Lathes Page: 66
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42 AIR CORPS
box are set to correspond to the number of threads per inch desired.
Generally there are two or more levers on the box along with a chart.
giving the position of these levers for the various threads to be cut.
In some of the older types of lathes no quick-change gear box is in-
corporated for gear changes. In this case the gears are separate from
the lathe, and proper combinations must be selected and attached by
the operator whenever it is desired to cut a thread. Selection is made
by referring to a gear table attached to the lathe. Iin the absence of
such a table the operator must calculate the proper gears to) be used to
produce the desired results.
b. In the following explanation of lathe gearing, the primary l)oints
will be considered first. These consist of the gear on the lathe spindle
and the gear on the lead screw. Since both are fixed inl position, an-
other gear on a swinging arm is used to make the connection between
them. This gear is known as the idler, and its size is optional as long
as it performs its function, which is to transfer the motion from the
spindle to the lead screw. The gear on the spindle is the driving gear,
while the gear attached to the lead screw is the driven gear. The gears
furnished with the lathe for thread cutting will generally range from
16 to 120 teeth by increments of four. Included in the set are a few
odd gears for cutting such pitches as 111/, 13, 27, etc.
(1) In order to select gears for a certain thread, a fraction must be
taken, the numerator of which represents the number of threads per
inch of the lead screw, while the denominator represents the number
of threads per inch desired. The numerator will then equal the num-
ber of teeth in the driving gear, and the denominator, the number of
teeth in the driven gear. For example, if the pitch of the lead screw
is 4 and the thread to be cut is 12, then 4 , 2 would be the fraction-4
representing the driving gear and 12 the driven gear. However, since
each number is smaller than the number of teeth of any gear in the
range, it is necessary to increase the numbers without changing the
value of the fraction. This may be done by Ilultiplying both the
numerator and denominator of the fraction by the same number:
12 X 4/4 ='%s4.
Thus, change gears of 16 and 48 teeth will be required to cuti 12 threads
per inch. The 16-tooth gear is the driver and is placed on tlie spindle,
while the 48-tooth gear is the driven alnd is placed on the lead screw.
(See fig. 60.)
(2) On some lathes the driving gear is not placed (directly on the
lathe spindle but on a stud that is geared (townl fromnl tlie spindle. In
cases where such additional gearing l)roduces a change of ratio, this
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/68/: accessed March 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.