at the end of the cone. The gears are so
proportioned that the ratio of the two engagements is as 10 to 1; that
is, when engaged with the cone gear (the back-gears being thrown in) the
mating gear will make ten revolutions to one of the spindle, so that
when the lathe is ordinarily geared to cut one thread per inch, it will,
when driven by the cone pinion, cut one thread in ten inches. This
construction dispenses with the extra strain on the reverse gears due to
moving the carriage at the rapid rate that would be necessary for such a
large lead, when not using an attachment. These attachments are not only
extensively used for the cutting of coarse screws but for cutting oil
grooves on cylindrical parts.

When cutting a thread of large lead or "steep pitch," the top of the
thread tool should be ground so that it is at right angles to the
thread; then the thread groove will be cut to the same width as the
tool.

=Testing the Size of a Thread.=--When the thread tool has been fed in
far enough to form a complete thread, the screw is then tested for size.
If we assume that a bolt is being threaded for a standard nut, it would
be removed from the lathe and the test made by screwing a nut on the
end. If the thread were too large, the nut might screw on very tightly
or not at all; in either case, the work would again be placed in the
lathe and a light cut taken over it to reduce the thread to the proper
size. When replacing a threaded part between the centers, it should be
put back in the original position, that is, with the "tail" of the
driving dog in the same slot of the faceplate it previously occupied.

[Illustration: Fig. 25. Testing Diameter of Thread with Calipers and
Micrometer]

As it is difficult to tell just when a thread is cut to the exact size,
special thread calipers having wedge-shaped ends are sometimes used for
measuring the diameter of a V-thread or a U. S. standard thread, at the
bottom of the grooves or the root diameter, as shown at _A_ in Fig. 25.
These calipers can be set from a tap corresponding to the size of the
thread being cut, or from a previously threaded piece of the right
size.

=The Thread Micrometer.=--Another form of caliper for testing threads is
shown at _B_. This is one of the micrometer type and is intended for
very accurate work. The spindle of this micrometer has a conical end and
the "anvil" is V-shaped, and these ends bear on the sides of the thread
or the surfaces which form