Threaded Tubes

While working on an ambitious but unfinished design, that I decided not to submit for the Makerbot giveaway contest, I came up with a way to pretension tubes based upon the idea that was raised in Forest Higgs' blog entry demonstrating that pretensioned square tubes can take a relatively large sideways load.

http://technocraticanarchist.blogspot.com/2010/03/some-crude-load-tests.html

My idea involves internally threading the tubes. A setscrew with a hole through the center would be used to tension a cable or wire that is fed and tied off through the hole, thereby holding several tube sections together.

I have provided a square and round tube and two different types of setscrews. One is just a regular setscrew (tubescrew.scad). The other variation (capturednut.scad) involves two nested screws where the bottom nut is “captured” by the top screw when it is screwed upward through the tube. A gap exists between the screws when it is being advanced up the tube. When statio