hreads are cut in the lathe a tool _t_ is used (see Fig. 2),
having a point corresponding to the shape of the thread, and the
carriage is moved along the bed a certain distance for each revolution
of the work (the distance depending on the number of threads to the inch
being cut) by the lead-screw _S_ which is rotated by gears _a_, _b_ and
_c_, which receive their motion from the spindle. As the amount that the
carriage travels per revolution of the work, and, consequently, the
number of threads per inch that is cut, depends on the size of the gears
_a_ and _c_ (called change gears) the latter have to be changed for
cutting different threads. The proper change gears to use for cutting a
given number of threads to the inch is ordinarily determined by
referring to a table or "index plate" _I_ which shows what the size of
gears _a_ and _c_ should be, or the number of teeth each should have,
for cutting any given number of threads per inch.

[Illustration: Fig. 1. Measuring Number of Threads per Inch--Setting
Thread Tool]

[Illustration: Fig. 2. Plan and Elevations of Engine Lathe]

=Selecting the Change Gears for Thread Cutting.=--Suppose a V-thread is
to be cut on the end of the bolt _B_, Fig. 2, having a diameter of 1-1/4
inch and seven threads per inch of length, as shown at _A_ in Fig. 1,
which is the standard number of threads per inch for that diameter.
First the change gears to use are found on plate _I_ which is shown
enlarged in Fig. 3. This plate has three columns: The first contains
different numbers of threads to the inch, the second the size gear to
place on the "spindle" or "stud" at _a_ (Fig. 2) for different threads,
and the third the size of gear _c_ for the lead-screw. As the thread
selected as an example has 7 threads per inch, gear _a_ should have 48
teeth, this being the number given in the second column opposite figure
7 in the first. By referring to the last column, we find that the
lead-screw gear should have 84 teeth. These gears are selected from an
assortment provided with the lathe and they are placed on the spindle
and lead-screw, respectively.

[Illustration: Fig. 3. Index Plate showing Gear Changes for Threading]

Intermediate gear _b_ does not need to be changed as it is simply an
"idler" for connecting gears _a_ and _c_. Gear _b_ is mounted on a
swinging yoke _Y_ so that it can be adjusted to mesh properly with
different gear combinations; after this adjustment is made, the lathe is
ge