Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

Hand operated or manual hole probes, usually consist of a split plastic body,
with the core and coil axis at right angles to the probe axis (see FIG. 5.11).
The split allows the probe to compress slightly to enter the hole, then expand
to press on the walls of the hole so as to minimize lift-off To test a hole, the
probe is inserted until the coil is just inside the far side of the hole, the depth
stop is adjusted to be flush with the near surface, and the set screw tightened.
The probe is then rotated approximately one and a quarter revolutions, to
ensure full coverage at that depth. The set screw is then loosened, the probe
withdrawn a given amount, commonly 1 mm to 2 mm, keeping the depth stop
flush with the surface, then the screw is retightened. The probe is again
rotated as before, and the process repeated until the coil is just inside the
near surface. This process is reliable if carefully performed, but time
consuming, and, especially if large numbers of holes are to be tested, motor
driven probes are preferred.
Charlie Chong/ Fion Zhang