MIL-P-288B
3.7.2.3 Grid scale. The grid scale shall consist of 1/2-inch squares
covering the free area of the body as shown on figure 3. The center vertical
line of the grid shall pass through the radial center in line with the 0- and
180-degree markings from the bottom edge of the body to a height of 3-inches
above the radial center. All other vertical grid lines shall be parallel to
the center vertical grid lines. The center horizontal grid line shall pass
through the radial center, in line with the 90 and 270 degree markings for a
length of 3-inches on each side of the radial center. All other horizontal
grid lines shall be parallel to the center horizontal grid line. Vertical and
horizontal grid lines passing through the radial center shall be in alinement
with the graduations as indicated, within a tolerance of plus or minus 15
minutes of arc. All other grid lines shall be parallel to the center lines
within the same tolerance. The grid lines shall be filled with magenta ink no.
X70248, manufactured by Hill-Hentshel Ink Company, St. Louis, MO, or magenta
ink no. M21135 manufactured by Sinclair and Valentine Company, Baltimore, MD,
or equivalent.
3.7.3 Arm. The arm shall be made of molded plastic (see 3.3.2) in accor-
dance with figure 4. The arm shall contain a beveled edge having a very light
matte surface suitable for pencil marking. The arm shall be graduated on the
underside of the beveled edge in 1/10-inch increments, with three lengths of
lines designating 1/10-, 1/2-, and 1-inch, beginning at a point 4.400 inches
from the center post hole and continuing to the end of the arm. The l-inch
lines shall be 1/8-inch long, the 1/2-inch line shall be 3/32-inch long, and
the 1/10-inch lines shall be 1/16-inch long. Each inch mark shall be numbered
beginning at 5 and ending at 17. Numerals shall be 3/32-inch high. The numerals
and graduations shall be either engraved or molded in the underside such that
they can be read correctly from the top side of the arm. Markings shall be
0.015-inch deep by 0.010-inch wide and filled with permanent waterproof ink or
ename1, which shall bond or fuse to the plastic. The arm shall be annealed
after molding, to relieve stresses.
3.7.4 Center post assembly. The center post assembly shall be made of
brass conforming to QQ-B-626, alloy 342. All parts shall be nickel-plated
conforming to QQ-N-290, class 1, grade E. The center post assembly shall
consist of a flanged center post with key, threaded retainer rig, keyed washer,
and knurled thumb nut. The center post shall be conically drilled so that a
sharpened lead pencil may be inserted through the post to the paper under the
protractor. The center post stem shall be threaded, approximately 7/16-inch
in diameter, with the conical hole 9/32-inch in diameter at the top converging
to approximately 3/32-inch in diameter at the bottom. A key shall be firmly
fastened to the center post flanged edge by pinning, brazing, or silver solder-
ing to prevent turning of the center post in the body. The center post shall
be shouldered and threaded to receive a retainer ring with a threaded hole
approximately 19/32-inch in diameter. The retainer ring shall have two holes
for fastening with a spanner wrench. The outside diameter of the center post
stem shall be threaded to receive the knurled thumb nut. A keyway shall be
provided on the stem to receive a keyed washer, which shall prevent the arm
from turning when the thumb nut is tightened. The thumb nut shall be approxi-
mately 3/4-inch in diameter by 3/8-inch high, with a threaded hole to fit the
center post stem. The hole in the top of the thumb nut shall be countersunk
approximately 3/32-inch deep. The general configuration of the center post
assembly is shown in section "A-A" of figure 3.
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