Vision+

Within the project a 3D measurement procedure for the measurement of the micro geometry of cooling holes in turbine blades has been developed. The correct shape of these cooling holes prevents the turbine blades from melting during their usage in the aircraft, making their inspection an essential part in air safety. The measurement procedure is based on the Focus-Variation principle, and allows, as part of the InfiniteFocus measurement device of Alicona, a fully automatic measurement of these complex parts, for which no optical measurement solution has existed before.

The problem: Geometry measurement of cooling holes in turbine blades

Turbine blades in turbines of aircrafts are highly complex parts, whose form and surface have an enormous impact on the functionality. In particular, these turbine blades contain a large number of cooling holes, through which air flows during their usage. This air builds a small cooling film between the combustion gases and the turbine blade and prevents overheating of the material. During quality control of these cooling holes it is essential to ensure that size, form and angle of the holes, which may all have different shape, exactly match the CAD dataset, that defines their ideal shape.

The problem with the measurement of these holes is that traditional tactile measurement methods, which obtain 3D geometry with a mechanical stylus, are not able to reach the full geometry of the thin holes, which are often smaller than 1mm in diameter. Optical 3D measurement methods on the other hand have become increasingly important in the last decade. However, traditional optical methods so far have not been able obtain 3D information at the steep inner walls of the holes.

Another open problem was the economic necessity to perform the measurement of all holes automatically in a row without additional user-interaction.

The solution: Fully automatic 3D measurement with Focus-Variation

Within the project, the 3D measurement procedure Focus-Variation has been extended, to measure very steep walls as they appear in turbine blades. The Focus-Variation method is a 3D measurement procedure for the measurement of micro parts, where an optical system with small depth of field scans the object vertically. Afterwards height is determined for each point of the object by searching for the height, where the object point appeared most in focus. This procedure is integrated in the InfiniteFocus measurement device by Alicona as shown in Fig. 1, where a turbine blade is mounted in a motorized rotation and tilt- unit. This allows to measure the cooling holes of the turbine blade automatically from different directions. A three-dimensional measurement of a cooling hole is shown in Fig. 2. This shows the complex geometry with very steep side walls, which can be measured at high resolution.

Fig. 1: Measurement device InfiniteFocus by Alicona
with a turbine blade mounted on a motorized rotation and tilt unit.

Impact and effects

With the proposed measurement method, Alicona has achieved an important solution for the large market of aerospace industry. Since Alicona also provides other measurement solutions for this market, such as the edge radius measurement of turbine blades or the measurement of the roughness of the coatings, it is possible to supply the aerospace industry with a large portfolio of inspection solutions. These do not only aim at increasing stability and lifetime, but also at aircraft safety, especially in context of the proposed solution for cooling hole inspection.

Fig. 2: Three-dimensional dataset of a cooling hole of a turbine blade,
measured with a 3D measurement technology realized within the Vision + project.