abutment surfaces the spherical abutment surfaces are located on abutment members which are press fit into cylindrical depressions in the clamping jaws in the foregoing manner 1 the conically tapered forward surface 50 of the shank is pushed rearwardly into an interference fit with a complementary tapered surface 52 on the bore 22 while 2 rearwardly facing abutment face 20 on the toolholder is brought into abutment with the forwardly facing surface 18 on the tool support member and 3 rear convex cylindrical surface 54 on the shank is expanded into abutment with a complementary concave cylindrical surface 56 in the bore by rotating the threaded member 28 in a second direction clamping jaws 24 and 26 converge inwardly toward one another to release the tubular shank a set screw 58 centrally mounted along axis x x in the bottom of the bore rearwardly of the clamping jaws provides an annular abutment surface 60 for abutment with either flange 62 and or 64 on the first and second clamping jaws respectively to assure that when the clamping jaws are moved radially inwardly they are sufficiently centered in the bore to avoid interference with the removal of the shank and the entry of a new shank flanges 62 and 64 have flat bottom surfaces 66 and 68 which engage and slide across an annular flat forward facing surface 70 in the bore to thereby hold the clamping jaws nonrotatable as the clamping.