Equipment and Procedures.
Magnetic particle inspection equipment which magnetizes with AC or DC is used to demagnetize parts after inspection,
depending upon the demagnetization features included in the equipment and the size and shape of the part.
The most common type of stationary demagnetizing equipment consists of an open coil through which alternating
current at line frequency, usually 50 to 60 hertz, is used. The demagnetizing coil may be equipped with a stand or may
be constructed and placed on a bench. Usually larger sizes also have a track or carriage on which parts can be placed to
To use a demagnetizer coil, such as that illustrated in Figure 3-36, the part is placed in the coil and the current turned
on. While the current remains on, the part is slowly withdrawn from the coil a distance of 4 to 5 feet before the current
is shut off. The axis of the part should be parallel to the axis of the coil for regularly shaped parts. On complex parts,
more complete demagnetization is sometimes possible if the part is rotated and turned end for end. For best result the
diameter of the demagnetizer coil should be just large enough to accommodate the part. However, for practical
purposes one or two sizes of coils will satisfactorily serve an inspection facility. When demagnetizing, small parts in a
large coil, keep the parts close to the inside wall or corner of the coil since the demagnetizing forces are strongest in
Figure 3-36. Part in Demagnetizing Coil.
Some stationary AC equipment with a coil on rails has a toggle switch, which enables the inspector to turn the current
on in the coil, and leave it on. This coil then becomes a demagnetization coil when a part is drawn through it while the
current is flowing. It is important to remember that the part must be drawn completely out of the magnetic field of the
coil before the current is shut off.
Care must be used in demagnetizing small parts using machines equipped with
"step-down" demagnetizers, which do not have adjustable current tap switches. A
small part such as a bolt being circularly demagnetized with this equipment may be
overheated with the initial high current steps.