A wood screw is a fastener that is threaded
into the wood. Wood screws are designated by
the type of head (fig. 6-84) and the material from
which they are made; for example, flathead brass
or round-head steel. The size of a wood screw is
designated by its length in inches and a number
relating to its body diametermeaning the
diameter of the unthreaded part. This number
runs from 0 (about 1/15-in. diameter) to 24 (about
Lag screws, called LAG BOLTS (fig. 6-84),
are often required where ordinary wood screws
are too short or too light, or where spikes do not
hold securely. They are available in lengths of
1 to 16 in. and in body diameters of 1/4 to 1 in.
Their heads are either square or hexagonal.
Sheet metal, sheet aluminum, and other thin
metal parts are assembled with SHEET METAL
screws and THREAD-CUTTING screws (fig.
6-84). Sheet metal screws are self-tapping; they
Figure 6-84.-Types of screws.
can fasten metals up to about 28 gauge. Thread-
cutting screws are used to fasten metals that are
1/4 in. thick or less.
Bolts and Driftpins
A steel bolt is a fastener having a head at one
end and threads at the other, as shown in
figure 6-85. Instead of threading into wood like
a screw, it goes through a bored hole and is held
by a nut. Stove bolts range in length from 3/8 to
4 in. and in body diameter from 1/8 to 3/8 in.
Not especially strong, they are used only for
fastening light pieces. CARRIAGE and
MACHINE bolts are strong enough to fasten
load-bearing members, such as trusses. In length,
they range from 3/4 to 20 in.; in diameter, from
3/16 to 3/4 in. The carriage bolt has a square
section below its head which embeds in the wood
as the nut is set up, keeping the bolt from
turning. An expansion bolt is used in conjunction
with an expansion shield to provide anchorage in
a position in which a threaded fastener alone is
Figure 6-85.-Types of bolts.