Just something to think about, regarding steel vs. aluminum: steels are about 2.5 to 3 times as stiff as aluminum alloys, but both vary greatly in strength and in the ways they respond to welding.
For aluminum, all tempered varieties will exhibit reduced mechanical properties in the heat-affected zone of a weld. As a rule of thumb for aluminum, expect it to have the properties of the O temper (as in AA 6061-O) in the region of the weld, and to retain the properties of the original temper (e.g. AA 6061-T6) elsewhere. This won't change the stiffness appreciably, but will change the strength dramatically. Additionally, some varieties of aluminum need to be welded with a dissimilar filler metal that will have different (sometimes greater, sometimes lesser) properties.
In steels, this effect can vary greatly depending on the alloy being welded, the hardening processes already applied to the workpiece, the filler material being used and even the type of welding process. In the case of things like AISI 4130, AISI 4140 or other chromium-molybdenum steels, you'll actually find that they're
supplied in an annealed (softest) or normalized (slightly stronger) state. That means that welding them isn't going to diminish the properties of these steels much further.
Failure of most structures can be said to occur when the part is permanently deformed (yielded)—so the yield strength is relevant. Normalized AISI 4130 has about the same yield strength (436 MPa) as tempered AA 7075-T6 aluminum (505 MPa—yes, "stronger than steel"), but if you weld them (producing something near the annealed condition), the steel will only weaken to 361 MPa, while the aluminum will drop all the way down to 105 MPa yield strength. So, if you can fasten the aluminum without welding it, choose the weld locations so they're under less stress, or in rare cases, weld it and then heat-treat the whole structure, you may elect to choose aluminum because of the weight advantage. If you need a welded structure with good strength even at the joints, then an alloy steel is probably a better choice (if you expect to see this sort of loading).
Cheap structural steel (what buildings are made of) is the worst of both worlds, however: low strength and high weight. For example, ASTM A36 is only rated for 250 MPa yield strength. (Actually, structural steels, like the old-fashioned ASTM A36, and the newer ASTM A572 and ASTM A992, have very variable properties—the minimum yield strength is specified, but the steel could actually be significantly stronger. In fact, nowadays, the majority of small-section steel shapes sold as A36 actually meet A572.) It does have low cost going for it, though.
On the other hand, if your structure isn't going to see levels of stress approaching the yield strength, maybe you're interested in designing for sufficient stiffness at minimum weight. In that case, because steels have a stiffness advantage roughly equivalent to their weight penalty (steel is about 2.75 times the density of aluminum), either one can be a good choice. At that point, if welding is possible for your team, you might as well go with the steel—you'll get similar stiffness and weight, but you'll be able to weld it rather than worry about more complicated joints. Indeed, it's rare for unwelded joints to be as stiff as welded ones, so welded steel will come out ahead, especially for complex shapes like that FSAE car pictured above. (Imagine if it was made from unwelded aluminum: as a practical matter, they would have been forced to use a stressed skin to stiffen it, or otherwise add more material. And if they'd welded it from aluminum tubes, it would have been weak at the joints.)
But what if you can handle the assembly of a stressed skin? In that case, you can probably get better results out of aluminum—sufficient stiffness by using thin webs between structural members, and sufficient strength by using high-strength aluminum structure. A steel structure constructed the same way would be infeasible, because of the difficulty of producing, fastening and maintaining paper-thin sheets of steel (compared to three-times-thicker aluminum sheets of the same stiffness and weight). (This is a big consideration with aircraft: you usually see aluminum planes, rather than steel ones. Only when there's another constraint—like air friction heating due to high-speed flight in the XB-70 and MiG-25, or cost as in some kit planes—do you see steel construction in aircraft.) As far as FIRST goes, stressed metal skins aren't typically very impact-resistant, and as a result, I'd be cautious about choosing them for the main structure of the robot. (I did have very good success in 2010 using polycarbonate sheets as the structural skin of a robot—but that was only possible because neither strength nor stiffness were major priorities in that part of the robot.)
Now, despite this talk of stiffness, sometimes it doesn't actually matter much. Have we ever known the field in Atlanta to be flat? (No, not a chance—those stupid tiles are worthless.) In that case, for some drivetrains, a bit of flexibility is actually very beneficial. (Holonomic ones that need all wheels on the ground are the best examples.) For the last few drivetrains I've designed (a 4WD wide-base and two 6WD long-base designs), we built them stiff longitudinally (so that they could maintain wheel and gearbox alignment), but flexible laterally, to account for variations in the field and to reduce the amount of structure. They weren't perfect for climbing (a crooked approach to an obstacle caused the frame to flex), but the largely open interior allowed us to keep the robot compactly packaged without worrying about structure.
Realistically, though, if you're willing to overbuild a little, you can ignore the conclusions of this sort of analysis and still make a perfectly serviceable robot. (It's probably for that reason that we see teams being successful with all sorts of designs that aren't necessarily constructed in the most technically optimal way.) Equally, different teams have different priorities when it comes to design. Some favour a rock-solid robot, others want one that can be taken apart easily. There is something to be said for getting the robot done quickly, and building it strongly enough that there's no chance the frame will need repair. That's something else that teams will have to think about—what's the tolerance for risk, and how capable are you of making a field repair to each of the possible designs?
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Originally Posted by Katie_UPS
How do you keep your 'bot -but more specifically, your chassis- together?
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To answer the original question: rivets. Lots of aluminum rivets, inserted into all manner of structural components. I've used aluminum extrusions, kit frame rails (sheet metal), aluminum-plastic composites, polycarbonate and high-density polyethylene as structural materials over the last couple of years. Typically, the way this works is frame rails are positioned at the outermost extents of the frame, and riveted to structural sheets on perpendicular edges. The sheets form the walls of the frame, and stiffen it somewhat, but the principal loads are borne by the frame rails themselves (to avoid drivetrain deflection).