Screws

Screws 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 diameter—meaning the diameter of the unthreaded part. This number runs from 0 (about 1/15-in. diameter) to 24 (about 3/8-in. diameter). 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 useless, Figure 6-85.-Types of bolts. 6-50