Screws

The use of screws, rather than nails, as fasteners may be dictated by a number of factors. These may include the type of material to be fastened, the requirement for greater holding power than can be obtained with nails, the finished appearance desired, and the fact that the number of fasteners that can be used is limited. Using screws, rather than nails, is more expensive in terms of time and money, but it is often necessary to meet requirements for superior results.

The main advantages of screws are that they provide more holding power, can be easily tightened to draw the items being fastened securely together, are neater in appearance if properly driven, and can be withdrawn without damaging the material. All screws can have slotted or Phillips heads, as shown in Figure 49.

Metal Screws

For the assembly of metal arts, use sheet metal screws. These screws are made regularly in steel and brass with four types of heads: flat, round, oval, and fillister, shown in that order in Figure 49.

Lag Screws

The proper name for a lag screw shown in Figure 49 is lag bolt. 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. Table 11 gives lengths and diameters of lag screws. Combined with expansion anchors, they are used to frame timbers to existing masonry.

Wood Screws

The common wood screw is usually made of unhardened steel, stainless steel, aluminum, or brass. The steel may be bright finished or blued; or zinc, cadmium, or chrome plated. Wood screws are threaded from a gimlet point for approximately two-thirds of the length of the screw and provided with a slotted head designed to be driven by an inserted driver. Wood screws, as shown in Figure 50, are designated according to head style. The most common types are flathead, oval head, and roundhead, shown in that order. Proper nomenclature of a screw, shown in Figure 50, includes the type, material, finish, length, screw size number indicating the wire guage of the body, drill or bit size for the body hole, and drill or bit size for the starter hole.

To prepare wood for receiving the screws, bore a body hole the diameter of the screw to be used in the piece of wood to be fastened as shown in Figure 51.

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 twothirds 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 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 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.

Wood screws come in sizes that vary from 1/4 inch to 6 inches. Screws up to 1 inch in length increase by eighths, screws from 1 to 3 inches increase by quarters, and screws from 3 to 6 inches increase by half inches. Screws vary in length and size of shaft. Each length is made in a number of shaft sizes specified by an arbitrary number that represents no particular measurement but indicates relative differences in the diameter of the screws. Tables 12 and 13 provide size, length, gauge, and applicable drill and auger bit sizes for screws.