Coil 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 must be laid against the inside wall of the coil, where the flux is concentrated, rather than centred on the axis.

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.