Page 161 - Robot Builder's Bonanza

130 ASSEMBLY TECHNIQUES

• Screws are designed for fastening together the parts of machinery, hence the name. Wood
screws and sheet metal screws have a pointed end and drive right into two (or more)
pieces of material, to cinch and hold them together. Machine screws do not have a pointed
end; they’re designed to be secured by a nut or other threaded retainer on the other end.
• Nuts are used with machine screws. The most common is the hex nut. The nut is fastened
using a wrench, pliers, or hex nut driver. Also handy in many robotics applications is the
locking nut, which is like a standard hex nut but with a nylon plastic insert. The nylon
helps prevent the nut from working itself loose.
• Washers act to spread out the compression force of a screw head or nut. The washer
doubles or even triples the surface area. Washers are available in sizes to complement the
screw. Variations on the washer theme include tooth and split lock washers; these provide
a locking action to help prevent the fastener from coming loose.

FASTENER SIZES

Fasteners are available in common sizes, either in metric or imperial.
Imperial
Imperial (also referred to as American, standard, or customary) fasteners are denoted by the
diameter, either as a reference number or in fractions of an inch. For machine screws and nuts,
the number of threads per inch is also given. For example, a machine screw with a size of
6-32 1/2″

has a diameter referred to as #6, with 32 threads per inch
and a length of 1/2″. Diameters under 1/4″ are indicated as
a # (number) size; diameters 1/4″ and larger can be denoted
by number but are more commonly indicated as a frac-
tional inch measurement—3/8″, 7/16″, and so on. See Fig-
ure 13-2 for the sizing parameters of the typical machine
screw fastener.
The number of threads per inch can be either coarse or
fine. With few exceptions, your local hardware store carries
Threads
just coarse thread fasteners. The one major exception is the
#10 machine screw, which is routinely available in either
coarse (24 threads per inch) or fine (32 threads per inch). Be
Number of Length careful which machine screws you buy, because nuts for one
threads in
one inch won’t fit the other.
Metric
Metric fasteners don’t use the same sizing nomenclature as
their imperial cousins. Screw sizes are defined by diameter;
Diameter size the thread pitch is the number of threads per millimeter.
Figure 13-2 The size of a machine This is followed by the length of the fastener. All in millime-
screw is specified by its diameter (in ters. For example:
inches, millimeters, or numbered scale),
threads per inch (or millimeter), and length. M2-0.40 5mm

13-chapter-13.indd 130 4/21/11 11:47 AM