Stiff metal ring and process for making it

A method for making an interdigitated stiff cylindrical ring starting with a flat piece of steel having a tongue projecting one end and a recess receding into the other end, said tongue and said recess being axially aligned with one another, each having a pair of locking segments at least one of which will interfere with the endwise separation of the ends of the ring, there being a maximum lateral spacing apart of said locking segments on the tongue and a minimum lateral spacing apart of said locking segments in the recess where said interference is maximum, the location of said maximum spacing apart of at least one of said tongue and recess being axially spaced from the respective end, said method comprising:

• using a first semi-cylindrical die with a concave half cylindrical die surface, bending said piece against and around a cylindrical mandrel to form a U-shaped structure with a bight and two parallel arms, and while holding the bight of said piece against the mandrel with the first said die, pressing the ends of said arms simultaneously against a unitary second concave half cylindrical surface on a second die along an axis which includes the centers of curvature of both half cylindrical surfaces and the center of the bight, thereby moving said ends of said arms along said second surface so as to approach and interdigitate one another in a motion which includes rotation, the stiffness of said tongue and of that part of the piece of steel at the end on which the recess opens, at least to said location of the maximum lateral spacing apart being such that it makes contact with said second surface during interdigitation, only at its axial termination, and then bringing the two dies against the ring and the ring against the mandrel to press the ring to a cylindrical shape, and removing the ring from the mandrel.