ared for cutting 7 threads to the inch. (The change gears of many
modern lathes are so arranged that different combinations are obtained
by simply shifting a lever. A lathe having this quick-change gear
mechanism is described in the latter part of this chapter.) The work _B_
is placed between the centers just as it would be for turning, with the
end to be threaded turned to a diameter of 1-1/4 inch, which is the
outside diameter of the thread.

=The Thread Tool.=--The form of tool used for cutting a V-thread is
shown at _A_, Fig. 4. The end is ground V-shaped and to an angle of 60
degrees, which corresponds to the angle of a standard V-thread. The
front or flank, _f_ of the tool is ground back at an angle to provide
clearance, but the top is left flat or without slope. As it is very
important to grind the end to exactly 60 degrees, a gage _G_ is used,
having 60-degree notches to which the tool-point is fitted. The tool is
clamped in the toolpost as shown in the plan view, Fig. 2, square with
the work, so that both sides of the thread will be cut to the same angle
with the axis of the work. A very convenient way to set a thread tool
square is illustrated at _B_, Fig. 1. The thread gage is placed against
the part to be threaded, as shown, and the tool is adjusted until the
angular sides of the point bear evenly in the 60-degree notch of the
gage. The top of the tool point should be at the same height as the
lathe centers, as otherwise the angle of the thread will not be correct.

[Illustration: Fig. 4. Thread Tools and Gage for testing Angle of End]

=Cutting the Thread.=--The lathe is now ready for cutting the thread.
This is done by taking several cuts, as indicated at _A_, _B_, _C_ and
_D_ in Fig. 5, the tool being fed in a little farther for each
successive cut until the thread is finished. When these cuts are being
taken, the carriage is moved along the bed, as previously explained, by
the lead-screw _S_, Fig. 2. The carriage is engaged with the lead-screw
by turning lever _u_ which causes the halves of a split nut to close
around the screw. The way a lathe is handled when cutting a thread is as
follows: After the lathe is started, the carriage is moved until the
tool-point is slightly beyond the right end of the work, and the tool is
fed in far enough to take the first cut which, ordinarily, would be
about 1/16 inch deep. The carriage is then engaged with the lead-screw,
by operating lever _u_, and the tool moves t