Abstract: A machine tool assembly formed as a shaft member having a cutter head assembly at an end thereof includes a pair of juxtaposed jaws defining a slot therebetween within which there is received a rotatable cutting tool. A T-shaped insert provided in the slot defines abutment surfaces for supporting and guiding the rotatable cutting tool, the T-shaped insert having a projection engaging into a transverse groove formed on the periphery of the rotatable cutting tool and a leg engaging into an axial bore of the shaft member, the leg abutting against a round key member located within a radial bore and adjustably engaged by a screw.