39 of groove 33 also is separated from the ring by a clearance space c 1 to assure movement into retainer groove 45 the centerline 51 of ring 47 is offset to a position outside the surface 34 of the bearing pin 19 by the preferred distances of 0 005 to 0 020 inch centerlines 41 and 51 do not lie on a common plane perpendicular to the bearing pin axis rather centerline 51 is spaced closer to the nose 43 than centerline 41 because of clearance c 1 the cutter 21 may move inward on bearing pin 19 to the position shown in fig5 here ring 47 is confined between sidewall 46 of groove 45 and the corner of sidewall 39 of groove 33 a clearance c 2 appears between the opposing surfaces of conical or tapered thrust bearing 52 the distnce from the offset annular edge of wall 39 of the assembly groove 33 to the opposite corner or outer wall 46 in retainer groove 45 is less than the thickness or diameter of snap ring 47 when the cutter is thrust inward to the position shown in fig5 that is the diameter of the snap ring in fig5 is represented by the letter x y the distance from the edge of wall 39 to the opposite corner of groove 46 is a lesser distance x hence when the cone is thrust inward the ring cannot be retracted into the assembly groove 33 but must remain in the retainer groove 45 to prevent accidental.