Aluminum Channel

Aluminum Channel

I'm having a heck of a time finding aluminum channel that is 4" or so wide, and 1/4" thick. Generally, aluminum channel doesn't come quite that thick. Sometimes, the vertical legs of the channel are more than 1/4" thick--but not the horizontal leg.

You see, the way that channels with low walls (as a channel that will cradle a large diameter round tube will be) get their stiffness, is primarily dependent on the thickness of the horizontal leg. The stiffness of a channel is roughly in direct proportion to the increase in width--but if you double the thickness of the base of the channel, it gets about five times stiffer with only an approximate doubling of the channel's weight. If you double the thickness of the vertical legs, you only get about a 30% improvement in stiffness.

What this means is that a 4" wide channel that is .25" thick (or perhaps even .30" thick) provides very nearly the perfect tradeoff between weight and thickness for this application--and I'm having trouble finding such a channel that is available off the shelf.

I had thought about cutting off one side of a 4" square, 1/4" wall tube, or perhaps a 4" x 1" rectangular tube with a 1/4" wall--but these also seem to be unavailable. Square or rectangular tubing that size is typically 1/8" wall or thinner--just not stiff enough for this purpose.

I looked at perhaps using steel channel instead, which is cheaper than aluminum--but steel turns out to be, for the same weight, no great advantage. Mild steel has Young's modulus typically of about 190 to 210 GPa; aluminum is 70 Gpa. The steel is therefore 2.7x to 3x the stiffness of aluminum--and 2.44x as dense. Because of the non-linear advantage of a thicker channel for enhancing stiffness, it turns out that steel doesn't buy me anything, except harder to machine, and a little cheaper on the raw material.

UPDATE: I've got a vendor offering me 3" wide, .25" thick, or 5" wide, .375" thick. The first choice gives me a .019" deflection (which, because there are two other members also providing support, means I will probably get closer to .010" of deflection), and a weight of 5.7 pounds. The second choice gives me a deflection of .0026" (which far stiffer than I need), but a weight of 14.8 pounds--which is just too much. If I don't find something better by tomorrow, I may go with the 3" wide channel. (The prices are really, really good, too.)

If I had a really big mill, I suppose that I could take the 5" piece and take an 1/8" off the inside of the vertical legs....

UPDATE 2: A reader suggested using the 5" piece, then drilling lightening holes in it to make it lighter--especially since it is far stiffer than I need. This is an intriguing thought. There's really no room in the vertical legs to drill anything but tiny, tiny holes, but putting a series of 2" holes through the horizontal leg every few inches might make this feasible. Of course, I've got to put some holes in the base for mounting the dovetail plate. This could get ugly, although not difficult.

One other thought just occurred to me: I have two 1" square aluminum tubes that I won't be using. I might be able to have the welding shop I used the other day weld these to the bottom of the 3" channel to increase stiffness, or have a 2 1/2" wide, 1/8" strip of aluminum plate welded into the bottom of the 3" channel.

UPDATE 3: It turns out that they can only really weld where the channel and the plate meet--so it sounds like using JB Weld as an adhesive between the channel and the plate would be the way to go--and that opens up a lot more possibilities. I could use a 4" wide by 1/8" channel with a 1/8" plate glued to it. This gives me nearly optimal tradeoff of stiffness and lightness.