MIL-P-288B
lines shall be as follows: 10 degree marks 0.19 inch, 5 degree marks 0.15
inch, 1 degree mark 0.11 inch, and 30 minute marks 0.07 inch. Numerals shall
be engraved every 10 degrees in both a clockwise and counter-clockwise direction,
starting with "O" at the fiducial point and continuing to 180 degrees. The
base of the numerals shall be toward the center of the circle.
3.5.3 Pivot center. The pivot center shall be phosphor bronze conforming
to QQ-B-750, composition D. It shall be secured to the center of the circle
frame by countersunk corrosion-resistant steel or monel screws. The pivot
center shall have a projection above the center disc of the frame for retaining
the collar of the reading glass, and a projection beneath the center disc for
retaining the collars of the two movable arms. The inside and outside surfaces
of the pivot center shall be true cylinders, concentric with the circle of the
instrument, and shall be ground to an accurate fit for the collars. The pivot
shall be free from imperfections and finished smooth.
3.5.4 Pivot plugs. Four pivot plugs shall be furnished for use in marking
positions. They shall be made of phosphor bronze conforming to QQ-B-750 compo-
sition D, free from imperfections, and finished smooth. The outside diameter
of the plugs shall allow for a sliding fit in the pivot center. The four
kinds of plugs shall be as follows:
(a) A cylindrical hollow plug, with a glass or clear plastic bottom,
having a hole in the center to receive a pencil point and
marked with cross lines at right angles on the bottom plastic
surface. The glass or plastic bottom shall be set flush in the
hollow plug. The hollow plug shall be painted with white
enamel on the inside.
(b) A solid cylindrical plug, with a plunger having a point on its
lower end, and fitted with a spring to hold the point clear of
the bottom when not pressed from above.
(c) A cylindrical plug with a glass or clear plastic bottom having
cross lines on the undersurface, at right angles and intersecting
accurately at the center on the bottom surface. The glass or
plastic bottom shall be set flush in the hollow plug, to avoid
parallax. The hollow plug shall be painted with white enamel
on the inside.
(d) A solid cylindrical plug having a short needle point in the
center of the base.
3.5.5 Arms. The arms shall be of nickel silver conforming to QQ-C-586,
alloy 752, half hard tempered or corrosion resistant steel conforming to
QQ-S-766 class 302 (passivated). The arms shall be flat bars having a length
of 18 inches minimum to 18-3/4 inches maximum from the center point of the
instrument. Each arm shall be l/8-inch in thickness with a top-edge bevel
angle of 20 to 45 degrees along the radial straightedge and a face width of
1/32 to 1/16-inch. Each arm shall be 0.6 inch in width at its outer extremity
and shall be tapered to accomplish the following construction. The fixed arm
shall be secured, by means of nickel silver screws and pins, to a rib of the
circle frame in such a position that the beveled edge will lie on the radius
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