SCREW MANUFACTURE. The blanks for ordinary screws are sometimes made of round rolled iron, cut into the required lengths, and pinched when red-hot between a pair of dies in the chaps of a vice ; while the heads are formed with a hammer, or the stamp of a fly.press. Another plan is to form the blanks of iron wire, cut by a machine, and have the heads struck up in a die without the application of heat.
After forming the head, the next process is filing or taming the necks and heads in a lathe ; after which the nick, or groove to receive the end of the screw-driver, is cut with a circular saw. The cutting of the worm is sometimes performed in a lathe, the blank being fixed in a chuck, and projected during its revolution between n pair of stationary cutters; the longitudinal motion of the blank, and the inclination of the thread, being determined by a regulating or pattern screw attached to the mandril. Small screws are frequently wormed by a similar apparatus turned by a winch-handle attached to the mandril; and sometimes by means of a steel tap-plate. In another plan the worm is formed by means of a pair of stationary cutting dies ; between which the blank is projected by an apparatus which gives it an alternating rotatory motion. The dies themselves regulate the size of the thread, without the use of a pattern screw, and they must therefore be changed for every variety of screw. The best screws are made to taper slightly from the head downwards.
Several attempts have been made to produce screws by casting. In the ordinary method, the chief obstacle in the way of casting screws consists in the difficulty of removing the pattern from the mould. Mr. Maullin has devised a method of overcoming this difficulty by an apparatus for screwing the patterns (of which a great number might be used together) out of the mould, so as to leave the impression of the thread uninjured.
Screw-bolts and other screws for working in metal are manufactured in a similar manner to those for working in wood, when the number required is sufficient to justify the expense of adjusting the machinery.
When this is not the case, they are, if small, often cut by hand, without the aid of a lathe. The die, or instrument for cutting an external screw, resembles a common nut, hut is usually divided into two parts, which are fitted into an iron stock or die-frame, with long handles. Notches are cut in the die, across the direction of the threads, in order to pro duce cutting angles, and to afford room for the escape of the portions of metal removed in cutting the worm. The die, which is formed of steel, and well tempered, is inserted in the die-stock, with its two halves a little distance apart, but capable of being brought together by regulating screws fixed in the die-stock. The bolt to be made into a screw is fastened in a vice, while its end is placed in the die. The operator then proceeds to turn the die-stock, so as to worm the die on to the bolt; not by a continuous motion in one direction, but by a series of turns backwards and forwards. When the die has proceeded as far as the worm is required to extend, it is taken off, screwed up a little closer, and again applied in the same manner ; the process is repeated, closing the die a little after each operation, until the worm is cut to the required depth. In working a similar apparatus by machinery, the dies are sometimes made in four pieces, the die-frame is stationary, and the bolt or screw-pin itself revolves. In this case the rotation is continuous, but in other respects the operation resembles that described. In cutting large screws, especially with a square thread, a steel cutter is sometimes used with the die, whether turned by hand or fixed in a lathe. Very small metal screws are cut by a steel tap-plate, wormed and notched in a similar manlier to the dies above described, but having several hold varying slightly in size ; the worm being formed progressively, by using at each operation a smaller hole than at the preceding one.