Half-cage for forming cages (10) for ball bearings, in which the half-cages (11, 21) comprise semi-spherical recesses (12, 22) arranged at predefined intervals along a circumference. The semi-spherical recesses are spaced out from each other by means of respective planar portions (14, 24), are arranged along the circumference, and comprise at least one rib (31, 41) having a certain width and extending along the circumference over at least the whole surface of said recesses.