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.
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.
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.
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.
Bolt Hole Preparation.
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.
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