Basically, as long as a structural material like plexiglass is not specifically prohibited in the manual, it is allowed. Depending on your application, polycarbonite/lexan might be a better choice, however.
I would use 1/4" plywood if I were you. It’s cheap and easy to work with. Plexiglass, while tough and cool, has a tendency to crack when you’re working on it. We also tried using sheet polycarbonate, and the stuff just cracked and shredded like no other. However, because it was cheap, we just got huge sheets of the stuff.
Acrylic/Plexiglass is pretty much the bane of my existance when doing electrical work. Thin acrylic shatters when drilling (except when drilling with fine-toothed holesaws… that’s the only thing I have found doesn’t shatter it), it shatters when cutting, it shatters when bolting, and it shatters when you look at it the wrong way. We used a lot of 1/8" acrylic on our 2006 robot (hopper walls, electrical board, and a few other things), and most of us decided never to use it ever again. Polycarb/Lexan is much easier to work with. It still shatters if you drill it really funkily or tighten a bolt too much, but it is nowhere near as bad as acrylic. I have even pop riveted 1/8" polycarb (which, I imagine, puts a lot of force on it) with no problems. Just use polycarb instead of acrylic, the cost difference isn’t that bad, and it is much easier to work with.
We used 1/4 in. ABS last year. That was a bit of overkill. However, I really liked the ABS because you can treat it like you would most any other material. We band sawed and drilled and it never showed the slightest crack. It’s pretty expensive stuff though.
Acyrilic is perfectly legal to use (follow the flow chart!). However, as others have stated, it’s not exactly an ideal material. Polycarbonate (lexan) is just as easy to work with and less likely to shatter or spiderweb on you.
Acyrlic does have one fantastic property- using a torch you can flame the edges. This finishes them off removing any tooling marks, rounding the edges and restoring it’s clarity. It makes parts look much more professional. Since that is a pretty minor consideration in FIRST, I’d focus on using polycarb. if you are set on using a plastic as a base.
In place of either material, I would suggest either plywood or, if you want to get fancy, my new favorite material, garolite. Garolite is a cloth and resin based composite, sort of like fiberglass. It is very rigid, easy to work with and transparent to radio. It’s used in some PCB’s. McMaster has it in various grades, sizes and thicknesses. It’s more expensive then plywood and comparable to polycarbonate. It is in between plywood and polycarbonate for toughness, but beats them both in rigidity. This means you can get away with a very thin, and thus lightweight, sheet. 3/32 or 1/8 would be fine, I think, depending on the overall size.
Whether you use acrylic (Plexiglass) or polycarbonate (Lexan) I would suggest that when drilling holes in it that you re-grind your drill bits to make them “pointier” than a typical jobbers drill (about 60 degrees sounds right). Alternatively you can purchase specially ground drill bits for plastics. Yes, you can drill the plastics with a standard bit, but the pointier bits cut nicer and reduce the likelihood of cracking in plexiglass.
We use lexan extensively on our robots and have found it formable and durable. Plexiglass tends to be subject to cracking if you don’t baby it carefully, while you can bend and fold lexan just like you can sheet metal.
We have also used a plywood, and recommend baltic birch, but… of course… you can’t see through it.
Question; Would not starting your hole with the smallest drill bit you have and working your way up to the hole size you want by redrilling with with the next size bit up also lessen the chance of cracking the Plexiglass ?
For those of you drilling acrylic / plexi / lexan, they make special bits specifically for the application. The only difference these bits really have is that they have a much more angled or “pointier” end. If you have an old set of bits around and a bench grinder you can quickly grind them so you have a set of plexi-bits.
Good question. Although that definitely makes sense, I’ll defer to someone with more practical experience with that technique for a definitive answer. (Edit: Literal translation of that sentence = “Duh… I dunno?”)
And yes… to a previous comment… you really don’t have to worry about Lexan cracking unless it has been exposed to UV for many years, in contact with nasty chemicals (never thought about loctite being one, but thanks to the poster above who mentioned it), or you are working in extremely cold temperatures.
We don’t use plexiglass on our robots… not that it can’t be done or is bad practice to use it… its just that lexan is SO much tougher.
Real polycarbonate should never just crack and shatter, unless it made contact with Loctite or acetone. (Both of these cause a chemical reaction that eats away at the material).
In 2006, Team 190 literally built their entire robot using polycarbonate, including the chassis, ball hopper, and shooter super-structure.
Besides the cost, it’s great stuff to work with. I’ve personally spent a lot of time milling parts out of Lexan on a milling machine, and it’s really easy to machine. I’ve never had problems with cracking, chipping or shattering, even when working with very small and thin parts.
If cost is an issue, then plywood is a sensible choice. But if you have more financial freedom with your robot, then polycarbonate is my preferred building material.
Polycarbonate is a lot more flexible than acrylic.
Whereas acrylic will shatter if you bend or hit it, you can gratuitously bend polycarbonate without worrying about it breaking.