MIL-DTL-24640C
a. A self-standing thickness gauge, for supporting the requirements as specified in 4.7.9.3, which shall exhibit
a measurement error of not more than ±0.001 inch. This gauge shall incorporate a flat horizontal platen upon which
the entire specimen shall be laid flat during measurement and shall incorporate a foot which rests upon the top of the
specimen during measurement. This foot shall be attached to a plunger which moves freely in the vertical direction
and which is attached to a dial or other indicating device from which the specimen thickness may be read. The foot
shall present a horizontal disk of +0, -1/64-inch diameter to the specimen, which shall bear down on the specimen
with a force of 85+0, -4 grams and shall be loaded with weights (see 4.7.9.3) to present additional force to the
specimen.
b. A micrometer (required for insulated conductor specimens only) with flat, parallel measurement surfaces
on both spindle and anvil, for the specimen thickness measurement as specified in 4.7.9.3(b)(1), which shall exhibit
a measurement error of not more than ±0.0001 inch.
c. A motor driven grinding wheel and a motor driven buffing wheel, or their equal, for the specimen
preparation as specified in 4.7.9.1.a and b(1). Guides shall be provided to assure that the specimen is pulled
tangentially to the surface of each wheel during grinding and buffing.
d. An oven, for accommodating the gauge with the specimen inserted and for supporting the requirements as
specified in 4.7.9.3. The oven air temperature shall be measured in the immediate vicinity of the specimen.
4.7.9.3 Procedure. The initial insulation thickness of each specimen (T in the formula below) shall be
determined (see a(1) and (b) below) and the oven shall be preheated to 121±1 °C. This oven temperature shall be
maintained for the remainder of the test. The gauge foot shall then be loaded with weights (see a(2) and b(2) below)
and the gauge placed within the oven. Not less than one hour later, one specimen shall be selected and also placed
within the oven. Not less than one hour after placing the specimen within the oven, the specimen shall be placed
beneath the gauge foot such that the gauge indicates the specimen thickness. Not less than 1 hour after placing the
specimen beneath the gauge foot, the final specimen thickness or diameter (as appropriate) shall be read from the
gauge and the final insulation thickness (t in formula below) determined (see a(3) and b(3) below). In an identical
manner, the remaining specimens shall also be tested. The percentage heat distortion of the insulation shall then be
calculated by using the following formula:
Percentage heat distortion = Median value of 100(T-t)/T for all tested specimens.
Where:
T
=
Initial insulation thickness of the specimen, in thousandths of an inch (see (a) and (b)
below).
T
=
Final insulation thickness of the specimen, in thousandths of an inch (see (a) and (b)
below).
The following additional details apply:
a.
Rectangular strip specimens:
(1) The initial insulation thickness (T) shall be the initial specimen thickness, as measured with the gauge,
prior to loading the gauge with weights and prior to inserting either the gauge or the specimen into the oven.
(2) The gauge foot shall be loaded with 2000±100 grams.
(3) The final insulation thickness (t) shall be the final thickness, measured as specified.
b.
Insulated conductor specimen:
(1) The initial insulation thickness (T) shall be defined as ½ of the difference between the measured
overall specimen diameter and the measured specimen conductor diameter. Measurements shall be made using the
micrometer. The overall specimen diameter shall be measured in the same radial direction as will be measured
when the final insulation thickness (t) (see b(3) below) is determined.
(2) The gauge foot shall be loaded as follows:
(a) Specimens with a conductor cross section of not more than 3999 circular mils nor less than 2300
circular mils: 400±20 grams.
(b) Specimens with a conductor cross section of not more than 21,999 circular mils nor less than 4000
circular mils: 500±25 grams.
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