Page 162 - Robot Builder's Bonanza

SCREWS, NUTS, AND OTHER FASTENERS 131

Figure 13-3 The three main types of
screws: machine, wood, and sheet metal.
Machine screws are used with nuts (or
threaded holes); wood and sheet metal
screws are intended to be fastened directly
Machine Wood Sheet Metal into the material.

means the screw is 2mm in diameter, has a pitch of 0.40 threads per millimeter, and has a
length of 5mm.

ABOUT WOOD AND SHEET METAL SCREWS
Machine screws and nuts are intended to fasten together. I like them because they let you
rebuild your robot an unlimited number of times— it’s easy to take them off and put them
back on.
But wood and sheet metal screws have their place, too. These are made to hold together
two or more pieces of wood or metal. Both types of screws are made to be self- tapping; that
is, they form their own threads in the material as they are being screwed in. See Figure 13-3
for a comparison.
• Wood screws are intended for wood and have a smooth part at the top of their shank. This
smooth part should be about the same thickness as the first piece of wood that’s been
assembled. You can also use wood screws with soft plastics, such as expanded PVC.
• Sheet metal screws are meant to hold together thin sheets of metal. With these, the
threads go all the way up to the head of the screw.

For most projects (and materials) you can often mix and match wood and sheet metal
screws, and things will still work. It might not be the “official” way of using these fasteners, but
as long as they don’t fall out or cause the materials to crack or break, there’s no harm. Use
what you have available.

SCREW HEAD STYLES
When buying screws (machine, wood, or metal), you have a choice of a variety of heads. The
head greatly contributes to the amount of torque that can be applied to the screw when tight-
ening it. Additionally, certain head styles are designed to have a lower profile, so they stick out
less than the others.

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