Page 151 - Robot Builder's Bonanza

120 WORKING WITH METAL

because the teeth on a wood file are far too coarse. Unless you plan on doing lots and lots of
metalworking, you can purchase an inexpensive set with a variety of files in it or get just what
you need. I’d start with just one or both of the following:

• For aluminum, get a single- cut mill, half- flat, half- round file; that is, on one side the file is
flat, and on the other it’s rounded. You don’t need a handle on the file unless you want one.
• For steel, get the same, except make it double- cut.

The number of teeth per inch defines the smoothness of the finished work. For general-
purpose deflashing and deburring, opt for files with 30 to 40 teeth per inch. This equates
roughly to “bastard” and “second cut” files, if the tools you’re looking at are marked that way
instead.
For small pieces, you might want to invest in a set of needle files, so- called because they’re
small like knitting needles. The set has different sizes and shapes.

Most files have teeth that face forward, away from the handle. So they do their work as you
G push it into the material, not as you draw it back. Use this fact to make your work easier. Don’t
try to use the file like you’re cutting a loaf of bread with a knife. Make even strokes, and bear
down only on the forward stroke.

Using Sandpaper
Use sandpaper if you want the smoothest edge possible. This is not just for looks, but for
function, too: the surfaces need to be like glass on pieces that slide against one another.
You need aluminum oxide or silicon carbide sandpapers for working with metal. For gen-
eral deburring and cleaning, use a fine or medium aluminum oxide paper; for finishing/polish-
ing, use a fine emery cloth ( doesn’t use a paper backing), and dip it in water as you work. The
higher the grit, the finer the finish.

Grit Key Name Grit

M Medium 80-100
F Fine 120-150

For a final smooth finish, buff the metal using 00 or 000 steel wool. You can get this at a
hardware or home improvement store. Look in the paint section. Which brings us to . . .

Painting Metal
Bases and frames of mild steel should be painted, to prevent rust. No painting or other treat-
ment is needed if the aluminum is already anodized— it’ll have a silvered, black, or colored
satin appearance. For bare aluminum, the metal can be left as is, but you may prefer to paint
it. There are several alternatives for painting aluminum and steel:

• Brush painting is simple, but brush marks are hard to avoid in metal. And the paint tends
to be applied too thickly.
• Spray painting with a paint designed for metal yields better results, but only if you apply
the paint in several light coats. Apply a primer coat first. Enamel paints work the best.
Paints for automobile engines are especially durable. Let dry completely before handling.

11-chapter-11.indd 120 4/21/11 11:45 AM