An apparatus for imparting compressive residual stress to at least a surface portion of a first plurality of balls includes a first body having a first surface, the first surface including a smooth contact portion, the smooth contact portion being substantially flat or convex and having a surface hardness greater than or equal to the initial surface hardness of the balls. The apparatus also includes a second body having a second surface, the first surface overlying the second surface, and at least one drive operably connected to the first body or to the second body and configured to move one of the first and second bodies relative to the other body at a substantially fixed distance, the at least one drive also being configured to move the first body toward the second body with a force or to move the second body toward the first body with the force.