the tube tu is inserted against the shoulder 18 which is necessarily formed during the milling operation and rests thereagainst in the embodiment of the invention shown in fig3 two spaced holes 14a and 14b are made in the steel disc sd into the aforementioned holes 14a and 14b push nuts pn may be pressed after the steel flange or disc sd has been inserted against the shoulder 18 the remaining portion 22 of the end of the tube tu is bent over or spun over or otherwise bent at right angles to the centerline cl of the tube tu to form an annular groove 20 in the tube tu in which the steel flange sd is firmly held the bent over or spun over portion 22 of the end of the tube tu accomplishes the dual purpose of holding the flange sd against the shoulder 18 and of acting as a pad 23 of relatively large surface area for one of the broad sides of the riser r the riser r is disposed against the pad 23 to form a right angle joint or connection as is shown in fig3 appropriate bolts 26a and 26b are inserted through the previously described openings 28a and 28b in the riser r and threaded into the push nuts of the aligned holes 14a and 14b of the flange sd appropriate securing torque is applied to the bolt heads of the bolts 26a and 26b to compress the riser r the portion 22.