The invention allows the manufacture of ring parts such as rings on which should be carried out machining. The method includes bending a sheet predimensionnee by means of a series of internal supports (8) mounted on a turntable (6) and by means of a pressure roller (4) pressing the surface the external sheet (2). A complete rotation of the turntable (6) allows the full bend-shaped ferrule of the sheet (2). The second main operation is to be performed on the same tooling the junction welding of the two ends (1, 3) of the sheet (2). This method enables to perform any machining on the first flat plate predimensionnee without these machinings are damaged by bending or welding operation. Application to the manufacture of the shells before casings surrounding the coldest channel of a turbofan engine.