real numbers of screws arc required in works of wood, ivory and metal, that cannot be cut with the taps and dies, or the other apparatus hitherto considered. This arises from the nature of the materials, the weakness of the forms of the objects, and the accidental proportions of the screws, many of which are comparatively of very large diameter and inconsiderable length. These and other circumstances, conspire to prevent the use of the diestocks for objects such as the screws of telescopes and other slender tubes, those on the edges of disks, rings, boxes, and very many similar works.
Screws of this latter class are frequently cut in the lathe with the ordinary screw tool, and by dexterity of hand alone; there* is little to be said in explanation of the apparatus and tools, which then consist solely of the lathe with an ordinary mandrel incapable of traversing endways, and the screw tools or the chasing tools figs. 401 and 405, page 519, with the addition of the arm rest; the details of the manipulation will be found in the practical section.
The screw tool held at rest would make a series of rings, because at the end of the first revolution of the object, the points A B Cof the tool would fall exactly into the scratches A B C commenced respectively by them. But if in its first revolution, the tool is shifted exactly the space between two of its teeth, at the end of the revolution, the point B of the tool, drops into the groove made by the point A, and so with all the others, and a true screw is formed, or a continuous helical line, which appears in steady lateral motion during the revolution of the screw in the lathe.
It is likely the tool will fail exactly to drop into the groove, but if the difference be inconsiderable, a tolerably good screw is nevertheless formed; as the tool being moved forward as equally he hand will allow, corrects most of the error. But if the difference be great, the tool finds its way into the groove with an abrupt break in the curve; and during the revolution of the screw, as it progresses it also appears to roll about sideways, instead of being quiescent, and is said by workmen to be "drunk," this error is frequently beyond correction.
It sometimes happens that the tool is moved too rapidly, and that the point C drops into the groove commenced by A; in this case the coarseness of the groove is the same as that of the tool, but the inclination is double that intended, and the screw has a double thread, or two distinct helices instead of one; the tool may pass over three or four intervals and make a treble or quadruple thread, but these are the results of design and skill, rather than of accident.
On the other hand, from being moved too slowly, the point B of the tool may fail to proceed so far as the groove made by A, but fall midway between A and B; in this case the screw has half the rise or inclination intended, and the grooves are as fine again as the tool; other accidental results may also occur which it is unnecessary to notice.
The assemblage of points in the screw tools proper for the hard woods, ivory and metals, renders the striking of screws in these materials comparatively certain and excellent, that is as regards those individuals who devote sufficient pains to the acquisition of the manipulation; but the softwoods, require tools with very keen edges of 20 to 30 degrees, and for these materials the screw tool is made with only a single point, as represented in figs. 377 and 378, page 516. With such a tool, no skill will suffice to cut a good useful screw by hand alone, as the guiding and correctional power of the many points no longer exists; and in consequence those screws in soft wood which are cut in the lathe, require the guidance to be given mechanically in the manner explained in the following section.*