Method of manufacturing yoke through multistage-drawing process

A method of manufacturing a yoke by drawing a plate material into a final product as the yoke by performing a multistage-drawing process, where the final product as the yoke is in the shape of a cylindrical container having a couple of thick wall portions opposed to each other in a cross section perpendicular to a central axis of the final product and a couple of thin wall portions that are thinner than the thick wall portions and connect the thick wall portions together, wherein a distance between the thick wall portions is shorter than a distance between the thin wall portions, the method comprising steps of:

• drawing the plate material into a first-drawing product in the shape of an elliptically cylindrical container at an initial drawing stage, the first-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section;
• drawing the first-drawing product obtained at the initial drawing stage into a re-drawing product in the shape of a generally circular cylindrical container at a middle drawing stage;
• drawing the re-drawing product obtained at the middle drawing stage into a late-drawing product in the shape of an elliptically cylindrical container at a late drawing stage, the late-drawing product including an elliptical cross section having a major axial length measured along a major axis of the elliptical cross section and a minor axial length measured along a minor axis of the elliptical cross section, the major axis and the minor axis of the elliptical cross section of the late drawing product obtained at the late drawing stage being reversed with respect to the major axis and the minor axis of the elliptical cross section of the first-drawing product obtained at the initial drawing stage, the late-drawing product having a couple of opposed smaller-curvature arc segments of a smaller curvature and a couple of opposed larger-curvature arc segments of a larger curvature;
• and
• drawing the late-drawing product obtained at the late drawing stage into the final product as the yoke at a final stage, such that the smaller-curvature arc segments of the late-drawing product are converted to the flat thick wall portions of the final product, respectively, and the larger-curvature arc segments of the late-drawing product are converted to the thin wall portions of the final product, respectively.