Table 3-1. Coil Size vs. Maximum Diameter for Parts Magnetized in Bottom of Coil

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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