How do optical 3D measurements of cooling holes work?

The quality control of cooling holes is crucial for increasing the efficiency and ensuring the safety of high-performance systems – thanks to cooling holes, the overheating of turbine blades that are used in aircraft turbines and gas turbines can be avoided. Even if heat-resistant materials are used, the surface must be cooled because the surrounding gases are in the range of the melting point. By blowing cooling air through these holes, a thin insulating layer is generated between the combustion gases and the blade. Thus, the position, orientation and shape of the holes play a major role in the safety of the turbine.

In addition to other measurement tasks like, e.g., the break edge measurement, the geometric verification of cooling holes is a challenging task in 3D metrology on aerospace parts.

The difficulty in the 3D measurement of these holes is based on several aspects: Turbine blades consist of many differently shaped cooling holes which are, on top, difficult to access. Measuring their geometry by means of computer tomography or fiber probes is either limited by their resolution, their ability to measure deep into the hole or their long measurement times. In contrast, Vertical Focus Probing, which allows the optical measurement of slope angles ≥ 90°, offers many advantages: Measurement times are shorter, the resolution is higher, and non-contact measurements as well as measurements deep into the hole are possible.?

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