SCREW MANUFACTURE. In the in fancy of screw making the thread was formed with a file ; but this process has long been superseded by the use of dies and cutters, which are applied in various ways. The cutting of the worm is sometimes performed in a lathe, the blank being fixed in a chuck, and projecting during its revolution between a pair of stationary cutters ; the longitudinal motion of the blank, and consequently the size or inclination of the thread, being deter mined by a regulating or pattern screw attached to the mandril, which must therefore be changed for every different degree of fine ness ; while the shape of the thread or worm depends on the form and position of the cutters. Small screws are frequently wormed by a similar apparatus turned by a winch handle attached to the mandril; and some times by means of a steel tap-plate. Many patented improvements have been introduced in this manufacture, chiefly in relation to the cutting of the worm. Several attempts have been made to produce screws by casting ; but not with much success.
Screw-Bolts and other screws for working in metal are manufactured in a similar man ner to common screws when the number re quired 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. In the screwing
of metal work, there are required two portions, the external and the internal screw: these require contrary processes in their =mire( tore.
In cutting screws in wood, an interns wooden screw is cut by an external screw iron or steel, and an external wooden scree by an internal screw of iron or steel : th cutter or tap being shaped according to th kind of screw to be produced.
The large iron screws used in vices, presses means of dies, turned with immense power bj 'very long levers ; the thread being mad( without cutting, by indenting and squeezinf up the metal. The very best scrap-iron is required for the screws formed in this manner on account of the twisting force to which are subjected.
In the ordinary method of cutting screws in a lathe, the size of the worm, or the dis; tanee between the threads, is regulated by a pattern screw, and cannot be varied from it; but an ingenious machine is used in the Woolwich dockyard for cutting a great variety of different screws from one pattern.
Birmingham is the great seat of the screw manufacture ; millions of dozens are made in that town yearly.