In cutting the internal thread the tool is advanced for the depth of cut, towards, instead of away from the operator, otherwise the manipulation of the apparatus is so similar to that for the external thread, as to require no description. Left handed screws, in which the thread winds around from left to right, either external or internal, are traced and cut under precisely the same conditions, and in the same manner as the right handed, except, that the tool travels along the cylinder in the reverse direction.
Metal screws with two, three or more threads winding around the same axis, have every individual strand cut after the same general method as a single thread; but the production of accurate multiplex threaded screws, is attended with increased practical difficulties, arising from their pitch, from the necessary attainment of uniformity in the depth and dimensions of their threads, and from some other causes; all of which require increased care in minute details of manipulation.
The threads of multiplex screws are more usually square, deep, and of a pitch that is considerable in proportion to their diameter. Hence, the tool theoretically, and frequently practically, requires an increased vertical inclination as it cuts from the larger to the lesser diameter, that is, from the top, the surface of the cylinder, to the bottom of the thread, the depth to which the tool is allowed to penetrate. This is sufficiently obvious in the diagram fig. 526, which shows that 2¼°, being the vertical inclination of the tool for a screw of the pitch of one inch diameter, an inclination of 4½°, or double, is required for a similar pitch upon half an inch diameter. Increased obliquity of pitch increasing the necessity. In some cases this difficulty may be surmounted by placing the tool at an angle suitable to a depth midway between the top and the bottom of the thread; but, when both pitch and depth are considerable, it becomes necessary to increase the inclination of the tool from time to time, during the progress of cutting the thread.
All the threads of a multiplex screw are commenced and their depth gradually increased, seriatim; as it is not advisable even with shallow multiplex screws, to proceed far with one thread, before the others have been considerably and equally advanced; the partial removal of material being liable to cause deflection. To avoid strain for the same reason it is frequently preferred to remove the bulk of the material from square threads, with a narrow tool, before proceeding to cut them to their full width. While for screws that are long compared with their diameter, and therefore more liable to bend from being comparatively weak, the cut may be gradually widened with a narrow tool, applied first to the one and then to the other side of the thread. These methods, employed primarily to diminish the strain of the cut to avoid deflection, also have the advantage of generating less heat from friction; the practical benefits of which have been already indicated. Notwithstanding every care however, the single or the multiplex threaded screw not infrequently becomes distorted or bent in the direction of its length; in which case it has to be removed from the lathe, and set straight by moderate blows, which are given with a lead or wooden mallet to avoid injury to the threads, the screw being laid upon a block of lead having a slightly concave surface. During this process the screw is replaced between the centers from time to time to observe the result; and the screw may even require setting more than once, prior to the termination of cutting the thread or threads.
Uniformity in the depth of the several threads of multiplex screws, may be assisted by turning down a portion of the blank at both ends, beyond the length of the thread, fig. 529, to serve as a gage. But in order to retain as much strength in the screw as possible, it is usual in the first instance to reduce the two ends to only about half the depth of the intended thread; and when that depth is reached by the tool, then to reduce them a second time to very nearly the finished diameter, but leaving still a very trifling reduction in this gage portion to be effected just before the threads are finally completed. A very close approximation to uniformity of depth may then be obtained by setting the tool to slightly touch the ends thus turned to the required diameter; but the threads should also be finally tested, by passing the tool, retained fixed at the same depth, along them all seriatim, that any differences, which may also have arisen from the wear of the tool, may be ascertained and corrected. Turning down both ends of the screw to serve as gage for size, is also very generally followed for screws having single square threads.
The nuts or internal screws for multiplex threads are usually cut first, and then the screws are fitted to them. Whenever possible, the nut should be finished with a thoroughfare tap ; but it is very generally necessary, that the threads in the nut should be first partially cut in the lathe, in order that the tap may meet with no difficulty in obtaining the lead. The partial cutting being essential, whenever the pitch is considerable.