roundhead, as illustrated in that order in figure 3-79,
All of these screws can have slotted or Phillips heads.
To prepare wood for receiving the screws, bore a
body hole the diameter of the screw to be used in the
piece of wood that is to be fastened (figure 3-80). You
should then bore a starter hole in the base wood with
a diameter less than that of the screw threads and a
depth of one-half or two-thirds the length of the
threads to be anchored. The purpose of this careful
preparation is to assure accuracy in the placement of
the screws, to reduce the possibility of splitting the
wood, and to reduce the time and effort required to
drive the screw. Properly set slotted and Phillips
flathead and oval head screws are countersunk
sufficiently to permit a covering material to be used to
cover the head.
Slotted roundhead and Phillips
roundhead screws are not countersunk, but they are
driven so that the head is firmly flush with the surface
of the wood. The slot of the roundhead screw is left
parallel with the grain of the wood.
The proper name for a lag screw (shown in
figure 3-79) is lag bolt or wood screw. These screws
are often required in constructing large projects, such
as a building. They are longer and much heavier than
the common wood screw and have coarser threads
that extend from a cone, or gimlet point, slightly more
than half the length of the screw. Square-head and
hexagonal-head lag screws are always externally
driven, usually by means of a wrench. They are used
when ordinary wood screws would be too short or too
light and spikes would not be strong enough. Sizes of
Figure 3-80.-Proper way to sink a screw.
lag screws are shown in table 3-11. Combined with
expansion anchors, they are used to frame timbers to
Expansion shields, or expansion anchors as they
are sometimes called, are used for inserting a
predrilled hole, usually in masonry, to provide a
gripping base or anchor for a screw, bolt, or nail
intended to fasten an item to the surface in which the
hole was bored. The shield can be obtained separately,
or it may include the screw, bolt, or nail. After the
expansion shield is inserted in the predrilled hole, the
fastener is driven into the hole in the shield,
expanding the shield and wedging it firmly against the
surface of the hole.
For the assembly of metal parts, sheet metal screws
are used. These screws are made regularly in steel and
brass with four types of heads: flat, round, oval, and
fillister, as shown in that order in figure 3-79.
Wood screws come in sizes that vary from 1/4 inch
to 6 inches. Screws up to 1-inch in length increase by
Figure 3-79.-Types of screws.