mating surfaces of the tip 14 and parent member 12 need not be exactly flush or true the manufacture and assembly of these elements are greatly facilitated these results are aided by the action of the spring 80 shown in fig3 besides holding the tip 14 securely in place during its fusion to the parent member 12 the spring 80 exerts a force at the joint 48 which facilitates the elimination of differences in the mating surfaces of these elements by forcing them together while they are in a melted softened state furthermore the axial force provided by the spring 80 insures a strong secure union between the tip 14 and parent member 12 fig4 illustrates the features of the dielectric heating apparatus 50 which prevents the burning or scorching of the tip 14 and parent member 12 as they are welded together to form the present cannula 10 since air and water are better conductors of the electric current generated by the dielectric apparatus 50 than the cannula material it is important that these elements be eliminated from the joint 48 and junction area thus prior to the fusion of the tip 14 to the parent member 12 both parts are thoroughly dried furthermore during fusion the hot expanding air existing in the junction area is vented from the interstices between the dies 54 and 56 and the cannula 10 in order to prevent burning such venting is accomplished outside of the cannula 10 by the tapered convex interior surface 62 of the outer die 54 which permits air to escape the junction area by means of its gradually increasing inner diameter inside the cannula 10 air is vented by means of longitudinal grooves 82 shown in fig4 formed in the exterior surface of the mandrel.