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TM 9-254
5-8.
Mil System - Continued
h.
The comparison of 30 minutes of arc to an angular displacement in mils is often necessary when checking
the optical alinement of instrument reticles against a collimator reticle.
Example (fig. 5-2):
Thirty minutes of arc is equivalent to the angle, or amount of deviation between the horizontal line of
the instrument reticle and the horizontal line of the projector reticle when one end of the instrument
reticle line is superimposed on the projector reticle line and the other end of the instrument reticle
line is 0.5 mil from the line when measured over a distance of 58 mils along the projector
collimator reticle line.
Figure 5-2. Angular Displacement
Thirty minutes of arc, at a distance of 29 mil, will produce a linear displacement of 0.25 mil between
the two reticle lines.
Thirty minutes of arc, at a distance of 58 mil, will produce a linear displacement of 0.5 mil between
the two reticle lines.
Section IV. MECHANICAL DATA
5-9.
Screw Thread.
a.
Several screw thread forms exist, but the most commonly used are those having symmetrical sides at equal
angles with a vertical center line through the thread apex, as shown in figure 5-3.
b.
The appropriate tap drill sizes for various screw sizes are listed in table 5-10 and table 5-11. The first column
in each table is the thread size. The fifth column is the drill tap size. The remaining columns contain general screw data.
5-21
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