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Figure 3-75.  Preparation for Magnetic Rubber Inspection
Table 3-9.  Magnetic Field Strength and Duration Recommendations

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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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.







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