There have been a couple recent discussion threads from people asking what kind of raw materials they should buy. Aluminum tube + gusset construction is by far the most common, but what about channel, angle, etc. - when might they be better (for weight savings or other reasons)?
One of the mentors on our team is a very experienced ME/EE/naval architect, and over the years he has regularly commented that tubes + gussets are overused and that channel & angle in combination with tube along with lap joints can be as good or better for many things. Example of the difference: vs.
I assume there are other people/teams out there who think about this stuff… Interested in your points of view when it comes to FRC applications. My own opinion (as someone who is still learning) is… we use tube & gussets because it’s easy & it works, & it’s pretty easily teachable… not necessarily because it’s always the best.
Angle can work pretty well for some things. Actually have a robot from some time back that shakes out like this:
Drive base, 1x1 tube
Lift, 2x1 C-channel (x8, 2 welded together for a center piece)
Upper arm, 2" square tube and 2x1 channel
Braces/mounting, 1" angle
Any problems with the channel used on the lift flexing on the open side? I may not be envisioning it right; using channel probably means different design options.
Team 4618 made good use of aluminum unistrut in 2018 to make a light weight elevator stage. The c-shaped profile makes it easy to build a linear slider without so many bearings
I’ll have to get some pictures later (read: remind me in a couple weeks), but no. I think we had more issues with bowing (the lift was rather strong and on at least one occasion managed to damage itself)–the inside set looks like it was replaced at some point due to bow or something else.
For each side, the two center channels were welded back-to-back to form an I-beam. The outer pair were normal. 1/8" wall IIRC. Anodized (helps with aluminum, gets it a bit harder).
I have definitely had good experience with both angle and channel, as well as tube, especially when not up against a tough weight limit and could go to 1/8" rather than 1/16" wall. It is a lot easier to put chain/belt runs and other mechanisms inside of channel than tube, and you get a “free” chain guard.
My favorite was our STEAMworks climber - a ratchet wrench on the shaft inside a bit of 2x1 C-channel merely needed to be kept from falling out with a velcro strap. Easy to set, easy to remove. Doing that with tubing would have required a good bit extra.
As an ME, tubes are the strongest options for framing given a constant weight. Other shapes may have advantages in ease of attachment or strength in situations where they are not primarily carrying load from along their length.
L-channel and C-channel will have much higher wall thicknesses than tubes of the same weight, so their sections are less likely to deform in some (but not all) situations where loads are transferred from one face to another rather than end to end. For example if you are joining to flat plates an 90 degrees, an L will usually make a stronger joint than a tube.
Open sections are much easier to access for bolting to a single wall and in the case of thicker walls, local deformation and breakage around the fasteners are less likely. So if ease of assembly is a priority, and strength over long distances is not needed, use the C or L.
Sometimes you need weird shapes that can be cut from a C or L. We keep some 3”x1/4” L and other odd sections around despite using primary 1x1 and 1x2 thin walled tube because some times we can make a single bracket out of the big stock that would otherwise use several pieces of tube, small L brackets, and custom gussets. In these cases the final part looks closer to a billet part than a long extrusion.
In your pictured example, the tube joint is actually stronger if gussets are used on both sides, the joint shown with a channel could also be made with all tubes, but it would be much harder to assemble. Really using channels is usually about ease of assembly not strength. Very valid approach, but if you are trying to squeeze every last ounce out of a design tube are usually your friend.
We used C-channel this past season as our outer-rail to support the ends of the shafts with our wheels. Worked well and allowed us to put a shaft-collar in there without worrying about rubbing against our bumpers. Would do it again.
We decided to support the other end of the shaft instead of cantilevering mainly because you have to have something backing the bumpers anyway, and that backing removes some of the ease-of-access you get from cantilevering.
When using C-channel, make sure your team is using spacers or tool that limits torque (like a drill set to a lower torque setting), as my history with it is that students get over eager with it and will torque it down to a D-channel with just hand tools.
On the topic of using lap joints instead of gussets for making tube frame corners, anyone do this? Of course, it requires some time cutting away some material at the end of each tube, and that alone may be a good reason to just butt the tubes together and fasten via gusset.
vs. 
