TM 1-1520-265-231.4.11.2.2Bolthole Inspection. Manual bolthole inspection probes usually consist of a split 90 degree probe with theexposed shaft inserted in an adjustable collar. The shaft is marked in increments and the collar secured at the desiredincrement by means of a set screw through the collar. The probe is then rotated 360 degrees around the hole at eachsetting until the entire surface of the bore has been inspected. These probes are available in federal or commercialcatalogs.1.4.11.2.3Scanning Fillets and Radii. Using appropriate radius probe, scan fillets and radii several times in eachdirection.1.4.11.3Eddy Current Instrument Standardization . Eddy current inspection equipment and standards required bythe procedures in this manual are listed in Table 1-7. Reference blocks, instrument settings, and standardizationinstructions for the eddy current instrument, are included in each eddy current procedure. Instrument settings, as theyare given in this manual, should be considered typical and present a test block display shown in Figure 1-7. Additionalnulling 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 onthe probe, if used.)1.4.11.4Sorting Metal Using Eddy Current. In addition to the more common usage for crack detection, eddycurrent equipment may be used for metal sorting. Electrical conductivity and magnetic permeability are the materialcharacteristics evaluated during this type of inspection. The sorting technique cannot directly identify alloy or even thetype of metal. But when there are limited possibilities, conductivity and/or permeability information may permit properclassification (see Figure 1-8). Typically the need for alloy sorting occurs when changes to parts are made to improveperformance.For example, a magnesium part that is experiencing severe corrosion is replaced by one made from aluminum. Anotherexample is the replacement of one aluminum part with another, also of aluminum, but made from an alloy havingimproved strength or corrosion resistance. In both these examples, there may be a need to verify that replacement hasbeen made, and the electrical conductivity of the alloys involved may be sufficiently different to permit verification by asorting inspection. Another situation is the requirement to NDI a part to confirm a visual indication where the material isnot known and cannot be easily determined. Eddy current sorting will quickly determine if the part is ferromagnetic andshould 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 currentequipment for crack inspection/ verification.1-33
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