MIL-P-288B
3.5.8 Clamps. Each clamp shall be secured to the circle frame by a thumb-
screw, and guided in motion by a tongue which slides in the circular groove in
the underside of the circle frame.
3.5.9 Tangent screws. The tangent screws shall be made of corrosion
resistant steel conforming to QQ-S-763 class 302 (passivated). They shall be
of a standard size #4 UNF 48 threads per inch and shall have knurled heads.
Each tangent screw shall be attached to its clamp, by a fixed ball working in
a socket joint, and to the movable arm, by a ball nut working in a socket
joint.
3.5.10 Reading glass. The reading glass shall be a 3.0X single lens mag-
nifier having a focal length of 1-1/2 inches and a clear aperture of 5/8-inch.
It shall be hinged to a collar which rotates around the pivot center, so as to
have motion parallel to the plane of the instrument. The field of view of the
reading glass, when the glass is parallel to the plane of the instrument,
shall be centered over the graduated edges of the graduated circle and the
vernier scale.
3.5.11 Magnifying glass. A pocket magnifying glass of 3.0X magnifying
power, conforming to GG-M-95, type 1, class A, shall be furnished with each
type I protractor.
3.5.12 Screwdriver. A screwdriver of good commercial quality of a size
fitting the screws on the instrument, shall be furnished with each type I
protractor.
3.6 Construction, type II, class 1 protractor.
3.6.1 Assembly. The assembled protractor shall consist of a body having
a graduated circle with a fixed integral arm, a pivot center, two movable arms
with vernier scales, and thumb nut clamps on each movable arm (see figure 2).
3.6.1.1 Accuracy. The total error of the assembled protractor shall not
exceed 6 minutes of arc.
3.6.2 Body. The body shall be made of plastic 0.040 inch (nominal) thick
(see 3.3.2). The body shall be a circle 12-3/16 inches in diameter (plus or
minus l/16-inch) with a fixed integral arm extending 13-1/2 inches (plus or
minus l/16-inch) from the center of the circle. The arm shall taper from a
width of 2-1/2 inches measured on the circumference of the circle to a width
of approximately 1 inch at the rounded end. A fiducial line shall extend from
the pivot center through the center of the fixed arm for the full length of
the arm. Two holes, 3/64-inch in diameter, shall be drilled through the fiducial
line, one at 7/16-inch from the outer end and the other at 7 inches from the
outer end of the arm. The circle shall be accurately graduated on the upper
surface, as specified in 3.6.4.
3.6.3 Movable arms. There shall be two movable arms made of plastic
0.030 inch (nominal) thickness (see 3.3.2), which shall pivot about the center
of the body. Both movable arms shall be the same length as the fixed arm and
shall be rounded on the outer ends. Each movable arm shall have a fiducial
line extending radially from the pivot center through the greatest length of
the arm. Each arm shall have a smooth straightedge parallel to and 5/16-inch
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