Lathes Page: 19
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correspond in extreme length with 24 parts or twenty-four fortieths
of an inch on the bar. Consequently each division on the vernier is
smaller than each division on the bar by one-thousandth part of an
inch. If the sliding jaw of the caliper is pushed up to the solid jaw so
that the line marked 0 on the vernier corresponds to that marked 0 on
the bar, the next two lines to the right will differ from each other by
one-thousandth of an inch, and the difference will continue to increase
(one-thousandth of an inch for each division) until they again corre-
spond at the line marked 25 on the vernier. To read the distance when
the caliper is open, commence by noting how many inches, tenths, and
parts of tenths the zero point on the vernier has been moved from the
zero point on the bar. Then count upon the vernier the number of
divisions, until one is found that coincides with the one on the bar,
which will be the number of thousandths to be added to the distance
read off on the bar. The best way of expressing the value of the
divisions on the bar is to call the tenths, one-hundred thousandths
(0.100), and the fourths of tenths, or fortieths, twenty-five thousandths
(0.025). For example, the vernier shown in figure 14 @ has been moved
to the right one and two-tenths divisions (1.200 inch) as indicated by
the bar, and the sixth line on the vernier coincides with a line on the
bar, thus making six-thousandths (0.006) to be added to the reading
from the scale, which would make the total reading one and two-
hundred and six thousandths (1.206) inches. American verniers have
scales on opposite sides of the beam for taking inside and outside read-
ings directly. In making inside measurements with the 12-inch Eng-
lish vernier, three-tenths (0.300) of an inch should be added to the
apparent reading on the vernier side for the space occupied by the
g. The micrometer caliper is made in various styles and sizes depend-
ing upon the purpose for which it is to be used. The most common
types are the outside, the inside, and the outside thread micrometers.
This tool is used where close precision is required, being capable of
taking measurements to within 0.0001 inch. It consists of a frame, an
anvil, or fixed measuring point, a spindle which has a thread cut 40
to the inch on the portion inside the sleeve or barrel, and the thimble
which goes outside the sleeve and turns the spindle. One turn of the
screw moves the spindle 1/40 (0.025) inch, and the marks on the sleeve
show the number of turns the screw is moved. Every fourth gradua-
tion is marked 1, 2, 3, etc., representing tenths of an inch and as each
mark is 0.025 inch, the first four mean 0.025 times 4 or 0.100 inch. The
thimble has a beveled edge divided into 25 parts and numbered 0, 5,
10, 15, 20, and back to 0. Each of these means 1/5 of a turn or 1/ 5 of 1/o,
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/21/: accessed January 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.