rriage by hand when the number of threads
being cut is not a multiple of the number on the lead-screw is as
follows: The tool is moved a little beyond the right end of the work and
the carriage or split nut is engaged with the lead-screw. The lathe is
then turned forward by hand to take up any lost motion, and a line is
made on the lathe bed showing the position of the carriage. The
positions of the spindle and lead-screw are also marked by chalking a
tooth on both the spindle and lead-screw gears, which happens to be
opposite a corner or other point on the bed. After a cut is taken, the
carriage is returned by hand to the original starting point as shown by
the line on the bed, and is again engaged when the chalk marks show that
the spindle and lead-screw are in their original position; the tool will
then follow the first cut. If the body of the tailstock is moved against
the bridge of the carriage before starting the first cut, the carriage
can be located for each following cut by moving it back against the
tailstock, and it will not be necessary to have a line on the bed.

[Illustration: Fig. 6. Indicator used when Cutting Threads]

=Indicator or Chasing Dial for Catching Threads.=--On some lathes there
is an indicator for "catching threads," as this is called in shop
language. This is a simple device attached to the carriage and consists
of a graduated dial _D_ and a worm-wheel _W_ (see Figs. 2 and 6) which
meshes with the lead-screw, so that the dial is revolved by the
lead-screw when the carriage is stationary, and when the carriage is
moved by the screw, the dial remains stationary. The indicator is used
by engaging the carriage when one of the graduation lines is opposite
the arrow mark; after a cut is taken the carriage is returned by hand
and when one of the graduation lines again moves opposite the arrow, the
half-nuts are thrown into mesh, as before, and this is repeated for each
successive cut, thus causing the tool to always come right with the
thread. If the number of threads per inch is even, engagement can be
made when any line is opposite the arrow, but for odd numbers such as 3,
7, 9, 11, etc., one of the four long or numbered lines must be used. Of
course, if the thread being cut is a multiple of the number on the
lead-screw, engagement can be made at any time, as previously
mentioned.

=Principle of the Thread Indicator.=--The principle upon which the
thread indicator operates is as follows: The n