is formed in sheet metal between depressions and projections of the die and projections and depressions of the punch. In forming, the punch moves toward the die such that the sheet metal is pressed between first convex sections and second and third convex sections of the die and the punch, and subsequently the sheet metal is pressed between fourth convex sections.

A fixed die and a punch, which moves toward and away from the die, are provided. A series of bulges is formed in sheet metal between depressions and projections of the die and projections and depressions of the punch. In forming, the punch moves toward the die such that the sheet metal is pressed between first convex sections and second and third convex sections of the die and the punch, and subsequently the sheet metal is pressed between fourth convex sections.

A protruding portion is formed in a metal plate material by use of a plurality of molding tools each of which has a die and a punch. In a first step, an initial protruding portion is formed in the metal plate material by use of a first molding tool. In this step, a top of the initial protruding portion is formed to be thinner than other parts. In a second step, a sidewall of the initial protruding portion is rolled out by use of a second molding tool, and the protruding portion is formed.