The patent screws manufactured by Mr. Nettlefold, of Holborn, are made with great attention to the perfection of the worm; the upper side, as intended to be represented in the following Fig. 1, is very flat, or inclined but very little from a plane, passing through its diameter, which causes a great resistance to its being forcibly pulled out of the wood; and the under side of the worm is considerably inclined, which greatly facilitates its entrance into the wood, rendering it unnecessary, in soft elastic woods, to bore any hole for its reception; the form is best seen in the sectional representation of a portion of the screw in Fig. 2, the black space being the screw, and the tinted parts on either side, a portion of the wood; also exhibiting the facility of entrance, and the difficulty of drawing out. The greater part of the common wood screws sold in the shops, are, however, very wretchedly constructed, in this as well as other respects. Fig. 3 shows a section of their shallow, rough, and imperfect worms, the heads, stems and nicks in which are fit accompaniments to articles so fabricated.
The chief defects of common wood screws, besides bad threads, are the having, at the termination of the worm, a projecting burr, which is apt to tear away the wood before it, and leave little or no solid matter for the screw to hold by; the nicks in the heads being too shallow, or highest in the middle, preventing the screw-driver from taking an efficient hold to turn them in and out. These defects are carefully obviated in Mr. Nettlefold's screws, and they are made with extraordinary truth and beauty, notwithstanding the very low prices at which they are manufactured.
The machinery by which wood screws are made vary considerably in the different manufactories, and numerous patents have been successively taken out for improved processes; in several of which, attempts have been made, with partial success, to fabricate them without the intervention of human hands, farther than furnishing the raw material to the machines. Several persons have succeeded in casting screws, - an operation of a very curious, and, apparently, difficult nature. A Staffordshire manufacturer of the name of Maullin, had a patent for an ingenious process of this kind, which is described in the 13th volume of the Repertory of Arts.
Immense quantities of screws, of the smaller kind, are made from wire, in the neighbourhood of Birmingham and Wolverhampton. They are chiefly made in the houses or cottages of the workpeople, wherein the children materially assist; the screwing being effected by turning a winch handle fixed to the end of a cylindrical mandril, the other end of which is furnished with a chuck to contain the screw blank, which is thrust forward and turned round between steel dies fixed in a puppet head, the action being regulated by a screw thread on the mandril, taking into a hollow screw fixed in an intermediate puppet head.
In the 41st volume of the Transactions of the Society of Arts, is described a very convenient and simple tap (invented by Mr. Siebe, of the Strand) for cutting hollow screws in wood, by which workmen are enabled with the same tool to form either right or left, single, double, or treble hollow screws of the same diameter. The screws capable of being made by this implement, are, however, far from being mathematically accurate, but will be found to be quite as good as the hollow screws made in the usual way, and adapted to the use of various wooden articles of domestic furniture, and to some common kinds of machinery.
The tap is a thin quadrilateral piece of steel, of the length and breadth of the required screw, having its longitudinal edges cut into saw teeth; the teeth in one row being opposite to the intervals in the other, and therefore representing a section of a screw, the teeth being sections of the threads. A cylinder of hard wood is turned, so as to correspond with the dimensions of the intended screw, and a longitudinal piece being sawn out from the middle, representing a section through the axis, the serrated plate is to be inserted and firmly rivetted in its place. The cylinder terminates in a flat head, for the purpose of receiving a key or lever, which enables the workman to overcome the friction experienced in cutting the screw.
In order to use this tool, a cylindrical hole, equal in diameter to the cylindrical stem, is to be bored in a piece of wood, and the serrated cylinder being then introduced, giving it a circular or spiral motion, will form a right or left-handed screw, according to the direction in which it is turned; and by entering two or three threads at once with a common V tool, the same tap will give a double or treble-threaded screw.
The hard wood being first made into a screw, the blade isrivetted in, and the teeth are made by a file to fit the wooden threads; the blade is then removed, the threads in the wooden cylinder are turned off, leaving it smooth; the blade is then tapered at the point, so that the first teeth are no bigger than the cylinder; it is then rivetted again in its place, and the instrument is complete, as represented in the subjoined Fig. 1. Fig. 2 is an end view of the same, which is exhibited to show that the cylinder is cut away a little where the teeth are inserted, to make room for the shavings. As the cylinder fits the hole, and the blade is taper, a gradual and steady cut is secured, which may either be to the right or left hand.
Mr. Siebe also proposes steel taps for metal screws to be made in the same manner, by filing away a solid screw of metal, so as to present two cutting edges, similar to those attached to the wooden cylinder.