For the convenience of arrangement, this section will be commenced with the description of the instrument which is commonly employed for making long screws in the soft woods, namely, the screw box, of which fig. 554 is the section, fig.555 the plan of the principal piece through the line a, and fig. 556 the cutter, shown the full size for a two-inch screw.
The screw box consists of two pieces of wood, accurately attached by two steady pins and two screws, so as to admit of separation and exact replacement; the ends of the thicker piece are frequently formed into handles, by which the instrument is worked. A perforation is made through the two pieces of wood; the hole in the thinner piece is cylindrical, and exactly agrees with the external diameter of the screw, or of the prepared cylinder; and the hole in the thicker piece is screwed with the same tap that is to be used for the internal screws or nuts, and which is shown in three views in fig. 557. The cutter or V, has a thin cutting edge sloped externally to the angle of the thread, usually about 60 degrees, and thinned internally by a notch made with a triangular file; the cutter is inlaid in the thicker piece of wood, and fastened by a hook-form screw bolt and nut.
In placing the cutter, four different conditions require strict attention. Its angular ridge should lie as a tangent to the inner circle; its edge should be sharpened on the dotted line b, or at an angle of about 100 degrees with the back; its point should exactly intersect the ridge of the thread in the box; and it should lie precisely at the rake or angle of the thread, for which purpose it is inlaid deeper at its blunt extremity.
The piece of wood for the screw is turned cylindrical and a little pointed; it is then twisted into the screw box, the cutter makes a notch, which catches upon the ridge of the wooden worm immediately behind the cutter, and this carries the work forward, exactly at the rate of the thread. The whole of the material is removed at one cut, and the shavings make their escape at the aperture or mouth m.
In cutting the smallest screws, with this well-contrived and effective inurnment, the screw box is held in the left hand, and the work is screwed in with the right] or the box is applied whilst the work remains upon the mandrel of the lathe. When the thread is required to be continued close up to a shoulder, the screw is cut up as far as the entire instrument will allow: the screw box is then removed, in order that the loose piece may be taken off from it, after which the screw is completed without impediment.
Screws of half an inch diameter and upwards, are generally fixed in the vice, whilst the screw box is handed round just like the diestock. For large screws exceeding two or three inches diameter, two of the V's or cutters are placed in the box, so as to divide the work; thereby lessening the risk of breaking the delicate edge of the cutter, the exact position of which is a matter of great nicety. The screw-box has been occasionally used for wooden screws of 4, 6, and 8 inches diameter, and upwards, and such large screws have been also made by hand, with the saw, chisel, mallet, and ordinary tools; but these large screws are now almost entirely superseded by those of metal, which, for most purposes, are greatly superior in every point of view.
In cutting the metal screw, or the bolt, the tools are required to be the converse of the tap, as they must have internal instead of external threads, but the radial notches are essential alike in each. For small works, the internal threads are made of fixed sizes and in thin plates of steel, such are called screw plates; for larger works, the internal threads are cut upon the edges of two or three detached pieces of steel, called dies, these are fitted into grooves within diesiocks, and various other contrivances which admit of the approach of the screwed dies, so that they may be applied to the decreasing diameter of the screw, from its commencement to the completion.
The thickness of the screw plate is in general from about two-thirds to the full diameter of the screw, and mostly several holes arc made in the same plate; from two to six holes are intended for one thread, and are accordingly distinguished into separate groups by little marks, as in fig. 558. The serrating of the edges, is sometimes done by making two or three small holes and connecting them by the lateral cuts of a thin saw, as in fig. 559. The notches alone are sometimes made, and when the holes are arranged as in fig. 560, should the screw be broken short off by accident, it may be cut in two with a thin saw, and thus removed from the plate.
In making small screws, the wire is fixed in the hand-vice, tapered off with a file, and generally filed to an obtuse point; then, after being moistened with oil, it is screwed into the one or several holes in the screw plate, which is held in the left hand. At other times, the work fixed in the lathe is turned or filed into form, and the plate is held in the right hand; but the force then applied is less easily appreciated. The harp-makers and some others, attach a screw plate with a single hole to the sliding cylinder of the popit-head. See page 564.
The screw plate is sometimes used for common screws as large as from half to three-quarters of an inch diameter; such screws are fixed in the tail vice, and the screw plate is made from about 15 to 30 inches long, and with two handles; the holes are then made of different diameters, by means of a taper tap, so as to form the thread by two, three, or more successive cuts, and the screw should be entered from the large side of the taper hole. It is, however, very advisable to use the diestocks, in preference to the screw plates, for all screws exceeding about one-sixteenth of an inch diameter, although the unvarying diameter of the screw plate has the advantage of regulating the equal size of a number of screws, and as such, is occasionally used to follow the diestocks, by way of a gage for size.