umber of teeth in
worm-wheel _W_ is some multiple of the number of threads per inch of the
lead-screw, and the number of teeth in the worm-wheel, divided by the
pitch of the screw, equals the number of graduations on the dial. For
example, if the lead-screw has six threads per inch, the worm-wheel
could have twenty-four teeth, in which case the dial would have four
divisions, each representing an inch of carriage travel, and by
sub-dividing the dial into eighths (as shown) each line would correspond
to 1/2 inch of travel. The dial, therefore, would enable the carriage to
be engaged with the lead-screw at points equal to a travel of one-half
inch. To illustrate the advantage of this suppose ten threads per inch
are being cut and (with the lathe stationary) the carriage is disengaged
and moved 1/6 inch or one thread on the lead-screw; the tool point will
also have moved 1/6 inch, but it will not be opposite the next thread
groove in the work as the pitch is 1/10 inch. If the carriage is moved
another thread on the lead-screw, or 2/6 inch, the tool will still be
out of line with the thread on the work, but when it has moved three
threads, or 1/2 inch, the tool will then coincide with the original cut
because it has passed over exactly five threads. This would be true for
any number of threads per inch that is divisible by 2. If the thread
being cut had nine threads per inch or any other odd number, the tool
would only coincide with the thread at points 1 inch apart. Therefore,
the carriage can only be engaged when one of the four graduations
representing an inch of travel is opposite the arrow, when cutting odd
threads; whereas even numbers can be "caught" by using any one of the
eight lines.

This indicator can also be used for "catching" fractional threads. As an
illustration, suppose 11-1/2 threads per inch are to be cut, and the
carriage is engaged for the first cut when graduation line 1 is opposite
the arrow; engagement would then be made for each successive cut, when
either line 1 or 3 were opposite the arrow, or in other words at spaces
equal to a carriage movement of 2 inches. As the use of the indicator
when cutting fractional threads is liable to result in error, it is
better to keep the half-nuts in engagement and return the carriage by
reversing the lathe.

=Replacing Sharpened Thread Tool.=--If it is necessary to sharpen the
thread tool before the thread is finished, it should be reset square
with the work