Lift Off Compensation

T.O. 33B-1-1 4-68 probe in a helical pattern through the length of the hole. This equipment maintains a constant speed of revolution. Results can be retained on a strip chart recorder or displayed on a storage oscilloscope. For most applications, no additional equipment is required. Occasionally, a special shim is necessary to provide a flat surface on which to position the probe stop or scanning unit to maintain good alignment between the probe and the hole. 4.6.1.8 Lift Off Compensation. The lift-off adjustment for bolt hole inspection is dependent upon the surface quality and dimensions of the hole. Optimum lift-off adjustment is that which just suppresses lift-off variations within the hole, but does not provide excessive compensation. Excessive lift-off compensation can reduce sensitivity and increase noise related to changes in lift-off less than the amount of adjustment. When using unshielded probes, specific amounts of lift-off adjustment can be obtained by using a shim between the coil of the bolt hole probe and the hole wall. The thickness of the shim must equal the amount of lift-off adjustment desired and must be relatively tough to prevent tearing during insertion and removal of the probe. Flexible plastic or Teflon tape can be used for this purpose. Lift-off adjustment is usually performed in the hole at a point at least 1/4 inch from the edge or at the center if the part thickness is less than 1/2 inch thick. The practice of performing a lift-off adjustment by pushing the coil away from the hole wall can lead to indefinite amounts of lift-off adjustment and should be used only with caution. More tolerance in lift-off adjustment settings is permissible when using automatic scanning equipment or shielded probes. 4.6.1.9 Sensitivity Settings. The sensitivity settings are based upon response to a specified reference standard. A wide variety of test standards are employed for bolt hole inspection. They include cracked parts, electrical discharge machined notches, notches cut with a jeweler's saw, differences in conductivity standards, and a multitude of other standards with larger notches and/or cracks. Each individual procedure usually specifies the standard to be used and the required response in terms of meter deflection or indication size on a recorder, strip chart, or scope. When it is necessary to find small flaws and the possibility exists that different types of probes (coil size and frequency) may be used, it is preferable to use a reference with the same approximate dimensions as the flaws to be detected such as the jeweler saw cuts or electrical discharge machined notches. Standards which affect a large volume of metal such as the conductivity standards and large notches do not provide consistency of response from the same crack for all probes. 4.6.1.10 Meter Testing And Scanning Speed. Scanning speed and pattern must be considered in the calibration procedure with meter equipment. Since the probe response with manual scanning will not be the same as that during mechanized scanning, calibration SHALL be performed at the same scanning rate employed during inspection. 4.6.1.11 Bolt Hole Preparation. NOTE Inspection must not be performed on holes which are offset at interfaces. Holes in mating surfaces must be realigned prior to eddy current inspection or redrilled to a larger diameter which is concentric through the mating parts. Prior to performing bolt hole inspection, all foreign material must be removed from the hole. Foreign material can include sealant, lubricants, metal slivers and paint chips. Usually this material can be removed using cotton swabs and a suitable solvent. Holes which are severely damaged during service or during fastener insertion or removal may require reaming prior to eddy current inspection. 4.6.1.12 Manual Bolt Hole Scanning Procedures. Manual scanning of bolt holes is performed at specified levels throughout the depth of the hole. Inspection is usually initiated with the probe core positioned immediately within the upper or lower edge of the hole so that the outside edge of the core is even with the surface of the part. The probe core position is adjusted to the specified level below the collar of the probe and the probe inserted into the hole until the probe collar rests against the surface of the part If inspection is performed for fatigue cracks parallel to the length of the hole, the inspector observes the meter for any