We may be brothers from another mother. HD also sells tubing that fits snugly over that threaded rod if you're nervous about the exposed conductors. With creative routing and additional support, you can get five rigs onto full-length verticals of uncut threaded rod in the lengths available at HD.
Oh ... what kind of tubing? How strong is it? Sounds like you are referring to some kind of plastic tubing?
I got some really thin wall aluminium tubing that I used a tubing cutter to make 2.2" spacers so I wouldn't have to use so damned many hex nuts. The result is questionable, it certainly has extra rigidity good for supporting 6 dual gpu cards (5970,6990,7990?). The design in the picture above doesn't have a whole lot of extra support on the outer most cards. The problem with the aluminium tubing is that it doesn't sit on a #6 washer very well. I was putting a washer between the extender pcb and the spacer and when I tightened the nut down on either end it did not straighten like I expected it to.
Plus there wasn't enough thread on a 12" peice of all-thread with nut, washer, pcb, washer, spacer, washer, pcb, washer, spacer.
So I removed all the washers except the ones on the outside ends. Tightened it up and it looked good, but the spacers ends are not much bigger then the pcb hole and they are rather sharp from the tubing cutter so I'm a little worried about extra strain on the pcb. Then again aluminium at this wall thickness is very very soft and it probably just flattened against the surface of the pcb.
I plan on placing motherboards so that I extend the back plane and can use a union to join one set of rails to another so I have motherboard to motherboard support while still allowing me to move 6 gpu segments off the rail for motherboard replacement, whatever.
In all I'm not totally happy with the aluminium spacers (not stiff enough), nor the zinc plated all-thread (too flimsy) or the 4" 6-32 bolts (too short) I'm using for posts. Plus I'm worried about corrosion and honestly I'm not a fan of cheap steal bolts. So I'm reworking the design to use stainless all-thread with stainless lock nuts and no bolts. I also want to use 1/4" stainless tubing, it's bigger and man is it stout! Dual gpu cards will be no problem. The only non-stainless is the aluminium L bracket supporting the rails. Also stainless + aluminium is a good combination.
