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Sign inCoil shot current, low fill factor
A coil (encircling) shot drives a longitudinal field along the axis of the part, so it reveals discontinuities lying transverse to that axis. What sets the field inside the coil is the magnetomotive force, the ampere-turns NI - current multiplied by number of turns - not the current alone. A five-turn coil at 1 000 A and a one-turn wrap at 5 000 A produce the same field.
The part fights back. Its own end poles create a demagnetising field that opposes the applied field, and that opposing field grows as the part gets shorter and fatter. The empirical codes capture this with the length-to-diameter ratio L/D: required ampere-turns fall as L/D rises. Below L/D = 2 the demagnetising field dominates and the formula stops being meaningful; above L/D = 15 there is no further gain, so 15 is the value used.
Low fill factor means the coil is much larger than the part - the coil bore cross-section is at least ten times the part cross-section. The part sits far from the windings, out where the field has spread and weakened, so it needs more ampere-turns than a snug coil would. For the same reason the part is normally laid against the inside wall of the coil, where the flux is concentrated, and the wall-position formula NI = 45 000/(L/D) applies. Where the part must instead hang centred on the coil axis - on a fixture, or from a crane sling - the standards give a different relation, NI = 43 000 R / (6 L/D - 5) with R the coil radius in inches: out on the axis the field the part sees depends on the size of the coil itself, so the coil radius enters the calculation.
The result is a starting point, not a guarantee. A coil's field is only usefully strong for roughly 230 mm (9 in.) either side of it, so long parts need several overlapping shots. Confirm the actual tangential field at the surface with a Hall-effect gaussmeter (30-60 G) or with artificial flaw shims before accepting the technique.
L/D ratio = part length / part diameter, limited to the range 2 to 15 Part against the coil wall: NI = 45 000 / (L/D) ampere-turns, ±10% Part in the coil centre: NI = 43 000 x R / (6 x L/D - 5), R = coil radius in inches, ±10% I = NI / N Effective coil field reaches about 230 mm each side of the coil
- Low fill factor applies when the coil bore cross-section is at least 10 times the part cross-section.
- In the wall position, lay the part against the inside wall of the coil, not on the coil axis. A part that has to hang on the axis is a centred shot - select the centred position so the coil radius is accounted for.
- Low fill factor still requires the coil bore area to be at least 10 times the part cross-sectional area, in either position.
- Ampere-turn values carry a ±10% tolerance.
- Field adequacy must still be demonstrated - Hall-effect probe reading 30-60 G tangential, or an artificial flaw shim / pie gauge on the surface.
- Longitudinal fields find transverse discontinuities. A second, circular shot is needed for longitudinal ones.
Reference: ASME BPVC Section V (2023 Ed.), Article 7 - longitudinal (coil) magnetisation, low fill factor: NI = 45 000/(L/D) for parts positioned against the coil inside wall, and NI = KR/(6(L/D) - 5) with K = 43 000 and R the coil radius in inches for parts positioned in the centre of the coil; ASTM E1444/E1444M-22 and ASTM E709-21 give the same relations. The 230 mm (9 in.) effective field extent is taken from the same standards.
