For the threaded portion, I am using 1/2 inch fine thread "rod coupling nuts". These are available at mcmaster.com, among other places. I ground the nuts down enough so they could slide inside the black iron pipe. I drilled 4 holes in the side of the pipe, I welded through the holes, as well as all the way around the nut, to make the joint even stronger. After I was finished welding the nuts, I ran a tap through each of them in order to fix any distortion caused by the welding.

I then made up a jig, using a piece of plywood and three bolts, to assemble, and weld the radius rods together.




nut with bag (640x480).jpgnut ground (640x480).jpgnut in pipe(640x480).jpgrod on board (640x480).jpg