MIL-DTL-24640C
(1) Each abrading element shall consist of a 5/16 inch square, (in2) high speed tool bit (Cleveland Twist Drill
Company number 855 or equivalent) which has been ground on two adjacent, longitudinal sides to produce a single,
sharp 90 degree longitudinal edge free of nicks. A medium grade grinding wheel (Norton Company number
39C6018VK or equivalent) shall be used for this grinding. Abrading elements shall be reground as required; in no
case shall an abrading element be used in excess of 5 hours without regrinding. Whenever the perpendicular
distance between either pair of opposite longitudinal sides of an abrading element become less than 0.3085 inch (as
by repeated regrinding) then that abrading element shall be discarded and replaced.
(2) The cylinder shall be 8± inch in diameter and shall be rigidly fabricated from metal.
(3) Each of the 2 abrading elements shall be maintained in electrical contact with the metal cylinder by
securing each in a notch cut into the cylinder, parallel to the cylinder axis; the 2 notches required shall be spaced
180±2 degrees apart around the circumference of the cylinder. These notches shall be cut such that the sharpened
90-degree longitudinal edge of each element shall be facing outward from the cylinder surface and such that the
midpoint of a straight line drawn between the midpoints of the 2 longitudinal edges adjacent to the sharpened edge is
tangential ±0.003 inch to the cylinder surface.
(4) An automatic counter shall be provided to total the number of times the test specimen is scraped by the
abrading elements during the test.
(5) A 1-pound ±0.5-ounce weight shall be provided for applying tension to the specimen.
b. Short circuit monitor A voltage source of not less than 12 V [either direct current (DC) or root mean
square (RMS) alternating current (AC)] which can be applied between the specimen conductor and both abrading
elements of the abrading machine as shown in figures 2 and 3. (The connection to the abrasion elements may be
provided by a wiping contact on the cylinder.) A means (such as an electrical relay) shall be provided whereby an
electrical connection between either abrading element and the specimen conductor shall automatically stop the
rotation of the cylinder on the abrading machine (as by removing motor power).
4.7.1.3 Procedure. Specimens shall be wiped with a clean dry cloth to remove any lubricant or dirt. Each
specimen shall then be tested (either simultaneously or at one time) as follows. Each specimen shall be hung
circumferentially over the cylinder of the abrading machine such that each specimen shall form an arc of not less
than 170 degrees around the cylinder for the remainder of the test. One end of each specimen shall then be secured
to a fixed surface. The abrading machine weight shall be attached to the opposite specimen end such that both this
specimen end and the weight be freely suspended. The automatic counter shall be set initially to zero, the short
circuit monitor shall be applied between both abrading elements on the abrading machine and the conductor of each
specimen. The cylinder motor shall then be turned on, allowing the cylinder to rotate beneath each specimen, from
secured end to weighted end.
4.7.1.4 Observation. Specimen failure shall be construed if electrical contact occurs between either abrading
element and any specimen conductor (as evidenced by cessation of cylinder rotation) prior to completing 150
abrasive scrapes (75 cycles).
4.7.2 Cable aging and compatibility (125 °C). This test shall be to detect any significant degradation due to
component incompatibility or prolonged overheating of completed cable containing thermosetting or tape insulation.
This test permits either of two methods (the current overload method or the hot air oven method) to be used for heat
aging.
4.7.2.1 Specimen. Specimens for cable aging and compatibility tests shall be as follows:
a.
For the current overload method, the specimen shall consist of a 40±4 foot length of completed cable.
b. For the hot air method, 2 specimens of completed cable shall be required. Specimen number 1 shall have a
length of 30±3 feet. Specimen number 2 shall have a length of not less than the sum of 12 inches +20 times the
maximum specified overall diameter of the cable (see 3.1). Both specimens shall be removed from the same length
of cable.
4.7.2.2 Special apparatus. Apparatus shall include the following:
a. The oven (required for either heat aging method) shall support the requirements as specified in 4.7.2.3, as
applicable. The oven air temperature shall be measured in the immediate vicinity of the specimen.
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