The screws cut upon hardwood, ivory and metal, for the direct attachment of one portion of work to another, which also greatly vary in their diameters and other dimensions, in many cases from choice or necessity, are cut or chased with screw tools in the plain lathe by the unassisted hand. The traversing mandrel, fig. 112, renders the operation of screw cutting mechanical and certain of success; the same screw tools are used in the hand, or the tool is applied to the work in the slide rest. Metal screws for bolts and ordinary purposes, previously cut or marked out by the diestocks, frequently have the principal portion of the material removed in the lathe also with the hand screw tools. Accurate and long metal screws are cut in the slide lathe, or by the apparatus connecting the mandrel with the slide rest screw, generally known as the spiral apparatus. Soft wood screws are cut with the traversing mandrel, or with the screw box. A description of the last tool is given in the second volume, to which the reader is also referred for particulars of the different varieties of screws, the methods by which they have been originated, and the various tools required for their production; the present chapter deals with the manipulation of the tools and apparatus above referred to.
The cutting edges of the inside and outside angular threaded hand screw tools, figs. 404. 405, are composed of a series of equidistant points, exact counterparts of the form or angle of the thread they are intended to reproduce. The angle on the face of the points, giving the depth of the thread, usually varies from about 45° to 60°, and for some purposes to 90°; in accordance with the material to be cut, and also in a minor degree with the dimensions and purpose of the screw. Screw threads of the deepest angles are probably to be found among those cut in steel, followed consecutively by shallower in wrought iron, gun-metal and brass; cast iron, in which the threads are liable to crumble, and are more usually tapped, requires still shallower threads; and lastly, thin brass tubes for optical and other instruments have very shallow threads, to avoid cutting too deeply into the substance of the work. Very deep threads in wood, are apt to break away at their edges, while the very shallow, are unsuitable to a material to some extent elastic and not generally allowing very exact fitting. The range of depth in screw threads therefore, may be considered as the least in wood, rather more in ivory, but the greatest in metal.
The angular teeth of the screw tools are cut upon hobs, fig. 551. Vol. II., tools made as short portions of angular threaded steel screws, grooved longitudinally to form cutting edges and hardened. The steel blank or shaft of the tool after being thoroughly annealed, is pressed against the revolving hob, the screw thread upon which penetrates the edge and causes the blank to travel along it. Arrived at the end, the blank is withdrawn and replaced at the commencement, the traverses being repeated until the blank is cut to the interval and form of the thread, as seen in the teeth on its face, and to their vertical angle or rake, as seen upon its end. In cutting large screw tools of coarse threads, the hob is usually economized by removing the bulk of the material with a file after the tool has been marked out upon the hob. The marks made upon the edge are deepened with a crossing or with a triangular file, until the teeth are tolerably well developed, the tool is then returned to the hob for completion. The screw tool in either case thus exactly fits the thread of the screw or hob; it exactly reproduces this thread, but also cuts it upon screws of larger or smaller diameter than that of the hob.
The tool formed by travelling along the hob, in turn is traversed along the cylinder to cut that into a screw, and the cutting action of the hand screw tool may be thus described. Every complete revolution of the hob in cutting the teeth in the screw tool, carries the tool to the left, a distance equal to the interval between two of its threads; and in cutting the screw upon the work, the tool in like manner for every complete revolution of the work, requires to be traversed along the rest towards the left, an exactly similar distance. If the first point alone of the screw tool be supposed to commence the screw, at the end of one revolution of the work, it will have travelled sufficiently to the left, to allow the second point to arrive exactly at the spot where the first began to cut. At the second revolution the third point, and at the third the fourth point, will have arrived at the same spot and so on, the first, second, and third points, passing on up the cylinder in regular progression, each continuing in the one and the same screw path just travelled by its predecessors. On the other hand, should the traverse of the tool be either too rapid or too slow for the interval between its points, these cannot take up each other's action, but falling instead somewhere between, produce a break or bend in every coil of the screw, or else a regular or irregular double thread, as the degree of error in the traverse may be large or small. The exact or sufficient traverse is most necessary at the moment the tool first touches the work, called that of "striking" the thread, that the teeth may at once impart their exact vertical angle or rake; the first short helical cut thus once correctly made, becomes the guide and gives the path to the teeth of the screw tool, to continue the screw in length, and to "chase" the form of its thread.
Uniformity in pace is essential; but the rate of the traverse varies with the coarseness or fineness of the particular screw tool in use. The coarse screw tool from the greater interval of its teeth, has to traverse a greater distance during every one revolution of the work; and therefore according to their relative difference, travels more rapidly than the fine. The diameter of the work also exerts some influence upon the rate of any particular tool, in as much as the lathe may revolve more rapidly with work of small, than with that of large diameter; the traverse of the screw tool is slightly accelerated on account of the increased speed of the mandrel, but the distance the tool is shifted is still the same, relatively to every revolution of the work. The traverse necessary for different threads, and the very slight modifications of speed required upon different diameters cut with the same tool, are readily appreciated with moderate practice; which it must also be said, is the only mode of acquiring the habit of striking and cutting screws with the hand screw too Is.