Re: Tube Versus Channel
 

We used 3 x 4 x 1/8 rectangular aluminum tube for side rails with wheels and chain runs mostly inside the tube. It wasn't easy to find but Metals Depot (metalsdepot.com) stock it and ship to Canada with no problem. By the time we finished with the tube there wasn't a whole lot left so it was light and very strong. Cross members were 1 x 2 x 1/8 rectangular tube with loads of lightening holes. The chassis is tig welded. 1 x 1 x .090 square tube was used for the superstructure. Here's a photo.

http://www.othsrobotics.ca/images/isabella.jpg

Rivnuts are great but do strip out easily if installed by the inexperienced. They also don't allow for two pieces to be tight together as there's a flange on the nut. Rivnut kits are available from Princess Auto for about $35 (http://www.princessauto.com/pal/prod...ting-Tool-Kit). That gives you the tool and then you can buy more rivets from McMaster Carr. Unfortunately the kit doesn't include 10-32. Making a 10-32 mandrel was straightforward once we bought a 10-32 LH die and much cheaper than buying a kit from McMaster Carr.

Re: Tube Versus Channel
 

It IS that much faster cutting aluminum extrusions. I'd used it on solid aluminum, and it's probably about equal on the cutting speed. But even then, you're saving time on post-processing. The circular saw leaves a cut that's much flatter and smoother, with deburring only necessary to remove the occasional thin burr and round off those sharp cut edges. Even past the processing speed, though, is the fact that it's a compound miter saw. If you maintain it and keep your fences square and in-line, you can pretty accurately cut whatever angle you want.

It's also a lot easier to get cuts to the exact length you want on this saw. Cut your part 1/16" longer and measure it. If it's right, stop. Otherwise, with the saw blade down and off, slide the piece over till it touches the flank of the blade. Hold your part, pull the blade up, turn it on and run it down and you've just taken off the tiny distance between the the flank of the saw blade and the outside of the kerf. It's something like 1/32" or less, so with a little extra effort, your nice square cut tube will be within 1/32" of where you want it.

The only downside is that a standard woodworking miter saw will only be good for cutting aluminum and your kerf is definitely larger than on a bandsaw. But it's still only 0.094", so unless you're cutting a heck of a lot of 0.5" pieces or something, you're not going to notice or care.

Re: Tube Versus Channel
 

I was thinking of the pre-cuts as basis for prototyping frames of superstructure. We use a primarily wood-shop, so prototypes tend to get made of scrap lumber. Cutting and fastening them together securely seems to suffer for the focus on getting right to trying the mechanism. If a reasonably secure framework can be made quickly, then maybe the ideas won't get lost because the prototype was too flimsy to tell if the idea were any good.

Sorry, that seems cloudy even to me. Just think making a sloppy pile of crates to get to the cookie jar. More attention to solidity means better validation of the demonstrated principle.

Re: Tube Versus Channel
 

Our 1/4 plate is .250 ± .001. We got it from McMaster, because we actually had to get it CNC milled by our machine shop. We hadn't tried to get a 2D sponsor before the season, but actually right after the end of build season, we got industrial Metal Supply to sponsor us, and they have a laser cutter that we will be able to use for the offseason and any future games. This is just one of the laser cutters that they have available: http://www.youtube.com/watch?v=-k8JGL8Muq4

We use the 1/4 plate for our ground pickup, shooter hood, and elevator gearbox, which we also made custom. This helped to simplify our frame quite a bit. What I was really talking about though, is it seemed like a lot of teams have way more complicated frames than needed. I like using 1/4 plate, wherever it seems like it would be beneficial to use sheet metal, as it has a lot of similar capabilities, and it adapts to whatever you need it for very well.

Re: Tube Versus Channel
 

Most of our machines are made from 1x1x.063 aluminum tube and 1/8" polycarbonate gussets that are fastened together with 1/4" rivets.

1/4" rivets are definitely overkill, but they make prototyping with 1/4" hardware and swapping in rivets at final assembly a cinch and our laser cutter doesn't always do such a great job when cutting smaller holes into some thicker parts -- a #10 clearance hole in 1/2" material gets pretty oblong by the end.

This year's drive pods were built from aluminum standoffs and 1/4" delrin plate; I was a bit worried about how well the plastic would handle side-loading, but it's been a champ so far.

With respect to tube vs. channel -- in almost all cases, I'd use tube if I could. Tube is much stiffer than channel in torsion.