Sheet metal member having a peripheral wall and method of thickening the peripheral wall thereof

A method of thickening a peripheral wall of a sheet metal member comprising a first forming step having plural small steps, each small step comprising a series of steps of:

• holding pressingly a disc-shaped sheet metal member in an interposed state, between a first rotating die and a second rotating die;
• making the disc-shaped sheet metal member rotate with a first rotating die and a second rotating die;
• pressing a first forming roller having a first groove-shaped forming surface wherein a pair of annular surfaces widening outwardly are connected via a sectional-curved annular bottom surface, in a radially inward direction against an outer periphery of a projection of the disc-shaped metal sheet projecting outwardly from the interposed portion;
• thickening the projection in an axial direction and a radial direction;
• and
• shaping the projection so as to fit in the groove-shaped forming surface of the first forming roller while thickening it,
• whereby the interposed portion is a base plate and the projection is a peripheral wall protruding axially from the outer periphery of the base plate to one side, and
• wherein, the first forming step is divided into said plural small steps, and, as relation between a previous small step in the first forming step and a post small step following the previous one, a radius of curvature of the annular bottom surface of the first forming roller employed in the post small step is greater than that of the first forming roller employed in the previous small step, and an opening angle made between a pair of annular surfaces of the first forming roller employed in the post small step is smaller than that between a pair of annular surfaces of the first forming roller employed in the previous small step and a depth of the first groove-shaped forming surface of the first forming roller employed in the post small step is less than that of the first groove-shaped forming surface of the first forming roller employed in the previous small step.