Robotics/Design Basics/Building Materials
Robotics: Design Basics: Building Materials
There is plenty of choice when it comes to picking the building materials for your robot. However not every material is a good choice.
There are three groups of materials. Each of these three groups have their own characteristics, possibilities and difficulties.
Note: There is a fourth group of materials called ceramics. However this group is only marginally useful for robotics.
Note In this book robot sizes are mentioned as small, medium and large. With small I mean any robot that can maneuver on a table. Medium means any robot that's too large to move on a table but still is light enough to lift on your own. Large mean anything bigger and heavier than that. Also robots that'll have to operate in real life (rough) condition should be counted as either medium or large.
Wood is probably the best material to start with. It's light, pretty strong and easy to work on. Not to mention cheap and easily available.
Even if you intend to use metal or plastic, wood can be handy for various purposes like prototyping and building jigs and other aids in working on metal or plastic parts.
The main reason that you will not see many wooden robots is because wood doesn't seem to fit in the picture of high tech machines. Funny thing is that (synthetic) composite materials have a very high coolness-factor (e.g Kevlar® Yes, Kevlar is a brand name. It's made of Aramide), yet wood, a natural composite material, hasn't.
- Useful for small or medium sized robots, prototyping and as construction aid.
There are 80 different pure metals each having different properties. However in the world of Robotics there are only a select few from the 80 that are useful. This list is increased by Alloying. Alloying is the process of combining either in solution or compound, two or more elements, at least one of which is a metal, and where the resulting material has metallic properties. The resulting metallic substance has different properties (sometimes significantly different) from those of its components.The properties of some metals and their alloys are below.
Some alloys are limited in supply in the market due to the limited demand for them. In order to obtain these materials it is often required to look further than the general consumer market.
Aluminium (or aluminum, both are correct) is commonly available in extruded forms in different shapes. It's pretty cheap, light, strong, resistant to corrosion and easy to work with. However welding aluminum isn't practical as it needs special welding equipment (MIG/MAG or TIG welding) and the bond isn't very strong. While soldering is possible, it doesn't make a strong bond. Rather use nuts and bolts or rivets.
- Useful for small or medium sized robots.
- Useful for non-load bearing parts in large robots.
- Not very good for bearings.
There is an alloy of aluminium called Duraluminium it is almost as strong as soft steel but very light thus making it a natural choice for the construction of Aircraft. However as a trade off for the combination of being strong and light it is quite expensive (if you can find it at all). The best bet is to search aircraft wrecking yards.
Commonly available steel is an alloy of iron. It's stronger than aluminum, but it's also heavier and harder to work with - in particular, it blunts tools more easily and swarf is always razor sharp. Welding isn't much of a problem. However heating steel (at welding temperatures) changes its characteristics (strength, hardness, rust resistance). Note that drilling steel requires cooling and a slow drill speed (both rotational and feeding). If you drill too fast, you'll see your drill heating up red hot. Drills that have been red hot lose their hardness and will be dull in no time.
- Useful for large robots and robots intended to operate in rough conditions. Too heavy for small or medium sized robots.
Heavy. Very good for bearings. Too expensive and heavy for most other purposes.
Heavier and more expensive than aluminum. Can be soldered.
- Useful for bearings.
Mostly available as wire or axles. Quite heavy, very good conductor.
- Useful for special parts and wires.
Synthetic Materials 
Like steel, synthetic materials are a name for a very large group of materials. There are hundreds of different plastics each with different characteristics and uses. We'll be covering only a few of them, but many techniques work just as well with other synthetics.
Most synthetic materials can be bent into shape after they are heated. A hot air gun (used to remove paint) can be sufficient for this purpose. Drilling and sawing these materials requires low speeds or they have to be cooled with water so the material doesn't melt. Soft plastics can be cut with an utility knife.
Polyvinyl chloride: Used for plastic tubes.
Polymethyl methacrylate: Transparent material. Can be bent when heated to 200°C.
Composite materials 
Polymer composite materials are materials consisting of a polymer matrix and a reinforcing material. (think of reinforced concrete: the polymer matrix is the steel grid and the reinforcing material is the concrete)
These materials are considerably stronger and stiffer than steel and aluminium alloys.
Material Specific strength specific stiffness
Steel 150*103 Nm/kg 20*106 Nm/kg
E-fiberglass/epoxy 300*103 Nm/kg 10*106 Nm/kg
Aramide/epoxy 500*103 Nm/kg 25*106 Nm/kg
The reason why composite materials haven't replaced steel is because of cost. Composites are only used where weight is a more important factor than price, e.g., airplanes: less weight means less fuel consumption and/or more useful payload.
Other Materials 
Generally, it's weak and looks ugly, but it's very practical for making fast prototypes. Can be cut with a knife or scissors and put together with duct tape or glue gun. When dry, it's an insulator and can be used as a prototyping board for circuits.
Often a cardboard mockup is built to make sure the parts of the robot all fit together properly.