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  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.