Direct Contact Demagnetization

T.O. 33B-1-1 3-81 NOTE Circular demagnetization is particularly effective on parts of complicated shape, such as multiple throw cranks or coil springs. This same equipment will also have a rheostat or current control switch which enable the inspector to select different magnetizing current levels as well as initial demagnetizing current levels. These switches may be provided with a motor drive. When equipment with a motor driven switch is used for demagnetization, the inspector places the part in the equipment and presses the demagnetization switch which causes the motor to drive the switch contactor from maximum to minimum current positions, giving a shot at each successively lower current value. This effectively demagnetizes the part and can be used either by passing the current through the coil on the equipment (longitudinal demagnetization), or by passing the current through the part itself (circular demagnetization). This process is referred to as "step-down" demagnetization. 3.6.6.1.5 Two methods are used to circularly demagnetize parts: the direct contact and central conductor methods. The method used depends upon the part's size, shape, and the method used to magnetize it. Generally the same method used to magnetize is used to demagnetize a part. Though the methods used may be the same, the kind of current required to demagnetize may differ from that used to magnetize. For example, parts having large cross sections, which have been magnetized using AC, may require step-down reversing DC to demagnetize them. The use of reversing DC overcomes the lack of field penetration, which occurs with AC. 3.6.6.1.6 Direct Contact Demagnetization. Demagnetization using the direct contact method is accomplished by alternately reversing and reducing the current in a part. The part may be clamped between contact heads on a stationary unit having provision for demagnetization, or cables may be connected to it and to a suitable demagnetizing current power supply. Starting with a current amperage greater than or equal to that which was used for magnetizing, the current is reduced to either zero or a very low amperage. Either AC or reversing DC may be used depending on the size, shape, and retentivity of the part. The AC demagnetization is usually less time consuming and is satisfactory for many small to medium-sized parts. However, for large parts or parts having thick cross sections, step-down reversing DC is required. A step-down reversing DC demagnetization is usually completed in about 30 seconds - one second per step. The one second at each step allows time for the field in the part to reach a steady state, at which time induced currents become zero, permitting maximum penetration of the field into the part. This can easily be done using a continuously variable auto-transformer or electronic decay circuitry to reduce the AC current to zero. 3.6.6.1.6.1 Parts having a complicated geometry or that have been magnetized using more than one current path through the part, may not be completely demagnetized in one demagnetizing cycle. The same number of demagnetizing cycles may be needed, and through the same current paths, as were used for magnetizing. Quite often with small, low retentivity parts, instead of such repeat demagnetization on the same part, a satisfactory and quicker demagnetization can be obtained using coil demagnetization with AC or reversing DC. 3.6.6.1.7 Central Conductor Demagnetization. The information contained in paragraph 3.6.6.1.6 and paragraph 3.6.6.1.6.1 also applies to central conductor demagnetization. Demagnetizing currents should start from the same or slightly higher amperages than were used for magnetizing. Placement of the central conductor or threaded-cable configuration should be the same as that used for magnetization. Sometimes different central conductor locations or configurations must be used and be determined by experiment. 3.6.6.1.7.1 To circularly demagnetize a part by direct contact, clamp the part between the contact heads. Demagnetization is accomplished by automatically passing shots of decreasing current through the part. Care must be taken not to demagnetize very small parts between the heads because the high initial current can overheat the parts. If longitudinal