a revolution
by placing the tail of the driving dog in the opposite slot of the
faceplate. This is a very simple method, but if the slots are not
directly opposite or 180 degrees apart, the last thread will not be
central with the first. Another and better method is to disengage the
idler gear from the gear on the stud, turn the spindle and work
one-half, or one-third, of a revolution, as the case might be, and then
connect the gears. For example, if the stud gear had 96 teeth, the tooth
meshing with the idler gear would be marked with chalk, the gears
disengaged, and the spindle turned until the chalked tooth had made the
required part of a revolution, which could be determined by counting the
teeth. When this method is used, the number of teeth in the stud gear
must be evenly divisible by two if a double thread is being cut, or by
three for a triple thread, etc. If the stud is not geared to the spindle
so that each makes the same number of revolutions, the ratio of the
gearing must be considered.

[Illustration: Fig. 12. Views illustrating how a Double Square Thread is
Cut]

=Setting Tool When Cutting Multiple Threads.=--Another method, which can
sometimes be used for setting the tool after cutting the first groove of
a multiple thread, is to disengage the lock-nuts from the lead-screw
(while the spindle is stationary) and move the carriage back whatever
distance is required to locate the tool in the proper position for
taking the second cut. Evidently this distance must not only locate the
tool in the right place, but be such that the lock-nuts can be
re-engaged with the lead-screw. Beginning with a simple illustration,
suppose a double thread is being cut having a lead of 1 inch. After the
first thread groove is cut, the tool can be set in a central position
for taking the second cut, by simply moving the carriage back 1/2 inch
(one-half the lead), or 1/2 inch plus the lead or any multiple of the
lead. If the length of the threaded part were 5 inches, the tool would
be moved back far enough to clear the end of the work, or say 1/2 + 5 =
5-1/2 inches. In order to disengage the lock-nuts and re-engage them
after moving the carriage 5-1/2 inches (or any distance equal, in this
case, to one-half plus a whole number), the lead-screw must have an even
number of threads per inch.

Assume that a double thread is being cut having 1-1/4 single threads per
inch. The lead then would equal 1 / 1-1/4 = 0.8 inch, and if the
c