Lathes Page: 68
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42 AIR CORPS
Instead of being idler gears, the one that engages with the spindle
is a driven gear while the other is a driving gear.
(2) As an example in the use of compound gearing, let it be as-
sumed that 48 threads per inch are to be cut, using a lead screw pitch
of 4. As previously explained, 4/48 will be the fraction to use for
figuring the gearing. The smallest number must be raised until it
equals at least the number of teeth of the smallest gear. Therefore,
as 16 is usually the smallest number of teeth available, the numerator
and denominator are both multiplied by 4:
4 4 16
48 4 172
In simple gearing the driving gear would have 16 teeth and the
driven gear 172 teeth; however, 172 teeth is far greater than the
available tooth range. For use in compound gearing the fraction
4/48 is changed to any two components whose product is equal to
4/48; for example:
1 4 4
3 16 48
As gears corresponding to these components are not available, it is
necessary to raise their numerators and denominators as follows:
1 16 16 4 6 24
X =- and X-
3 16 48 16 6 96
Thus, 16 and 24 represent the number of teeth of the driving gears,
while 48 and 96 represent the number of teeth of the driven gears.
In this connection, it must be remembered that the lathe spindle is
the driving member and the lead screw the driven member, while the
gear that meshes with the spindle on the compound stud is a driven
gear and the one that meshes with the lead screw is a driving gear.
d. When cutting metric threads, a pair of transposing gears is
used on the compound stud. One of these gears has 50 teeth while
the other has 127 teeth. In the English system, threads are specified
in threads per inch, whereas, in the metric system they are specified
in threads per centimeter. 50/127 of an inch is equal to 1 cm.; there-
fore, if the lathe is geared for a certain number of threads per inch
in simple gearing and the transposing gears of 50 and 127 teeth are
placed in compound in the gear train, instead of cutting a certain
number of threads per inch the same number of threads per centi,
meter will be cut.
e. When a thread pitch must be cut that is not given on the thread
index plate, it is necessary to change the sector gear. The following
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/70/: accessed March 27, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.