Page 295 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

272 Making Things Move

CAUTION When cutting paper or cardboard (or anything), keep all body
parts out of the path of the knife. This may seem obvious, but if you’ve ever
held some paper with your thumb flared out and then cut right into it, you
know that safe cutting practices don’t always come naturally. Save the dissection
for biology class and make safety a priority before you cut anything.

Casting and Molding
To cast a part, you first need to create a mold around it that will have the imprint of the
part in it. The original part is then removed, sometimes by cutting the mold or with the
help of some kind of mold release. Then the cavity in the mold can be filled to create a
positive cast that matches the original part. This is generally accomplished by pouring a
liquid plastic compound into the mold and letting it cure (harden) before removing it
from the mold. Casting is an excellent way to clone an off-the-shelf or 3D printed part.
It can be very economical if you need to make several copies of the same part.

One good method is to use silicone rubber for the mold (like Mold Max from
www.smooth-on.com), and then a liquid plastic casting compound like Smooth-Cast
300 to make the positive cast. Smooth-Cast has a pretty fast setting time, a 1-to-1
mixing ratio, and it’s easy to work with. You can use dyes to create any color you
want, and it can be painted and machined when cured. The Compleat Sculptor
(www.sculpt.com) is a great source for all of these materials, and the shop frequently
holds classes in its New York City store.

For the preparation of the negative mold, the main objective is to make sure there are
no air bubbles and that the mold can be separated in a logical way. Before the silicone
rubber is poured around the original part, you can glue small wooden dowels to it to
create channels that will allow air bubbles to escape. A larger, primary dowel also
needs to be glued to the part somewhere in the center to create a pour hole.

Working with Wood
Wood can but cut and manipulated with many of the tools mentioned earlier, but
there are some additional tools that are specific to woodworking. Planers come in
hand-operated and powered versions. They take ragged pieces of wood and shave off
the top surface until it is parallel with the bottom surface.