An apparatus including rigid plate having a first face and a second face, a plurality of through seats arranged as a circular crown about an axis of symmetry (A), perpendicular to each seat and shaped as a circular crown segment; a plurality of knives carried by the plate, each being axially slidingly guided into a seat, independently of the other knives; each knife has a head which protrudes from the seat from the side of the first face, a prismatic body having an axial length greater than the seat, and a wedge-shaped end, opposite to the head, which protrudes from the second face when the head substantially abuts against the first face; the wedge-shaped ends circumferentially having a first profile, complementary to that of the frontal toothing to be obtained, and the knives can be replaced totally or partially with others having a second profile different from the first profile.