e time in setting up the machine for each job.

Thread-grinding machines were being designed concurrent with the
development of hob-grinding machines. Many were entirely concerned with
features peculiar to the problems of wheel-dressing and to automatic
characteristics. An invention to embody the use of a master screw and
concerned with the precision grinding of worm threads, for use in
gearing, was patented by Frederick A. Ward in this era.[8] That part of
the invention pertaining to the use of a master screw, "a rotary work
holder mounted on said carriage and provided with a driving spindle, an
exchangeable master screw and stationary nut detachably secured to said
spindle and head,..." is shown in figure 23.

[Illustration: Figure 23.--DETAILS OF A WORK SPINDLE WITH WORK, showing
the use of a master lead screw to control the pitch of a precision worm
thread being ground. From the 1933 U.S. patent 1899654, of F. A. Ward's
worm-grinding machine.]

Machines embodying the principle of the master lead screw are found in
constant use by industry at the present time for specialized
application. Whenever technological changes again reopen the topic of
thread-cutting to a new degree of accuracy or call for a reevaluation of
popular methods for any other reason, we may expect to see another
resurgence of the master-screw method, for no other design eliminates so
many variables or rests on such firm and fundamental natural principles
as the machine of _Das mittelalterliche Hausbuch_ of 1483, the earliest
such machine now known.

FOOTNOTES:

[1] JACQUES BESSON, _Des instruments mathematiques, et mechaniques,
servants a l'intelligence de plusiers choses difficiles, & necessaires a
toutes republiques_, 1st ed. (Orleans, 1569). [Also available in later
editions in French, German, and Spanish.]

[2] J. FOSTER PETREE, introduction, _Henry Maudslay, 1771-1831, and
Maudslay Sons and Field, Ltd._ (London: The Maudslay Society, 1949).

[3] _American Machinist_ (September 28, 1916), vol. 45, no. 13, pp.
529-531.

[4] U.S. patent 10383 issued to Joseph Nason of New York, January 3,
1854.

[5] U.S. patent 293930 issued to Charles Vander Woerd of Waltham,
Massachusetts, February 19, 1884.

[6] U.S. patent 1874592, filed June 8, 1929, issued to C. G. Olson of
Chicago, Illinois, August 30, 1932, and assigned to the Illinois Tool
Works, also of Chicago.

[7] U.S. patent 1901926, filed February 16, 1928, issued to C. G. Olson
of