pic: Another Cheap Wood Frame Idea

[Rick TYler]

I've built three small boats and been a project manager for a Boy Scout project which built seven canoes -- all out of plywood/epoxy/fiberglass composite. I'm 100% with Bruce, if you decide you need fiberglass use epoxy resin, not polyester. If you want some solid advice and a good source of materials, check out the boat-building forum at bateau2.com. The folks there don't have much experience with robots (except me...) but what they know about strong, light, easy-to-make composites is pretty impressive.

As for materials science, there's a great article in "The Nature of Boats" by Dave Gerr on materials commonly used in small-boat building. The weakness of wood is that it is labor-intensive (not an issue in robots), eventually will rot (not an issue in robots) and has fairly low impact resistance (an issue for Battlebots, but probably not for us). According to Gerr, Douglas fir is stiffer than steel, aluminum or laid-up fiberglass for the same weight of materials.

Fiberglass does not add stiffness in a wood/fiberglass composite. Epoxy-saturated fiberglass is a great low-weight fastener for joints, and the epoxy seals the wood to reduce water penetration (not an issue for robots, unless we get the long-awaited aquatics game!). Fiberglass itself is not stiff -- the wood is the component in the lay-up that adds stiffness. The glass holds it all together, protects from weather, and adds some impact and abrasion resistance. There are additives for epoxy, like graphite powder, that can improve abrasion resistance.

Generally, I'd avoid Kevlar cloth (it fuzzes and is a pain to sand smooth), or carbon-fiber cloth (very hard to work with, fuzzes, and supernaturally expensive) (unless you make carbon-fiber poles as arm material, in which case I'd like you to send me pictures). Other cloths like Dynel are more for abrasion resistance than strength and will do nothing for a robot chassis than add weight.

A chassis made from 6mm okoume plywood with joints taped with 9-ounce fiberglass set in SystemThree or WEST epoxy would be amazingly strong and light. My sons and I built a 17'4" canoe with 4mm okoume, 6-ounce glass, and SystemThree epoxy that weighs less than 60 pounds without seats and hardwood trim. I think a robot base could be made that is less than 10 pounds.

One really important downside to composite materials (like a chassis) is that repairs during a tournament will be nearly impossible. Epoxy sets up in a chemical reaction that takes days for maximum strength. If you break a joint in a competition you can't just whack another piece of stock off with a hacksaw, drill holes in it, and bolt it into place.

With some planning you could get the same effect with careful material selection (okoume plywood is more expensive but a lot lighter than birch, for example) and glued corner blocks. Those gussets are big and heavy. Epoxy and glass might impress the judges, but wouldn't really add much value in a structure that doesn't have to stand up to weather and waves.

I wonder if FIRST would approve the use of wood in FTC?