Step 11: Tubes and Extrusions: Joining Plates and Structural Applications

If you're crafty, you can actually use premanufactured tubes and extuded shapes as structural members backed by plates or by themselves. This section will be a bit biased towards hobby robots, machines, and other larger implements because I really cannot think of an Arduino enclosure hardcore enough to warrant aluminum and steel.

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!
Yet another incredibly informative and well written instructable, nice job! Love the FEA’s and especially enjoyed your notes on set screws. My goto sources are always McMaster and ServoCity. Similar to your RoyMech site I’ve used http://www.gizmology.net/ for reference many times.
I knew I was forgetting something! Gizmology has been added to the end - I may sprinkle relevant links in the middle too.
http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html is the correct link now, it's a great resourc, especially for offline use. THANK YOU FOR THIS INSTRUCTABLE!
<a href="http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html" rel="nofollow">http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html</a>&nbsp;whoops.&nbsp;
nice one
Re using one part to template tthe other, &quot;dimpling&quot; -- mention transfer punches here?
Probably worth it. I was definitely in &quot;slummin' high school&quot; mode then, when we didn't have a set of center punches much less transfer punches! I'll look to adding it in.
Wow that was incredibly comprehensive! Are there still robot combat competitions going on?
Hell yeah. Primarily small weight classes and these days grassroots-level and builder run. The big event is RoboGames: http://robogames.net/index.php and Combots: http://combots.net/, and on the east coast, NERC: http://www.nerc.us/ <br> <br>Various other local clubs and organizations exist also. A current listing of events is on buildersdb: http://buildersdb.com/
This might be my new favourite Instructable. Great info!
Thanks for sharing that. <br>One suggestion to add for using set screws in transmitting torque on shafts (only works if the shaft and hub are the same material and ends flush) is to use the set screw as a key - drill and thread the keyway parallel to shaft on the joint between the shaft and hub.
Wow, that is some invaluable mechanical design info. I thoroughly enjoyed the read and I feel like I just took an engineering class, an incredibly fun one. Seriously, amazing detail, thanks a bunch! <br> <br>Simply out of pure curiosity, why weren't taped holes used over T-slots more often? I'm guessing that it didn't fit the 2D fab theme of the class? <br> <br>PS: Working in that shop must have been like a dream come true, I'm olive green with envy :)
Purely as a matter of convenience. The t-nutted holes are not nearly as strong as a properly drilled and tapped hole due to the number of inside square edges. It's a matter of recognizing when the structural loads in the device can be borne by material-on-material interference (the slots and tabs) and then having the fasteners (t-slots) only be there to keep it all together. There are far more instances when drilling and tapping is stronger than using t-nuts.
Excellent! Thank you! <br> <br>Suggestion for an addition: how do you align parallel guide rails on which bushings will slide? Also, how do you keep things sliding freely when temperatures change? <br>Specifically, in my 3D printer project I have an aluminum carriage supported by 4 bronze bushings that slide on guide rails. The print-bed is bolted to the carriage and is heated. As the print bed heats up, it expands, applying force to the bolts that stand it off the carriage, which in turn bend the carriage, which in turn misaligns the bushings.
The design you describe is a classic &quot;overconstrained slide&quot;. It's very sensitive to change in the center distance (gap) between the two rails regardless of what you do to the bushings. <br> <br>Generally you have only 3 bushings - two on one axis to constrain it against planar motion (up/down, left/right) and against tilting/pitching, which are two rotations. And one on the other to make sure it does not pivot on the first axis (rolling). The result is only one motion possible (along the rail). Four bushings adds another constraint which is technically unnecessary, and for it to not impede the motion of the slide, they all have to act perfectly in line and on the same axes. Any misalignment of the rails or of the bushings, then would seize up the slide, as you've noticed. <br> <br>The solution is usually to use 3 bushings - two on one rail, one on the other, and also 'float' the 3rd bushing on a mount which is compliant to misalignment in the center distance. For small applications it's sufficient to just use one of those rubber-mounted self-aligning bushings. <br> <br>In addition, your issue seems to involve flexing of the entire carriage structure which can bind up the two-bushing side too, unless they are also self-aligning. Short of isolating the hot build bed from the carriage, perhaps one or more of the bushings on the two-bushing rail should be also flexible types. It's less rigorous machine design but also a practical solution.
Sup everyone, <br> <br>Feel free to chat amongst thyselves and ask questions. Interesting discussions could very well get folded into the document for everyone to reference.
Well done.
That was great! Now can you come over and help me build my Spencer Aircar?

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