These are used for applying hardware of all descriptions; also for panel work, cabinet work, all types of fine finish work, and support of electric and plumbing fixtures. Screws have greater holding power than nails and permit easy removal and replacement of parts without injury to the wood or finish. Screws avoid danger of splitting the wood or marring the finish, when the screw holes are bored with a bit.
Screws are made in a large variety of sizes and shapes to suit different uses (Fig. 11.86) and they are made of different metals to match various materials. A muchused type of head, other than the ordinary single-slot type, is the Phillips head, which has two countersunk slots at right angles to each other. The head keeps the screwdriver exactly centered during driving and also transmits greater driving power to the screw, while holding the screwdriver firmly on the head. Phillips heads are smoother at the edges, because the slots in the head do not extend to the outer circumference.

Steel screws for wood vary in length from 1⁄4 to 6 in. Each length is made in a variety of thicknesses. Heads may be ordinary flat (for countersinking), round, or oval.
Parker screws for securing objects to thin metal are self-threading when screwed into holes of exactly the correct size.
Sizes of screws are given in inches of length and the gage of the diameter.
Lengths vary by eighths of an inch up to 1 in, by quarters from there up to 3 in, and by halves from 3 to 5 in. Unlike wire gages, the smallest diameter of a screw gage is the lowest number; the larger the number, the greater is the diameter in a screw gage. Gage numbers range from 0 to 30.

Lag screws are large, heavy screws used for framing timber and ironwork (Figs. 11.86 and 11.89). Lengths vary from 11⁄2 to 12 in and diameters from 5⁄16 to 1 in.

Two holes should be bored for lag screws, one to take the unthreaded shank without  binding and the other (a smaller hole) to take the threaded part. This smaller hole is usually somewhat shorter in length than the threaded portion. Lag screws usually have square heads and are tightened with a wrench.

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