T.O. 33B-1-1
3-118
c. Select method of magnetization. Magnetism may be applied with portable electromagnets (yokes),
permanent magnets, or conventional magnetic particle inspection equipment. In areas of limited
accessibility, 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 other
small parts whose shape makes magnetization difficult with an electromagnet. The magnetic fields
produced in large parts by permanent magnets are often quite low and unpredictable; therefore, they
should not be used on such parts, unless a specific procedure has been developed and verified. Central
conductors are best suited for fastener and attachment holes particularly when there are multiple layers
of materials.
d. Select the method of magnetic contact. Field strength is greatly reduced when there is poor contact
between the magnet and the test piece. To improve contact, auxiliary pole pieces are useful as
illustrated 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 space
requirements.
Figure 3-76. Using Pole Pieces to Improve Magnetic Contact.
e. Determine the magnetic field requirements. The magnetic field recommendations (field strength and
duration) for inspection of holes and surfaces are shown in Table 3-9. Note that these are recommended
starting points; actual requirements are those that produce acceptable inspection replicas.
