TM 1-1520-265-23
1.4.11.2.2
Bolthole Inspection. Manual bolthole inspection probes usually consist of a split 90 degree probe with the
exposed shaft inserted in an adjustable collar. The shaft is marked in increments and the collar secured at the desired
increment by means of a set screw through the collar. The probe is then rotated 360 degrees around the hole at each
setting until the entire surface of the bore has been inspected. These probes are available in federal or commercial
catalogs.
1.4.11.2.3
Scanning Fillets and Radii . Using appropriate radius probe, scan fillets and radii several times in each
direction.
1.4.11.3
Eddy Current Instrument Standardization . Eddy current inspection equipment and standards required by
the procedures in this manual are listed in Table 1-7. Reference blocks, instrument settings, and standardization
instructions for the eddy current instrument, are included in each eddy current procedure. Instrument settings, as they
are given in this manual, should be considered typical and present a test block display shown in Figure 1-7. Additional
nulling will be required to reestablish the position of the "flying spot" with the probe on the part/area to be inspected.
(Use Teflon tape listed in Table 1-8 on the probe to reduce wear. Instrument settings shall be made with Teflon tape on
the probe, if used.)
1.4.11.4
Sorting Metal Using Eddy Current. In addition to the more common usage for crack detection, eddy
current equipment may be used for metal sorting. Electrical conductivity and magnetic permeability are the material
characteristics evaluated during this type of inspection. The sorting technique cannot directly identify alloy or even the
type of metal. But when there are limited possibilities, conductivity and/or permeability information may permit proper
classification (see Figure 1-8). Typically the need for alloy sorting occurs when changes to parts are made to improve
performance.
For example, a magnesium part that is experiencing severe corrosion is replaced by one made from aluminum. Another
example is the replacement of one aluminum part with another, also of aluminum, but made from an alloy having
improved strength or corrosion resistance. In both these examples, there may be a need to verify that replacement has
been made, and the electrical conductivity of the alloys involved may be sufficiently different to permit verification by a
sorting inspection. Another situation is the requirement to NDI a part to confirm a visual indication where the material is
not known and cannot be easily determined. Eddy current sorting will quickly determine if the part is ferromagnetic and
should be inspected using the magnetic particle method. Also, if the part is nonferromagnetic, which test block
(standard) most closely matches the conductivity of the part and, therefore, should be used to adjust the eddy current
equipment for crack inspection/ verification.
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