MIL-P-288B
(d) One screwdriver
(e) Four pivot plugs
(f) One stowage and carrying case
3.5 Construction, type I protractors.
3.5.1 Assembly. The assembled protractor shall consist of a circle frame
with an inlaid graduated circle and three arms, one of which shall be fixed
and attached to the circle frame and two that shall be movable and attached to
a pivot center. A tangent screw with vernier scale shall be attached to each
of the two movable arms. A reading glass shall be hinged to a rotatable collar,
attached to the pivot center. Figure 1 is an example of an acceptable design
of the ribs for the circle frame.
3.5.1.1 Accuracy. The graduations on the body of the type I protractors
shall be such that no graduation mark will vary from its true position by more
than 2 minutes of arc. The movable arms shall be mounted in the assembled
instrument so as to rotate on an axis at the center of the graduated circle,
with an accuracy such that the maximum error due to eccentricity shall not
exceed 3 minutes of arc. The total error of the assembled protractor shall
not exceed 5 minutes of arc.
3.5.2 Circle frame. The circle frame shall be brass, conforming to
QQ-B-626 alloy 342. The upper surface of the frame shall be machined from the
outside diameter to an inside diameter and depth, to match a graduated circle.
A graduated circle (see 3.5.2.1) shall be inlaid flush with the top surface of
the circle frame. As an option, instead of graduations on the frame, graduations
can be combined on a full width graduated circle made of corrosion resistant
steel, conforming to QQ-S-766 class 302 (passivated), and silver soldered to
the frame. A numbered scale shall be engraved on the top surface of the frame
such that, when assembled with the graduated circle, numerals will appear at
every 10 degree graduation starting at 360 degrees at the fiducial point and
continuing clockwise in decreasing increments of 10 to 180 degrees, then in
increasing increments of 10 to 350 degrees. Numerals shall be 0.070 inch high
and shall be filled with black enamel. The base of the numerals shall be
toward the center of the circle. The frame shall have a circular groove on
the undersurface, to receive the guide pins of the clamps. The center of the
circle frame shall be a disc not greater than 1-7/16 inches in outside diameter.
It shall be connected to the outer circle of the frame by three radial ribs.
The center, ribs, and outer circle of the frame shall be fabrocated as one
integral piece and shall form a rigid support for the arms. The center of the
circle frame shall be fitted for the pivot center.
3.5.2.1 Graduated circle. The graduated circle shall be nickel silver
conforming to QQ-C-585, alloy 752, half hard tempered or of corrosion resistant
steel conforming to QQ-S-766 class 302 (passivated). The graduated circle
shall be not less than 1/2-inch wide, with the outside diameter not less than
6-5/16 inches and the inside diameter not greater than 5-7/32 inches. The
entire circle shall be accurately graduated at the outer circumference in
degrees and half degrees. The graduated lines shall have a depth of 0.003
inch minimum to 0.005 inch maximum, shall have a width of 0.005 ± 0.001 inch
and shall be filled with waterproof ink or enamel. The length of the graduated
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