Screw, a device constituting one of the mechanical powers. It is in two forms: one, known as the external, convex, or male screw, is a cylinder of wood or metal surrounded with either a spiral groove or ridge, which makes equal angles with lines parallel to the axis of the cylinder; the other, called the interior, concave, or female screw, is a hollow cylinder with grooves around its interior fitted to the ridges of the corresponding solid screw. When very short and used as a fastening upon the external screw, it is called a nut. The spiral ridges are called the thread of the screw, and these are made more or less close together according to the purposes for which the screw is designed. The action of the screw is indefinitely extended and its power increased by adding to it a wheel and axle, so arranged that the teeth of the wheel engage in the threads of the screw and are brought round continually while the screw is made to turn in a fixed position against the wheel. In this arrangement it is known as the endless screw. (See Mechanics.) The small screws in general use, answering instead of nails, are commonly known as wood screws, and are made of all sizes from 3 or 4 in. in length to 1/4 in. or less.
Screws of the same character for the special uses of the watchmaker and instrument maker are of still smaller sizes. The wood screw tapers slightly from the head downward, and the thread usually occupies about two thirds of the length from the point. The under side of the head is of a true taper, and when the screw is set in its place accurately fits the hole that has been rimmed out for it to the same taper. The upper side is flat, and is crossed by a narrow slit for the edge of the screw driver, by which it is turned round. The most approved form has the gimlet point, which allows of the screw entering the wood without first boring a hole for its reception. The thread is a thin fillet left by removing the intervening metal. - Several methods have been devised for making screws. By one, now rarely used, the cylindrical lengths cut from rolled iron or iron wire were at a red heat headed in dies, and the thread was then cut by a file, its place being previously marked around the cylinder by one of several devices employed for this purpose. By the machine process in use in Birmingham, England, the first operation consists in clipping off the pieces from a coil of wire and striking up each piece at one end to form the head.
The blanks thus formed are in the next operation placed one at a time in a lathe, and proper shape is given to the head and neck by cutting away the superfluous metal. Each blank is then placed by hand in a receptacle which holds it firmly, and is raised by a lever so as to present the head to a steel circular saw, which cuts in an instant the slit for the screw driver. The cutting of the thread, called worming, is done in a lathe, the mandrel of which at one end carries an iron box which works upon a fixed regulating screw. This gives the required longitudinal movement to the blank which is secured to the other end of the mandrel, and is pushed by the revolution point first through the steel cutters. These are made by levers to press more or less firmly upon the blanks, and their action is to turn out a shaving of the metal, leaving a sharp thread or worm. By other methods the cutters are dies having the same thread as the required screw, and corresponding for external screws to the nut or tap, except that they are either in two or four parts, which may gradually be brought together to close the circle while the shank of the blank is worked down in them to the size required.
Each variety of screw in this method of cutting requires its own die, and various sizes are cut by hand with dies. Internal screws are cut by steel tools called taps, having the thread of the corresponding external screw, but partly cut away along the whole length in order to produce cutting edges and afford room for the escape of the shreds of metal removed.