Frequently trouble is experienced through the screw we ing badly, and it is by no means an unusual thing to find the screw quite slack after a few weeks' working, even in new machines. Doubtless in many instances this wear is due to rough workmanship, want of similarity between thread and nut, for after all the cutting accurately of screws with double or triple square threads is not the easiest task that meets the machinist. If there is any want of equality between either of the threads of a multiple-threaded screw, then it is obvious that upon one or more of the threads an undue amount of work must fall, with a proportional effect upon the life of the thread. For this reason it is usual to mark the thread and the space in which it works, so that if for any reason the screw has to l>e taken out of the nut it can be easily replaced in the same position with regard to the nut as it occupied originally.

Fig. 31.

In the best practice, of course, the threads are accurately cut, both in the spindle and in the nut, so that any thread will work in any space with equal facility.

The difficulties met with in obtaining a true thread may be understood by considering the following two special cases illustrated in tigs. 31 and 32. In the first of these is shown a series of parallel grooves cut in a cylindrical bar of steel by a square-nosed tool held stationary in the rest at right angles to the axis of the bar, and only moved inwards as the work progresses. The result is a series of grooves, the spaces S, S1, S2, S3 are of equal widths, and have sides exactly perpendicular to the axis of the bar. These grooves actually represent a square-threaded screw whose pitch is zero.

Fig. 32.

Fig. 33.

Now, turning to fig. 32, another cylindrical bar is seen, upon which exactly the same tool has been used, but instead of its being held stationary, it is moved along parallel to the axis of the bar, and the bar is stationary, the result being a longitudinal groove a of the same size and baring the same sectional shape as the grooves in fig. 31. If the bailie turned through exactly 90 deg. after each groove is completed, then there will be four grooves a, a1, a2, a3 equidistant from each other and of equal size and similarly shaped, and this may be looked upon as a square-threaded -screw having four threads and of infinite pitch.

Next, compare the shape of the projections T, T1, and T2 in fig. 31 with those in fig. 32 which are marked T, T1, T2.

Fig. 34.

and T3. In fig. 31 the sides of each projection are parallel, but in fig. 32 they are not parallel, but a considerable angle is enclosed, and the reason for this difference is easily appreciated after considering the two figures. Further, it may be noticed that in order that the shape of both the projection or external screw thread and the space or the internal screw thread may be as nearly alike as possible, it is necessary to make the pitch as small as possible, for they can only be exactly the same when the pitch is zero.

Assuming that a thread of sensible pitch is cut with a square tool T4 (see fig. 33), the projection must have sides converging towards the axis of the bar (see Sl, fig. 33), and if this operates in a space with parallel sides N1, then the pressure must inevitably fall upon the extreme edge of the thread, and rapid wear will occur, resulting in the appearance of slack very soon after the machine is put into regular operation. Sometimes a tool is used shaped as shown at T2, fig. 34, which produces threads of the shapes shown there, and thus overcomes the difficulty to a certain extent. Examples of similar threads to these may be found sometimes in the leading screws of screw-cutting lathes.

Fig. 35.

Fig. 36.

For these reasons it is easily seen that unless care be used in cutting the threads of the screw, that trouble from bad working and undue wear is sure to ensue. In figs. 35 and 36 are shown examples of multiple-thread screws, that in fig. 35 having four threads, whilst that in fig. 36 has three threads.

Fig. 37.