the circular wedge member 44 on the inserted work unit the conical gripping surface 54 is preferably as shown made as a plurality of circular barb shaped ribs 66 concentric with the longitudinal axis 60 of the pin 36 more particularly and as seen in fig7 in proceeding away from the apex 56 each rib 66 has first and second ends 68 and 70 located in planes perpendicular to the longitudinal axis 60 the first end 68 of each rib is located closer to the apex 56 then the end 70 further for each rib 66 the end 70 is of larger diameter then the end 68 and such end 70 is also of larger diameter then the end 68 of the next adjacent rib each rib 66 between its end 68 and its end 70 further includes a conical intermediate external surface 72 having a cone angle c substantially larger than the cone angle a in the illustrated case of fig7 this cone angle c is 60 degrees while the cone angle a is 30 degrees a suitable spring means urges the circular wedge element 44 downwardly on the pin 36 toward the fixed gripping surface 58 preferably and as illustrated this spring means consists of a helical compression spring 74 received on the pin 36 and working between the top of.