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Five-axis milling of turbine impellers

Turbine impellers with complex shapes require a 5-axis milling machine to machine every corner of their geometry.

In a modern design system, the geometry that needs to be processed is designed by the CAD system and then converted into the motion trajectory of the tool in the CAM system. The output results are converted into motion for each of the 5 axes of the milling machine through a post-processing program. For turbomachinery impellers, it is necessary to consider dedicated CAM software. From a technological perspective, impeller machining mainly consists of the following five distinct operations:

1. Rough machining of the flow channel between the blades

2. Milling hub part

3. Round the entrance and exit edges

4. Finish the blade surface under small cutting amount

5. Variable radius fillet processing at the blade root

Special attention needs to be paid to areas on the blade surface that the tool cannot reach. Because it is easy to cause interference between the tool and adjacent blades at this time.

Before the adoption of 5-coordinate CNC machining centers, most turbine machinery manufacturers used 3-axis or 4-axis machine tools to process impellers, and most of them used point processing methods, that is, every point on the blade surface is processed into a point through the tool tip. When the tool moves along the blade surface, it will leave some pits or residual sharp corners, and the height value of these pits or sharp corners depends on the programming skills. Point processing is also a feasible method, but this method has some inevitable shortcomings:

1. The surface of the blade is not smooth and some small grooves will be left. These small grooves must be parallel to the direction of flow (see Figure 1).

2. For blades with severe bending, due to the short spatial distance between the blades, it is difficult to avoid interference with adjacent blades during processing.

3. If you want to reduce the impact of grooves on the flow field, a long processing time is required. That is, the tool must move around the blade many times.

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