MIL-P-288B
of the circle, which passes through the fiducial point of the graduated circle.
The two movable arms shall be secured to collars which rotate about the pivot.
center. Each movable arm shall be fitted with a vernier, clamp, and tangent
screw. The collars shall be reduced in thickness, so that when the arms are
secured in place they will come flush with the bottom of the pivot center and
lie in the same plane.
3.5.5.1 Straightedges, type I, class 1. Using the center point as datum,
the straightedge of the left movable arm for the type 1, class 1 protractor
shall lie on a radius of the circle and shall pass through the fiducial point
of the graduated circle, when the left arm of the protractor is closed. The
straightedge of the right movable arm for the type 1, class 1 protractor
shall lie on a radius of the circle and shall be at an angle not greater than
11 degrees, 30 minutes from the fiducial point, when the right arm of the
protractor is closed.
3.5.5.2 Straightedges, type I, class 2. Using the center point as datum,
the straightedge of the right movable arm for the type I, class 2 protractor
shall lie on a radius of the circle and shall pass through the fiducial point
of the graduated circle when the right arm of the protractor is closed. The
straightedge of the left movable arm for the type I, class 2 protractor shall
lie on a radius of the circle and shall be at an angle not greater than 11
degrees, 30 minutes from the fiducial point when the left arm of the protractor
is closed.
3.5.5.3 Angle range, 260 degrees. The straightedge of the left movable
arm for the type 1, class 1 protractor and the straightedge of the right movable
arm for the type I, class 2 protractor shall be capable of laying off angles
up to 260 degrees.
3.5.5.4 Angle range, 270 degrees. The straightedge of the right movable
arm for the type I, class 1 protractor and the straightedge of the left movable
arm for the type I, class 2 protractor shall be capable of laying off angles up
to 270 degrees.
3.5.6 Extension arms. Three extension arms, made of the same material
specified in 3.5.5, shall be provided. Each extension arm shall be 13-1/2
inches long, tapering in width from 0.6 to 0.4 inch at the outer end, and shall
be joined to the permanent arms by scarfing pieces. The scarfing pieces shall
be brazed or riveted to the extension arms and secured, when needed, to the
permanent arms by means of a threaded thumbscrew and two dowel pins, affixed to
the scarfing pieces. Extension arms shall be matched marked to the mating arm.
3.5.7 Vernier scales. The vernier scales shall be of nickel silver con-
forming to QQ-C-585, alloy 752, half hard tempered or of corrosion resistant
steel conforming to QQ-S-766 class 302 (passivated). The upper surface of the
vernier scales shall be flush with the surface of the graduated circle. The
vernier of each movable arm shall have its zero graduation positioned over the
straightedge of the arm. Each vernier shall embrace 14 degrees, 30 minutes of
the graduated circle and shall be so graduated as to permit reading of the
scale to 1 minute of arc. Each vernier shall have one additional mark on each
end of the scale. The graduation requirements shall be the same as those of
the graduated circle (see 3.5.2). The engraved numerals shall be numbered 0,
10, 20, and 30.
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