s, because of the
internal and external stresses originating in minute deviations from the
ideal of tooth form and spacing. The problems were not entirely new but
had hitherto been solved by increasing the size of the parts, an avenue
of limited utility to designers in these fields where total weight as
well as the effects of mass and inertia are so important. By making
these parts of heat-treated steel, the strength could be made suitable
while the size and mass of the parts were kept within bounds. The
necessary processes of heat-treating were not always applicable to
finished parts as they sometimes destroyed both finish and accuracy.
Grinding, which was well developed for the simple plane, cylindrical,
and conical surfaces so widely used in mechanisms, had to be extended to
threads and gears so that they could be finished after heat-treating.
Sometimes the gear teeth themselves were ground; for other applications
it was sufficient to improve the accuracy of the gear cutters.

[Illustration: Figure 20.--A HOB-GRINDING MACHINE patented in 1932 and
incorporating the master-screw principle. Carl G. Olson's U.S. patent
1874592.]

Attempts to produce gear hobs free of the imperfections and distortions
introduced by heat treatment led to another return to the use of the
master lead screw. Figure 20 illustrates a machine having this feature
which was patented in 1932 by Carl G. Olson.[6] In speaking of the
spindle-driving mechanism disclosed in earlier patents, the patent goes
on to say:

This driving mechanism includes an integral spindle 20, one
extremity thereof being designed for supporting a hob 22 and the
other extremity thereof being formed so as to present a lead screw
24. The spindle 20 is mounted between a bearing 26 and a bearing
28, the latter bearing providing a nut in which the lead screw 24
rotates.... From the description thus far given it will be apparent
that the rotation of the lead screw 24 within the bearing or nut 28
will cause the hob to be moved axially, the lead of the screw 24
being equal to the lead of the thread in the hob.

Claim 8 which concludes the descriptive portion of the patent states in
part:

In a hob grinding machine of the class described, a rotary work
supporting spindle, means for effecting longitudinal movement of
the spindle, a tool holder for supporting a grinding wheel in
operative position with respect to th