Direct Contact (Head Shot) Technique

T.O. 33B-1-1 3-33 b. Central Conductor Technique: Electric current is passed through a central conductor that into an opening in the part. These techniques are discussed in more detail below. 3.3.11.2 Direct Contact (Head Shot) Technique. This technique produces circular magnetization by passing electric current through the part itself. Direct contact to parts is generally made by placing them between clamping heads. Lead faceplates and/or copper braid pads must be used to prevent arcing, overheating, and splatter. Wetting of the contact plates with the suspension vehicle before current application helps prevent overheating. On large parts, current contact is sometimes made by clamping lug- terminated cables to the part using ordinary C-clamps. Regardless of how it is made, the electrical contact should be as good as practicable. This will minimize any heating or arcing at the juncture. This requires that the contact surfaces be clean and free of paint or similar coatings and have adequate pressure applied to achieve good mechanical and electrical contact over a sufficient area of the part's surface. Any excessive heating at the contact points may burn the part, affect its temper, finish, etc. 3.3.11.3 Central Conductor Technique. This technique produces circular magnetization by passing electric current through a conductor that has been placed coaxially in an opening, frequently in the center of a part. A magnetizing field does exist outside a central conductor carrying current, so the walls surrounding a central conductor become magnetized making possible the detection of discontinuities that parallel the central conductor. Central conductors are any conductive material such as a copper bar or cable placed in the center of the part to be magnetized. The central conductor technique SHALL be used if longitudinal discontinuities on the inside of tubular or cylindrically shaped parts are to be detected. Theoretically, the magnetic field is zero on the inside surface of such parts unless a central conductor is used. The direct contact technique may not produce reliable results in this case, particularly if the part is a concentric tube or cylinder with good current contact at each end. Either the central conductor or the direct contact technique can be used to detect discontinuities on the outside surfaces of such parts. Because the circular field around a central conductor is at right angle to the axis of the conductor, the central conductor technique is very useful for the detection of discontinuities that lie in a direction generally parallel with the conductor. The central conductor technique is also very useful for detecting discontinuities, usually cracks, which emanate radially from holes. A part having a hole or opening that is to be inspected for inside and outside discontinuities is usually positioned with the central conductor centered coaxially in the hole or opening. On very large parts having large openings the central conductor maybe located close to the inside surface and several inspections made around the inside periphery of the opening. Placing the conductor close to the inside surface reduces the current requirement since the strength of the circular field increases with decreased distance from the conductor. 3.3.11.4 Selection of Current Level. 3.3.11.4.1 General. A number of factors must be considered when determining what current amperage to use for circular magnetization. Some of the more important of these factors are: a. The type of discontinuity being sought and the expected ease or difficulty of finding it. b. The part's size, shape and cross-sectional area through which the current will flow. c. The amount of heating that can be tolerated in the part and at the current contact areas. 3.3.11.4.1.1 Another factor is the relationship between the current and the leakage fields at the surface of the part. The magnetizing force at any point on the outside surface of a part through which electric current is flowing will vary with the current. The greater the current, the greater will be this magnetizing force. Inside the part, just under the point on the surface, the magnetic flux density will be the product of this magnetizing force and the magnetic permeability of the part at that point. It is this magnetic flux density which determines the leakage field strength at discontinuities. Thus, current is directly related to the strength of leakage fields at discontinuities, and it is these leakage fields that capture and hold magnetic particles. The more difficult the discontinuities are to detect, the weaker the leakage fields will be for a given