The diestock, in common with other general tools, has received a great many modifications that it would be useless to trace in greater detail, than so far as respects the varieties in common use, or those which introduce any peculiarity of action in the cutting edges. A notion of the early contrivances for cutting metal screws will be gathered from the figures 561 to 564, which are copied half-size from Leupold's Theatrum Machinarum
Generale, 1724.*: For instance, fig. 561 is the screw plate divided in two, and jointed together like a common rule; the inner edges are cut with threads, the larger of which is judiciously placed near the joint, that it may he more forcibly compressed: there is a guide, a, a, to prevent the lateral displacement of the edges, which would be fatal to the action. Similar instruments are still used, but more generally for screws made in the turning lathe.
In one of these tools, the frame or stock is made exactly like a pair of flat pliers, but with loose dies cut for either one or two sizes of threads. Plier diestocks arc also made in the form of common nut-crackers, or in fact, much like fig. 561, if we consider it to have handles proceeding from a a, to extend the tool to about two or three times its length; the guide a a is retained, and removable dies are added, instead of the threads being cut in the sides of the instrument. Screwing tools arc also made of one piece of steel, and to spring open, something like fig. 1 page 232, Vol. I., but shorter and stronger: the threads are cut on the sides or ends of the bosses, which are flat externally, for the convenience of compression in the tail vice.
In general, however, the two dies are closed together in a straight line, instead of the arc of a circle: one primitive method, fig. 564, extracted from the work referred to, has been thus remodelled; the dies arc inserted in rectangular taper holes in the ends of two long levers, which latter are connected by two cylindrical pins, carefully fitted into holes made through the levers, and the ends of the pins are screwed and provided with nuts, which serve more effectually to compress the dies than the square rings represented in fig. 564.
• Moxon, Plumier, and others, describe similar tools, and also the screw box.
The diestock in its most general form has a central rectangular aperture, within which the dies are fitted, so as to admit of compression by one central screw; the kinds most in use being distinguished as the double chamfered diestocks, figs. 565 and 566;
and the single chamfered diestock, figs. 568 and 569, the handles of which are partly shown by dotted lines. In the former, the aperture is about as long as three of the dies; about one-third of the length of the chamfer is filed away at the one end, for the removal of the dies laterally, and one at a time. In the single chamfered diestock 569, which is preferable for large threads, the aperture but little exceeds the length of two dies, and these are removed by first taking off the side plate b a, which is either attached by its chamfered edges as a slide, or else by four screws; these, when loosened, allow the plate to be slid endways, and it will be then disengaged, as the screws will leave the grooves at a, and the screw heads will pass through the holes at b.
Sometimes dies of the section of fig. 567 are applied after the manner of 566, and occasionally the rectangular aperture of fig. 569 is made parallel on its inner edges, and without the side plate b a; the dies are then retained by steel by steel plates either riveted or screwed to the diestock, as represented in fig. 570, or else by two steel pins buried half-way in the sides of the stock, and the remaining half in the die, as shown in fig. 571. These variations are of little moment, as are also those concerning the general form of the stock; for instance, whether or not the handles proceed in the directions shown (the one handle s, being occasionally a continuation of the pressure screw), or whether the handles are placed as in the dotted position t. In small die-stocks, a short stud or handle is occasionally attached at right angles to the extremity, that the diestock may be moved like a winch handle: and sometimes graduations are made upon the pressure screw, to denote the extent to which the dies are closed. These and other differences are matters comparatively unimportant, as the accurate fitting of the dies, and their exact forms, should receive the principal attention.
In general only two dies are used, the inner surface of each of which includes from the third to nearly the half of a circle, and a notch is made at the central part of each die, so that the pair of dies present four arcs, and eight series of cutting points or edges: four of which operate when the dies are moved in the one direction, and the other four when the motion is reversed; that is when the curves of the die and screw are alike.
The formation of these parts has given rise to much investigation and experiment, as the two principal points aimed at require directly opposite circumstances. For instance, the narrower the edges of the dies, or the less of the circle they contain, the more easily they penetrate, the more quickly they cut, and the less they compress the screw by surface friction or squeezing, which last tends to elongate the screw beyond its assigned length. But on the other hand, the broader the edges of the dies, or the more of the circle they contain, the more exactly do they retain the true helical form, and the general truth of the screw.
The action of screw cutting dies is rendered still more difficult, because in general, one pair of dies, the curvatures and angles of which admit of no change, are employed in the production of a screw, the dimensions of which, during its gradual transit from the smooth cylinder to the finished screw, continually change. or instance, the thread of a screw necessarily possesses two magnitudes, namely, the top and bottom of the groove, and also two angles at these respective diameters, as represented by the dotted lines in the diagrams, figs. 572, 574, and 576, (which are drawn with straight instead of curved lines). The angles are nearly in the inverse proportion of the diameters; or if the bottom were half the diameter of the top of the thread, the angle at the bottom would be nearly twice that at the top. (The mode of calculating the angles, is subjoined to figs. 614 - 618, page 657.)