The inspection design for the Air Force’s Retirement For Cause (RFC) program uses automated eddy current and ultrasonic inspection systems to detect flaws in the compressor and turbine components of the F100 engine. The requirements of the program include the inspection of bolt holes, cooling holes, and oil drain holes in the various engine components. The hole diameters range from 0.060″ to 0.600″ with many of the components having 60 to 70 holes of a given size. The spacing between holes is fairly accurate; in general, variations are less than 0.010″. Since the initial system accomodates approximately 20 different engine components, the only practical method of clamping the components to the systems’ turntable utilizes a set of jaws that clamp outwards on a component’s bore or rim. This method requires the operator to first align the part on the turntable. Accuracies on the order of 1 to 2 degrees are expected (0.200″ at a 7″ radius). The inspection of holes is accomplished using rotating eddy current probes having a diameter 0.010″ less than the hole diameter. For a reliable inspection, the probe must be centered in the hole to within 0.001″ of true center. As stated above, initial probe placement may be 0.200″ off center for the first hole and up to 0.010″ off center for succeeding holes (see Figure 1). This means that centering must occur while the probe is outside of the hole. The resulting requirements for a hole centering algorithm are:
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