There are a great many varieties of screws, but the principal one is the wood screw (Fig. 1ll), which is made by machine. Wood screws were originally made with blunt points. It was then necessary to make a hole in the wood before the screw could be driven. In the nineteenth century, the invention of the gimlet-pointed screw obviated the necessity for this preparatory process. When first manufacturing these screws by machinery, the metal was cut out between the threads. This method tended to weaken them, and they frequently broke when driven into wood. Later the method of manufacture which is in use today was introduced. The modern process consists of raising the thread by a system of rolling and compression. An operator feeds into a screw-making machine wire of various sizes, and the machine cuts off the wire at the desired length and turns the screw. The hammer part of the machine then strikes the exposed end of the wire, shaping the head of the screw. This method makes a screw that is strong and that possesses good holding power. Screws are usually made of steel, and are finished in many ways, so that we have on the market blued, brassed, and bronzed screws. Wood screws are specified by their length, and by a number which is the gauge number of wire from which they are made. They are sold by the gross. The screw is capable of resisting a much greater force than the nail, and is therefore a much better fastening agent. It is, however, more expensive than the nail, and cannot be driven into wood so rapidly.
Fig.107. Wire Nails.
Fig. 108. - Cut Nails.
Fig. 109 - A Nail Factory. The wire is fed into machines which cut and shape the wire into nails.
Fig. 110. - Kegs of Nails.
In addition to being used as fastening agents, screws are also used for communicating motion, as is the case of the lead screw of a lathe or the screw of a jack screw. These screws are produced by a cutting process in which the thread is formed from solid pieces of stock; that is, a single-pointed cutting tool, harder than the stock, cuts it, or it is cut by means of taps and dies. The tap and die are tools of hard steel used to produce internal and external threads respectively.
Fig. 111. - Wood Screws.