Page 53 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

34 Making Things Move

nce you’ve decided to make something that moves, you’ll need to find parts and
Omaterials to build it. Aside from the usual constraints of availability and budget,
how do you choose materials for your project? What’s the difference between
types of wood or kinds of aluminum, and are there any other options?

In this chapter, you’ll learn about the various types of materials, how to use them, and
where to find them. But first, we need to talk about how materials are described.

Describing Materials

In order to choose materials for projects, you need to learn how to describe materials
and how strong they are. Each type of material is characterized by material properties.

Material Properties

A material property is just something about the material that’s the same regardless of
its size or shape. For example, density is a material property, but weight is not. Density
is equal to mass divided by volume, so no matter how much stuff you have, that ratio
stays the same. However, the more stuff you have, the more it will weigh, so weight is
not a distinguishing material property.

Another useful material property is yield strength. A material that yields, or stretches,
before it breaks is called ductile. One that breaks right away is called brittle.
As an example, take a paperclip and try to bend one of the legs just a little, so it
returns to the original shape. This leg will have deflected, or deformed, but because it
returned to the original shape, it hasn’t actually yielded yet. Now take that same leg
and bend it a lot—way out to the side. It stays in its new position, and returning it to
the original shape would be hard (if not impossible). At this point, the paperclip has
yielded and deformed in a way that is not temporary.
If you can imagine paperclips made of different materials yielding at different angles,
you can see how yield strength is a good material property to use when comparing
the strength of materials relative to each other. Look on the MatWeb site
(www.matweb.com) to see the yield strength (among many other material properties)
of just about any material you might want to use.