l t directed in the opposite direction to the axial thrust it will be noted that such hollowing out is obtained by combining rotation of the tool about axis yy 39 with an axial thrust without exercising any percussion contrary to what happens in the initial action of a conventional drill as can be seen in fig1 the blade extends from the endmost face through a slot f in the preferred embodiment described this slot is offset with respect to the center o of said endmost face the result is that the end part of body c comprises a reinforced side on which the reaction of the blade is exerted during rotation of the tool around axis yy 39 thus any risk of breakage of the blade through deformation of the surface of the tool on which it bears is eliminated fig1 shows that said end face of body c forms a spherical cup c s of a diameter very slightly greater than that of the ball such a shape for the end portion of body c not only ensures centering of the ball but also provides a rigid support for the blade on the wall portions of the slot which thus project beyond the transverse plane tangent to the bottom of the cup when the blade is relatively little slanted with respect to the axis yy 39 fig1 further shows that the axis a is offset with respect to the axis of symmetry yy 39 of body c.