Figure 4.—The working area of [figure 3], showing the tool and signature. (Smithsonian photo 46525A.)
Perhaps the most startling feature of this machine when compared with the machine of the Hausbuch, is the absence of a cross-slide for adjusting the tool. Possibly this can be explained by the blunt scraping edge on the tool. In actual use, recently, to cut a sample screw, using a tool similar to the one found in the machine ([fig. 7]), it was found advantageous to be free of a cross-slide and thus be able to feed the tool into the work by feel rather than by rule, as would be done with a slide rest. In this way, it was possible to thread steel without tearing, as the cutting pressure could readily be felt and the tool could release itself from too heavy a cut. Size on several screws could be repeated by setting the tool to produce the desired diameter when its supporting arm came to rest against the frame of the machine. The screws used in the machine itself were apparently made in just such a way. They were not cut with a die as the thread blends very gradually into the body of the screw without the characteristic marks left by the cutting edges of a die. Threads cut with a single-point tool controlled by a cross-slide usually end even more abruptly than those cut by a die, while it would be quite simple with a machine of the nature we are considering to bring the thread to a gentle tapering end as seen in [figure 8] (another view of the screw A in [fig. 3]) by gradually releasing the pressure necessary to keep the tool cutting as the end of the thread was approached.
Figure 5.—Spring for keeping the follower spindle against the work, showing the method and range of adjustment. Note the rectangular projection to engage a mating socket in the spindle, to prevent spindle rotation. (Smithsonian photo 46525.)
Figure 6.—Work spindle and its nut removed from the machine to illustrate how easily another spindle and nut of different pitch could be substituted. (Smithsonian photo 46525C.)
That machines of this general type having the lead screw on the axis of the work were competitive with other methods and other types of machines over a long period of time may be seen from figures [9] and [10]. The machine, left front in [figure 9] and in more intimate detail in [figure 10], can be seen to differ little from that shown in Das mittelalterliche Hausbuch of 1483. The double work-support is, of course, a great improvement, while the tool-support is regressive since it lacks a feed screw.
The development of engineering theory, coupled with the rising needs of industry, particularly with the advent of the Industrial Revolution, brought about accelerated development of screw-cutting lathes through the combination of screw-cutting machines with simple lathes as seen in [figure 9] and in detail in [figure 11]. One important advance shown here is driving the machine by means of a cord or band so that any means of rotary power could be applied, not just hand or foot power. Of greater interest and technical importance to this study is the provision, seen to better advantage in [figure 11], for readily changing from one master lead screw to another. This had already been achieved in the Manuel Wetschgi machine, as far as versatility is concerned, although not in quite such a convenient way.