the device includes a base plate with a recessed seating for the shoulder of a tube with a threaded neck and a compressing mechanism engaging the open end of the tube body , the compressing mechanism being mounted axially with respect to the tube . the compressing mechanism is a reciprocable rod which , on the end thereof facing the base plate , has a clamping head for clamping the open end of the tube body . in a conical recess in the base plate , there is provided an internally threaded bore for screwing in the threaded neck of the tube .