T.O. 33B-1-13-118c. Select method of magnetization. Magnetism may be applied with portable electromagnets (yokes),permanent magnets, or conventional magnetic particle inspection equipment. In areas of limitedaccessibility, steel extensions or pole pieces are used to transfer magnetism into the inspection area.Permanent magnets are useful in certain specialized applications, such as threaded bolts, gears, or othersmall parts whose shape makes magnetization difficult with an electromagnet. The magnetic fieldsproduced in large parts by permanent magnets are often quite low and unpredictable; therefore, theyshould not be used on such parts, unless a specific procedure has been developed and verified. Centralconductors are best suited for fastener and attachment holes particularly when there are multiple layersof materials.d. Select the method of magnetic contact. Field strength is greatly reduced when there is poor contactbetween the magnet and the test piece. To improve contact, auxiliary pole pieces are useful asillustrated in Figure 3-76. These may be machined from soft iron and attached to the poles of magnets.Pole pieces should be designed to have the least reduction in cross-section, consistent with spacerequirements.Figure 3-76. Using Pole Pieces to Improve Magnetic Contact.e. Determine the magnetic field requirements. The magnetic field recommendations (field strength andduration) for inspection of holes and surfaces are shown in Table 3-9. Note that these are recommendedstarting points; actual requirements are those that produce acceptable inspection replicas.
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