Table 3-1. Coil Size vs. Maximum Diameter for Parts Magnetized in Bottom of Coil
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Electromagnet Technique
Table 3-2. Typical Coil-Shot Current for a Five-Turn Coil with Part in Bottom of Coil
TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1
3-37
A =
p
(6)
^{2}
A=
p
(1)
^{2}
A = 113 sq. inches
A = 3.14 sq. inches
3.3.12.6.2.1
When the cross-sectional area of the part is less than one-tenth of the cross-sectional area of the coil, the part should be
magnetized lying in the bottom of the coil.
3.3.12.6.2.2
When the cross-sectional area of the part is greater than one-tenth of the cross-sectional area of the coil, the part must
be magnetized in the center of the coil.
3.3.12.6.2.3
When using a cable wrap or when the cross-sectional area of the part exceeds one-half of the cross-sectional area of the
coil, the part should be centered in the coil and the formula for high fill factor coils SHALL be used for estimating the
required amperage.
3.3.12.6.2.4
Table 3-1 lists the diameter of the largest part that can be magnetized lying in the bottom of a coil or placed next to the
coil wall for some typical coil sizes. For any given coil diameter, parts with
diameters
larger than those listed must be
magnetized by some other method, such as centering them in the coil, using a cable wrap, or using a larger coil.
Table 3-1. Coil Size vs. Maximum Diameter for Parts Magnetized in Bottom of Coil.
Coil Diameter
(inches)
Maximum Part
Diameter (inches)
8
2.5
12
3.8
15
4.8
18
5.7
20
6.3
24
7.6
3.3.12.6.3
Calculating
Coil
Current
.
Two rule-of-thumb formulas have been developed for use in estimating the coil current levels to be used for
longitudinal magnetization
. One formula is for a part centered in the coil and the other for a part lying in the bottom of
the coil. These formulas apply to cylindrical and irregularly shaped parts and at one time were thought to estimate the
required current to within 10%. Recent studies show that in almost all instances they overestimate the required current
by at least 50%. They use the part length-to-diameter (L/D) ratio. The useful magnetizing field produced by an
encircling coil extends approximately 6 to 9 inches to either side of the coil. For parts longer than the effective field
distance, one or more inspections are required along the length of the part. When repositioning these longer parts in
the coil, allow a 3-inch effective field overlap. The formulas are intended for part with a L/D ratio between 3, and 15.
For inspecting parts having L/D ratios of 3 or less, see paragraph 3.3.12.6.3.7. For parts with a L/D ratio greater than
15, use 15 as the value for the ratio.
3.3.12.6.3.1
Formula
for
Part
Lying
in
Bottom
of
Coil
.
The following formula can be used when the cross-sectional area of the part is less than one-tenth the cross-sectional
area of the coil(s) and SHALL be used whenever the part is lying in the bottom of the coil, or is placed next to the coil
wall during magnetization. If the part has hollow portions, replace D with D
_{eff}
(see paragraph 3.3.12.6.3.4).
I
KD
NL
=
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