The process comprises forming a tube (1) of an electrically conductive material having on its inner surface axial grooves (2) and axial projecting portions (3) between the latter, injecting a hardenable insulating material (4) into the tube (1) so as to fill the latter, this material penetrating the grooves and surrounding the projecting portions, and removing the outer portion of the tube (1) after the hardening of the insulating material so as to reveal the insulating material of the grooves (2) and form between the latter commutator segments which are insulated from one another. The tube (1) is formed by the extrusion of two complementary half-tube shells (1a, 1b) and the assembly of the latter one against the other on an axial joint plane (A--A).