Steel on a FRC Robot

[Tristan Lall]

I'm not a fan of ordinary structural steel for FRC robots. The strength to weight ratio is awful, and the available sections are generally inconvenient. You see it occasionally, and you always wonder what set of design tradeoffs made that worthwhile for the team. (To be completely blunt, it's probably more like a lack of design that precipitated it.)

Alloy steels, particularly chromium-molybdenum steels like AISI 4140, are very useful, however. In the annealed condition, the strength to weight ratio is around the same as AA 6061 wrought aluminum. The difference is that you can weld annealed AISI 4140 with negligible loss of strength, while AA 6061 will drop dramatically in strength when welded. That makes welding a viable construction method for high-stress, high-strength parts.

The trouble is, because the steel weighs more, you have to use thinner material. This can be difficult to work with. Also, since 4140 typically comes in solid and tube sections (and doesn't work very well as sheet metal), you'll see it used in space frames, rather than stressed skins.

And of course, steel (of all kinds) is stiffer (in terms of elastic modulus) than aluminum, so when distortion is a major criterion, you may want to choose it.

But as for stainless steel, I've got to ask: why? (There are lots of grades of stainless, but in general, while it's probably a slightly better material than structural steel for an FRC structural application, it has a lot of other undesirable properties—for instance cost and machinability, under most circumstances—that make it a surprising choice.)