Step 11: Tubes and Extrusions: Joining Plates and Structural Applications
Aluminum extrusions especially tend to be manufactured to a high degree of precision and perpendicularity compared to steel, since steel shapes are more often roll-formed or continuously cast. Small steel shapes also tend to have a "scale" finish which is rough as well as messy. I tend to not use steel shapes as much because of the increased weight, in most cases without that much extra strength (Compare the yield strength of A36 steel, a common structural steel found in rolled products, to 6061 structural aluminum).
Steel, however, is definitely more conducive to common welding techniques (whereas aluminum generally requires very expensive and highly skill-dependent TIG welding); welding is not my forte, so I defer to others there. For instance, observe this marvelous use of aluminum extruded rectangle in the form of a small go-kart (SAM by Amy Qian).
This section will focus instead on creative uses of extruded aluminum shapes. Besides the usual square, rectangular, angles, etc., there are also manufacturers who sell specialized extrusions with quick-fastening systems, one of the most common being 80/20.
Using Extrusions as Beams
Rectangular, circular, and other obtuse closed-profile tubing is well-suited to long spans such as those found in machine frames. Extrusions are efficient in this role because their materials tend to be distributed at the outer edges (for things like tubing), maximizing its stiffness per weight as a result maximizing the profile's second moment of area. A few of the example pictures on this step show machines and contraptions built using a variety of extrusion profiles.
Image 1 is a FEA simulation of several different profiles (L-angle, U-channel, and square tube) in a single cantilevered bending load. The L bracket, being both unbalanced in the bending plane and without another vertical side to resist it, is pretty horrible. Notice the rotational deformation which comes from being unbalanced - if the load was stronger, then the final shape would be V-shaped as it rotates to become symmetric! L brackets, as you might have guessed from that, will only perform in bending if there is another L-bracket that is its mirror image. In that case, it begins approximating a U-channel.
There's fairly little difference between the U-channel and box tube due to the fact that the wide sides take up the majority of the load in bending. The missing top of the U-channel means it misses out on some compressive strength, hence it deforms slightly more.
Image 2 shows some 80/20 X-shaped T-slotted extrusion in comparison to a square aluminum tube of nearly the same material area. 80/20 is prized for its strength in complement to its ease of assembly (many different brackets, hinges, plates, etc. are available). It is, sadly, not the strongest possible profile because of the need to have the T-slots. The difference is minor enough that 80/20 can be a great choice for fast-building larger doohickeys.
Not Using Extrusions as Torsion Springs
One application which most (non-circular) tube is poor at is torsional loads. Images 3 and 4 are some cantilever torsion loads on the ends of the common aluminum extrusions. The L-angle is clearly very very hopeless, with the open profile of the U-channel fairing better but still deflecting substantially. The box tube, being a closed loop, responds the best. 80/20 is fairly poor in torsion because of its lack of a closed loop and the fact that there is less material at the outer edges otherwise
However, keep in mind the deformation is highly exaggerated to show the material movement.
Image 4 shows a 100 lbforce-in (about 10 Nm) torque on the end, and the actual deformation is about 0.04 inches (like 1mm), so it's not like 80/20 will instantly turn pretzel on you! In contrast, the square tube barely moves. A round tube of equal sectional weight will show even less deformation.
Overall, it's important to not load your structural tubes in torsion unless you specifically design it to handle such a load, the details of which are beyond the scope of this guide.
Clamping to Extrusions with Plates
Ever get into a situation where you're not really sure where something is going to end up on an extrusion frame? Using two piece of plate and some bolts, you can build a siding adjustable mount that can be tightened down anywhere and possibly drilled and screwed in place later.
Image 5 shows the "Small Bike", an appropriately named transportation device, also from Amy Makes Stuff, which has many adjustable components such as seat mounts, chain tensioners, and even the position of the wheels, adjustable on a thick square extrusion.
Using Extrusions as Standoffs
This is a variation of using plates on an extrusion that focuses on the plates. The industrially manufactured perpendicularity of tubes can also be used to hold plates together. Several of the example pictures show rectangular and square aluminum tubing used in this fashion. They are generally made to better than +/- 0.01" dimensions, so this can be a remarkably accurate way to make frames, manipulators, drivetrain pods, etc. It is fairly common practice to build a mechanical power transmission setup entirely within a thick-walled tube, with bearings fitted into the walls of the tubing.
Check out the example images for some implementations of tubes, extrusions, and various misadventures involving them!