ur
explication_ (Paris: 1762-1772), vol. 9, plate 1.]

[Illustration: Figure 10.--DETAILS OF THE MACHINE in the left foreground
of figure 9, showing the crude tool-support without screw adjustment.
From _L'Encyclopedie_, vol. 9, plate 2.]

This reduction in the length that could conveniently be threaded was no
great drawback on many classes of work. This can be realized from figure
16 which shows a traverse-spindle lathe headstock typical of the
mid-19th century. During the years intervening between the machines of
figures 12 and 16, the general design was greatly improved by removing
the lead screws from the center of the spindle. This made possible a
shorter, much stiffer spindle and supported both ends of the spindle in
one frame or headstock rather than in separate pieces attached to the
bed. The screws were now mounted outside of the spindle-bearings, one at
a time, while the mating nuts were cut partially into the circumference
of a disk which could be turned to bring any particular nut into working
position as required. With this arrangement, a wide variety of leads
either right or left hand could be provided and additional leads could
be fitted at any future time. Screw-cutting lathes of this design were
popular for a very long time with instrument makers and opticians who
had little need to cut screws of great length.

[Illustration: Figure 11.--DETAILS OF THE THREADING LATHE seen in the
right foreground of figure 9 showing the method of drive and support for
the work. From _L'Encyclopedie_, vol. 9, plate 1.]

The demands of expanding industry for greater versatility in the
production of engineering elements late in the 18th century set the
stage for the evolution of more complex machines tending to place the
threaded spindle lathes in eclipse. Maudslay's lathe of 1797-1800 (fig.
15) appeared at this time when industry was receptive to rapid
innovation. Unfortunately, the gearing which once existed to connect the
headstock spindle with the lead screw has long been lost. At this time
it is quite difficult to say with certainty whether the original gear
set offered a variety of ratios, as was true of slightly later Maudslay
lathes, or a fixed ratio. The plausibility of the fixed ratio theory is
supported by the very convenient means, seen in figure 15, for removing
the lead screw in preparation for substitution of one of another pitch.
All that is required is to back off its supporting center at the
tailstock