Screws offer more holding power than nails. The advent of power screwdrivers and new screw types has made the nail almost obsolete in many shops. There are four factors to consider when you choose a screw: size, head type, screwdriver type, and thread type. Each is described below.
Screw Sizes
Screw sizes are designated by a number that indicates the diameter and the length of the screw in inches (Table 10-2). The smallest diameter screw is 0, and the largest commonly available is 24. For bench work, the most useful sizes are 4 through 12. Of those sizes, 6, 8, and 10 are probably used more than any others.
Head Types
Screws have one of three basic types of head: flathead, roundhead, and panhead (Illus. 10-5). Flathead screws are meant to be countersunk flush or below the wood surface. You must countersink the hole before driving the screws (Illus. 10-6 and 10-7). The bugle head screw is a modern variation of the basic flathead screw. It is designed to pull down flush with the surface without being countersunk beforehand.
The ordinary bugle head screw will pull down flush in softwoods, but it will remain slightly above the surface in hardwood. Some bugle-head screws have sharp nibs below their heads to cut away the wood below the head as you drive the screw. This type will pull flush even in hardwood. Roundhead screws have a flat bottom, so they don’t have to be countersunk. The rounded top extends above the wood surface. Roundhead screws are usually used for attaching hardware. Panhead screws are similar to roundhead screws, but the head is flattened on top so it doesn’t extend as much above the surface.
Screwdriver Types
Screws are commonly available with recesses in their heads to fit the following three types of screwdriver: straight blade, Phillips, and square (Illus. 10-8). Screws that take a straight-blade screwdriver are often called slotted-head screws. This is the oldest type of screw. Slotted-head screws work fine when you are driving screws by hand, but they are hard to use with a power screwdriver.
The Phillips-head screw has a cross-shaped recess in its head. This type is much easier to drive with a power screwdriver because the bit doesn’t jump off the screw head as easily. Phillips-head screws have become increasingly popular as more and more people use power screwdrivers.
The square-head screw has a square recess in its head. Although it is not as widely available as the Phillips-head screw, it is increasingly popular among wood- workers because it offers an even better grip for the screwdriver bit. There are several other types of screwdriver designs, but at present they are mostly used in industrial applications.
Thread Types
The traditional wood screw thread pattern is an old design that was based mostly on the limitations of thread-cutting machinery of a century ago. It has two disadvantages. First, it requires a two-step pilot hole, one size for the threaded portion and a slightly larger size hole for the shank. Second, the threads are fairly shallow, so the screw’s holding power is limited.
Sheet-metal screws have deeper and sharper threads, so many woodworkers use them, but a new type of wood screw is quickly becoming very popular. The case-hardened, extruded-thread wood screw has threads that are very deep and sharp (Illus. 10-9). They cut through wood easily and offer a lot of holding power. The shank of the screw is the same size as the root diameter of the threaded portion, so a single-diameter pilot hole can be used. These screws were originally developed for attaching drywall in building construction, so they are often called drywall screws, but new types developed specifically for cabinetmaking are now available. Originally, these screws were only available in a black finish, but now they are available in several types of plated finishes as well. You can drive these screws into softwood and plywood without drilling a pilot hole. For hardwoods, a pilot hole is still recommended.