TM 1-1520-253-23
2.15.3.7
Inspection procedure. Refer to Eddy Current Method, paragraph 1.4.11, TM 1-1520-253-23.
NOTE
A nonmetallic insert (a short piece of PVC pipe, Styrofoam etc.) may be inserted snugly into the hole
to aid as a probe guide to prevent edge effect signals.
a. Place the probe on a good area of the web under inspection and null. Adjust the phase angle as re-
quired to obtain horizontal lift-off.
b. Recognize the possible lift off variation associated with the hubs paint thickness in comparison to the
the layer of tape on the calibration standard. Lift off variations will be evident by horizontal movement of the flying dot
across the CRT display. Reference Figure 2.15C.
c. Adjust the vertical positioning of the flying dot to maintain the hub null point on the CRT display if its
conductivity is greater than the reference standard.
d. Scan around the perimeter of the lightening holes as close as possible without achieving edge effect.
Continue scanning the area surrounding the lightening holes to sufficiently cover an area of one half inch from the edge
of the hole.
e. Any signal similar to the notches in the test block is cause for rejection.
CAUTION
Excessive probe wear will be encountered if the teflon tape on the probe tip is allowed to deteriorate.
Light sanding of the inspection area with crocus cloth to smooth the painted surface is permissible.
Wipe the inspection area clean after sanding.
Figure 2.15C Lift off variation due to nonconductive coating
NOTE
Either probe in the probe kit identified in paragraph 2.15.3.1 can be used depending on the ease
of accessibility and user friendliness. If the probes are changed, repeat paragraph 2.15.3.6 steps
1, 2, 3, and 4. The probe contact surface should be kept as flat as possible on the test surface. The
probe shafts are made of thin copper tubing allowing the probe shaft to be carefully modified as
necessary to optimize the probe contact angle.
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