A bot can have all sorts of interesting internals, top-of-the-line motors, and an incredibly powerful weapon.. But if its armor is made of paper, it probably won't last long. When it comes to armor, different bot designs require different properties. A ramming bot, for example, may have the weight for steel armor, which it may need since the idea is to out-ram the opponent. And since it has no weapon, it can use the saved weight for heavier armor (or more of a light armor). But if you have a weapon, you have less weight to play around with, and you'll probably have to settle for aluminum. A springy material may make good armor when combined with a different material for puncture resistance, but it probably wouldn't be good for a frame, which must be stiff to hold everything together.

But a problem with browsing through materials is that there are a lot of terms to wade through while searching for a perfect fit for your application. What is the difference between tensile strength, shear strength and hardness??? Well, that's what you're here for.

Tensile strength is how much a material resists being pulled apart. The way I see this is that it is on the same plane as the material. If you have a sheet of plastic, drilled a screw through the sheet and drilled it flat into the wall, then tensile strength would be how much the sheet would resist if you pulled on the bottom of it, before deforming. You wouldn't be pulling away from the wall - that's the strength of the screw holding the material on - but down, up, or side to side. Well, anyway the important thing is it's the pulling force required to deform a material. But with bot armor, this isn't usually a huge concern: not many forces will be pulling your materials like that (although a low tensile strength usually isn't a good sign: something else may be weak elsewhere).

Shear strength is a different plane than tensile strength. This would be like ripping your paper: perpendicular to the flat plane. If you go back to the example where I drilled a piece of plastic into the wall and held it with a screw (by the way, don't try drilling plastic sheets into the wall like this at home, and if you do, I HAD NOTHING TO DO WITH IT), the force the screw would experience is a shear force. This is usually a greater problem with screws than with anything else, though. In battle, if screws fail, it is often because the screw head or threads got sheared off. With as many impacts as there are in fighting robots, the screws that hold the bot together experience more forces than you'd think.

Speaking of impacts, impact strength is how well a material holds up to direct impact. Glass, for example, has a pretty cruddy impact strength. Some materials are unbreakable: Ultra-High Molecular Weight polyethylene, for example, and under some circumstances polycarbonate (a.k.a. Lexan, Hyzod and other brand names) can be close to it. This is important to use as armor in Battlebots, obviously: don't build your bot out of glass unless you want to see your bot transmogrify into lots of purty crystals shattered across the arena.

Hardness is how difficult it is to permanently deform a material. The harder the material, the better, usually. Hammer heads (the tool) are very hard, for example. Hardness isn't the same thing as elasticity, or how springy a material is, just how resilient it is. But at the same time, hardness isn't everything: some very hard materials are known to be brittle at times, so sometimes it's best to aim to the middle of the scale or so.

Heat treatability is whether or not a substance can be heat-treated. There are lots of different heat treatments which I won't go over here, but mostly involve drastically changing the material's temperature, which changes properties. Usually, this affects hardness the most, as well as things like ductility (how workable is the material by hand.. Clay is ductile, metal tends not to be much so) and machinability.

Machinability is how easily a material is machined. If a sander can effortlessly drill through the material by its mere friction (polycarbonate), the machinability is better than if sabersaw blades are routinely melted in an attempt to cut the material (titanium).

Density is how heavy a material is with a given volume. (Remember D=M/V? So, if you know D and V, M=DV. Good for estimating weight.)

And last, we have the term alloy. With metals especially, if a material is used in its pure form (say, pure aluminum), it lacks some desirable qualities. But suppose another metal (like copper or brass or zinc) has a quality that your aluminum lacks? The two can be melted together to form an alloy, or two or more materials as one, which sometimes have both qualities, and sometimes may not. That's a science called metallurgy, and I'm no metallurgist (maybe someday as a hobby!). Obviously, it's hard to alloy metals by hand, or even without special equipment. But most metals have different alloys available for sale, with differing properties. Take aluminum for example. Alloy 6063, which is often used in those aluminum angles you might pick up in a Lowe's, is of a moderate strength, but can be low-strength when it comes to Battlebots. But 6061, possibly the most common Al alloy is twice as strong, 2024 (my personal favorite, an aircraft-grade alloy with moderate hardness and moderate price) is even stronger, and 7075 is strongest of all. But each alloy has its own pros and cons. Confusing? Don't worry, it will all be explained more further on.

Now, since plastics and metals are meltable, they are often sold in different forms (and wood at times is sold in some of these forms, but only simpler ones such as rod or bar, not channel or square tubing to my knowledge). Here are a few of the forms you may find plastics or especially metals.

Sheet - This is probably what you'll be using most, as armor panels, for example. Plastics, woods and metals are all available in this form, which is a flat, thin material in the dimensions you want (ie. 12"x24"x1/16"). The thickness is much less than the other sizes.

Plate - Exactly like a sheet, only thicker. The transition thickness from sheet to plate is 1/4", usually. You may also use this as armor.

Bar (square, rectangular, hexagonal, octagonal, etc) - Ever seen those Play-Doh toys where you push the Play-Doh through a square or star-shaped hole and get a long square bar of Play-Doh? (Ever notice that Play-Doh was an ancient Greek philosopher?) This pushing through a hole is much like these metal bars are made (they're called extrusions, you will see this a lot in computer design programs later). The square bar is hot metal pushed through a square hole, hexagonal is through a hexagon, octagonal through an octagon. Rectangular, though, is usually a very thin rectangle, where the thickness is much less than the width of the bar. In any case, you may use these in a frame or as a spinning weapon, or some other use. Wood is available like this (at least, in the square form, though it's not called a square bar, I don't think), and plastic may be, although not incredibly often (I'm not sure). Metal is the most common material where this form is found.

Tubing (square, rectangular, round)- A long, extruded tube with a cross-section of a circle, square or rectangle. Circular tubes are most common, then square, then rectangle. The trick is finding a tube of a good material for robotics: often weaker alloys are used with aluminum tubes. These are available in plastic and metal, and can be used for a lighter solution for a frame (we used Al square tubing with Ni: it caused a heckuva lot of work and some sweating for me as I had to have the dimensions right the first time (and caught several errors in my work, which made me nervous), but it held up very well in combat once completed).

Angle - An L-shaped extrusion which is very good for holding things at right angles. I believe it can be found in plastic, know it's widely available in metal. You can use a long L-bracket for an entire side of your bot's armor, or just cut small pieces if you're low on weight and put them in strategic places. It's dirt cheap (relatively, that is), so it's probably worth investing in a good quality Al or steel alloy for this.

Channel - This is a C-shaped extrusion, kind of reminiscent of an unused staple if viewed from one side. I know metal is available like this and think plastic is. There are many uses, but probably the one I heard that makes the most sense is as an entire side for a bot! The legs of the C attach to the base and top, the body of the C make up the side. It makes for a sturdy side armor, but I imagine it would be heavy, and it looks expensive for that size. Anyway, there are other uses too. Use your imagination!

Rod - Or round bar. Just a pure cylinder. These are available in wood, plastic and metal, and could be used for all sorts of things.. Drums, wheel axles or hubs, make-shift fencing swords (touché).. And so on.

Discs - Flat slices of rod, basically. Probably available in wood and plastic, but I know it's available in metal. Good for wheel hubs as well, or other parts where you need a thin rod. And what about for a weapon? Nah, that would never work.

Shims- Incredibly thin sheets. Available in wood, plastic and metal, but they're usually used for aligning things perfectly (Got a missing leg on your desk? Stick shim under it until it's exactly level. That sort of thing) and they're so thin that they probably won't be of much use (or strength) in your bot.

There are other forms out there as well, but these are the most common, and the rest you can probably figure out for yourself.

So, what kind of materials are frequently used in fighting robots? Or are often avoided?

-Wood-

Plywood - cheap, light, but not very strong compared to metals. It does have a little give to it, so might make decent shock protection, but when it breaks it splinters.. Like everything else, there are always tradeoffs. Best quality is marine plywood, but I'm not sure how huge a difference there is between marine and what you get at Lowe's or Home Depot. Worked for us under aluminum with Ni.

Balsa - very cheap, very light, normally used for hobby R/C airplanes. Incredibly weak, though. Don't use this for anything structural (or if you do, don't blame me when it breaks). I think Steven Wright describes balsa wood's main advantage best: "My house is made out of balsa wood, so when I want to scare the neighborhood kids I lift it over my head and tell them to get out of my yard or I'll throw it at them." Or you could make a prototype with it.

-Plastic-

Acrylic - a good plastic for modelling things and for an almost glass-like clearness... but not much else in the robot-fighting world. Notorious for being fragile, and the see-through isn't worth it (see polycarbonate below)

Polyethylene (Ultra High Molecular Weight - UHMW) - very light, very low friction (good for skids), unbreakable, fairly cheap.. Gosh, it's a wonder material! Er.. Except it's very, very easy to cut. So it may not be your best outer armor. But, there's always tradeoffs. If you doubt there will be much risk for it being cut, give it a shot.

Polycarbonate (Lexan, Hyzod, etc) - The infamous Lexan, what most arenas' see-through walls are made of. High impact resistance, radio signals can go through it, light and low to moderately priced. It's just easily cut, unfortunately, and when it breaks, it can get ugly (may look more damaged than it is). And also sometimes is sensitive to UV: avoid laying it out in the sun. Still, so many advantages. This is what I prefer to have as the top of my bots, for reception-purposes (plus you can see the insides.. gnarly).

Nylon - Another low-friction plastic, with less friction than UHMW (I think), and very light, but can be somewhat expensive. Worth a look if you ever consider skid material, though.

-Metal-

Steel - I'll be honest, I don't know as much about steel as I'd like. I know it is very weldable, and a little about some of the alloys - spring steel is very elastic, but can be expensive. Chromoly, or chromium molybdenum steel seems to be seen as the most well-rounded steel as well. Carbon steel I have seen used (a friend of mine built a bot with insanely thick walls of carbon steel.. Nothing could touch it. And tool steel is what tools are made out of: made to be harder than almost anything. Hammers are made from tool steel, so when it's hardened to the right hardness, it can be great as a weapon! Overall, steel is a heavy metal (no relation to the music), which often limits or eliminates it from lighter classes, but it is very common in Middleweights and up. It's also cheap for metal, another plus, and usually the strongest material out there.

Aluminum -
2024 - my personal favorite as a well-rounded high-strength Al alloy, 2024 is aircraft-grade (which just means they are comfortable using it on airplanes). And because of the popularity of homebuilt airplanes, another hobby out there, a place called Aircraft Spruce supplies this very cheaply compared to other sources: $20 for a 2'x2'x1/16" sheet of it, about half of what others charge. It is more expensive than the more common 6061, but in my opinion worth the extra money if you can spare it. High strength for Al (comparable to lower grades of steel for half the weight of steel), low to moderate cost, plus it looks cool (shiny). My only known combat experience of this was with Ni's wedges against the drumbot War Drums: it hit our wedge, and took a chip out of it, but the thing held up for the most part against a scary weapon. I love this alloy!
6061 - a well-rounded, common aluminum alloy, the cheapest of the three alloys I'm mentioning here. It's the most commonly used in BattleBots in the aluminum family as far as I know, and is light, fairly cheap, and fairly strong (good strength-to-weight ratio). Weaker than steel, but lighter than steel as well. Up to 12 or 30 lbs, this is fine around 1/16" to 1/8", so keep it cheap!
7075 - The more expensive of the 3 alloys I'm talking about here, 7075 is also the hardest and the highest strength, making it a favorite of many of the competitors who choose more exotic alloys. It's also aircraft-grade, but a little harder to find than 2024, and the extra hardness makes it a little brittle. I've heard good things about this metal, though.

Magnesium - very expensive (comparable to Ti, below), but lighter than Al and stronger than Al. And for those of you who haven't reached it in Chemistry yet or haven't seen the magnesium whatchamacallits, it's VERY VERY FLAMMABLE. Huge white flames. Not good against a flamethrower, I wouldn't think! But it supposedly machines well too.. Its cost and availability are the biggest drawbacks (I could only find it in one place, and they only had three sheet thicknesses, all three obscure.

Titanium - (6AL-4V is the only alloy to have!) This metal is probably the posterchild for fighting robots. Nothing like Ti on a robot to make it look so much more dangerous (and cool). And it's true: it's lighter than steel (although heavier than Al) and has strength comparable to steel, is very elastic, and according to the razor commercials is sharp enough to cut a car in two (well, almost anything can be that sharp, Ti isn't "born" that way). But it's incredibly expensive and hard to machine. (I know personally. I got some thinking I got a deal.. After paying for the Ti I had to have it professionally machined since I realized I couldn't do it with my tools.. Now I regret using it.) But if properly machined, it can be very usable! And it makes purty sparks when spinning blades hit it (Though not for me)! So if you have the patience and the extra budget, you might spring to Ti. At least there's fewer alloys to choose from!

-Other-

Kevlar - This is what those famous bulletproof vests are made of. It's really cloth, but can be stretched across spaces for armor. It's really light, and very strong (especially to puncture). It is moderately priced, but its main drawback is setup. It involves epoxy, and apparently requires some skillful techniques to pull off the best. But if you're feeling adventurous, this may be a good material for you.

Carbon fiber - This incredibly light, incredibly stiff and incredibly strong material would be perfect! If it were cheaper and safer. It's high-priced, but it is also very dangerous to machine. The materials within the Carbon fiber can cause cancer, so a splinter can be life-threatening, and it typically isn't good to inhale the stuff. It's popular, but I personally don't think it's worth the risk. And I strongly recommend that you don't use this for your first bot, and strongly consider not using it for any later bot.

So, there you have it. A nice run-over the wide world of materials. Now you can see why I've been putting off writing this for a couple of days now! I'll give you a few links, then retire to a well-earned night's sleep.

Links:

Aircraft Spruce (http://www.aircraftspruce.com/)
A great provider for aluminum (best prices I can find!), and they also sell Kevlar, carbon fiber and high-quality screws.

Wicks Aircraft (http://www.wicksaircraft.com/)
They don't have a huge variety of material, but they do sell steel and aluminum (and the sacred, rare, 6061 square tubing. Most square tubing is 6063, but 6061 is twice as strong for the same weight. We used their tubing for Ni's frame)

Online Metals (http://www.onlinemetals.com/)
Good for small orders, this site has plenty of different kinds of materials and forms.

Kenward Plastics (http://www.kenwardplastics.com/lexan.asp)
Even though I haven't released this site to the bot-building public, I think they would still have a psychic feeling that I was leaving out Kenward Plastics and murder me if I didn't place this link in. Low-priced Lexan, albeit in bulk. There. HAPPY!?

McMaster Carr (http://www.mcmaster.com/)
THE site. I mean, MAN what a lot of stuff this company sells. It's definitely worth a look. Type "Metals" or "Plastics" (or "wood") into their search engine to see their selection!

Titanium Joe (http://titaniumjoe.com/)
This is where I got my Ti from in my rookie year. I didn't much like Titanium since I can't machine it, but I liked the service, and they sponsor a BattleBot team! So if you're interested in using Ti, consider them first.

MatWeb (http://www.matweb.com/)
A good resource for finding material properties. They've changed part of it to paying mode, but the free mode still is useful.

SozBots (http://www.sozbots.com/)
SozBots sells small chunks of material, sized for antweights. This includes Lexan, Ti, 7075 Al and carbon fiber, among others.

Robot Marketplace (http://www.robotmarketplace.com/)
What can I say? This place has everything you could need.. And then some. Check their Metals and Materials section to see what they've got. (Polycarbonate and Carbon Fiber, plus some links to other suppliers, both of which I've already mentioned above.)


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