A rotary cutting tool for cutting a spherical seat on a surface of a workpiece comprises a body adapted to carry cutting inserts. The body is fixed to a drive shaft which rotates the body about a longitudinal center axis of the body. The body includes a forwardly facing front surface of generally convex shape, and a plurality of insert seats formed in the front surface in circumferentially spaced relationship about the axis. Each of the seats faces forwardly and comprises recesses of generally pyramidal shape. A screw threaded hole extends through each of the seats in a rearward direction and is adapted to receive an insert fastening screw. The cutting inserts include projections of generally pyramidal shape received in respective ones of the recesses. The projections are formed by a first plurality of V-shaped parallel grooves intersected by a second plurality of V-shaped parallel grooves arranged perpendicular to the first groove.