Figure 7-32.--Damage measurement techniques.
angle of look, and circumferential orientation by
Polaroid Camera Equipment. It is relatively easy
which you obtained the defect photo.)
to make a comparative measurement if Polaroid
equipment is available, such as the photograph
4. By comparative measurement, apply the
shown in figure 7-32. For the Polaroid method of
magnified scale increments from the photo of the
estimating damage, use the following steps:
scale to the photo of the actual defect. These two
photos should be at the same relative magnification.
1. Position the rotor to obtain the best view of the
defect, usual] y normal (at a right angle) to the defect
Comparative Assessment. If photographic
and centered in the field of view.
equipment is not available, the comparative
assessment becomes more difficult. You can still
2. Obtain a Polaroid photo of the defect.
make a comparative assessment successfully by using
the following procedure:
3. Using a full-scale cross section of the engine
for reference, locate a scale (machinist 6-inch rule) in
1. Position the rotor at the optimum rotation
the relative axial and circumferential position outside
angle to view the defect.
the compressor case. Withdraw the borescope probe
with the camera attached. Hold the borescope aligned
2. Use a sample blade (if available) and mark a
with the center line of the borescope port and obtain a
similar or depiction of the blade defect. Place your
photo of the measurement scale. (Holding the
sample blade in the relative position of the installed
borescope aligned with the center line of the
defective blade on the outside of the engine.
borescope port means using the same position, axial,
