arriage is moved back 0.8 / 2 = 0.4 inch, the tool will be properly
located for the second cut; but the lock-nuts could not be re-engaged
unless the lead-screw had ten threads per inch, which is finer than the
pitch found on the lead-screws of ordinary engine lathes. However, if
the movement were 0.4 + 0.8 x 2 = 2 inches, the lock-nuts could be
re-engaged regardless of the number of threads per inch on the
lead-screw. The rule then, is as follows:

_Divide the lead of the thread by 2 for a double thread, 3 for a triple
thread, 4 for a quadruple thread, etc., thus obtaining the pitch; then
add the pitch to any multiple of the lead, which will give a movement,
in inches, that will enable the lock-nuts to be re-engaged with the
lead-screw._

Whenever the number obtained by this rule is a whole number, obviously,
the movement can be obtained with a lead-screw of any pitch. If the
number is fractional, the number of threads per inch on the lead-screw
must be divisible by the denominator of the fraction.

To illustrate the application of the foregoing rule, suppose a quadruple
thread is to be cut having 1-1/2 single threads per inch (which would be
the number the lathe would be geared to cut). Then the lead of the
thread = 1 / 1-1/2 = 0.6666 inch and the pitch = 0.6666 / 4 = 0.1666
inch; adding the pitch to twice the lead we have 0.1666 + 2 x 0.6666 =
1.499 inch. Hence, if the carriage is moved 1-1/2 inch (which will
require a lead-screw having an even number of threads per inch), the
tool will be located accurately enough for practical purposes. When the
tool is set in this way, if it does not clear the end of the part being
threaded, the lathe can be turned backward to place the tool in the
proper position.

[Illustration: Fig. 13. Indexing Faceplate used for Multiple Thread
Cutting]

The foregoing rule, as applied to triple threads or those of a higher
number, does not always give the only distance that the carriage can be
moved. To illustrate, in the preceding example the carriage movement
could be equal to 0.499, or what is practically one-half inch, instead
of 1-1/2 inch, and the tool would be properly located. The rule,
however, has the merit of simplicity and can be used in most cases.

Special faceplates are sometimes used for multiple thread cutting, that
enable work to be easily and accurately indexed. One of these is
illustrated in Fig. 13; it consists of two parts _A_ and _B_, part _A_
being free to rotate