MIL-P-288B
distance from the fiducial line. The smooth parallel edge of one arm shall be
to the left of the fiducial line and on the other arm, to the right of the
fiducial line, when the two fiducial lines are in coincidence. The shape of
each arm on the opposite side to the smooth parallel edge shall be such that
the arm covers approximately 33 degrees of the protractor graduations, when
the fiducial lines of the movable arms are in coincidence with the fiducial
line of the fixed arm. This side of the arms shall extend past the periphery
of the protractor body a distance of 3/8-inch, to accommodate a clamp (see
3.6.6), and shall taper toward the outer end at such an angle that the total
width of the arm at the outer end is approximately 1 inch. TWO holes, 3/64-inch
in diameter, shall be drilled through the fiducial line, one 7/16 inch from
the outer end and the other 7 inches from the outer end of each arm. When the
fiducial lines of the two movable arms are in coincidence with the fiducial
line of the fixed arm, their holes shall coincide with the holes in the fixed
arm. The movable arms shall be accurately graduated on the under side to
serve as verniers, as specified in 3.6.4.
3.6.4 Graduations. The protractor body shall be accurately graduated
and numbered on the upper surface. The graduations shall be either engraved
or printed with a tolerance of plus or minus 5 minutes of arc. The outside
diameter of the graduations shall be 11 inches (plus or minus 1/16 inch). The
fiducial line of the fixed arm shall form the zero line, and the graduations
shall be marked in one degree increments and numbered every 10 degrees. Numer-
als shall have their base toward the center and shall read from 0 to 350 degrees
in two rows of numerals, one in a clockwise and one in a counter-clockwise
direction. Clockwise numerals shall form the outside row. The movable arms
shall be accurately graduated and numbered on the underside, such that the
numerals can be read correctly from the top side. The graduations on the
movable arms shall serve as verniers, readable to 2 minutes of arc. They shall
be numbered every fifth graduation, starting with zero at the fiducial line and
continuing from 10 through 60. The base of the vernier numerals shall be towards
the center of the body. The vernier graduations shall be concentric with the
circle graduations and shall have their base in coincidence with the outside
diameter of the graduated circle. All graduation marks and fiducial lines shall
be 0.004 to 0.010 inch wide. The length of the graduation marks shall be as
follows: 1-degree and 2 minute marks, 5/32 inch; 5-degree and 10 minute marks,
1/4 inch; 10-degree marks, 5/16 inch. Numerals on the body shall be 5/32 inch
high and on the movable arms, 1/8 inch high. All graduation marks, fiducial
lines, and numerals shall be filled or printed with permanent waterproof black
ink or black enamel. They shall be protected with a transparent plastic cover
sheet from 0,003 to 0.005 inch thick of a material compatible with the base
material. When tested as specified in 4.4.4, there shall be no evidence of
the cover sheet separating.
3.6.5 Pivot center. The pivot center shall consist of a brass center
post, knurled ring nut, and plastic washer arranged to allow for free axial
movement of the movable arms. The center post and knurled nut shall be made
of brass and shall have an overall height not to exceed l/4-inch, when assem-
bled. The bottom of the post shall have a flange 3/8-inch in diameter which
shall project 1/32-inch below the underside of the protractor body. The edges
of the flange shall be slightly rounded, in order to facilitate sliding of the
protractor over the chart surface. The stem of the post shall be 1/4-inch in
diameter and shall be threaded to receive the knurled nut. The top of the
post shall have an opening approximately 3/16-inch in diameter and counterbored
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