Impeller machining represents unique challenges. Please take a minute to carefully observe the following model.
As Michelangelo observed, we must remove all the excess material from the blank in order to free the design model. First, a simple cylinder will be turned to get the raw blank.
Note that there are two kinds of blades on this split impeller, full and half blades. Both are curved and warped possessing a variable fillet on their leading edges. The intersection between the hub and blade surfaces forms a variable radius fillet. The blades are not perpendicular to the hub surfaces and the intersecting angle is changing as you move down the blade. The height of the blades triples from one end to the other.
The first challenge is roughing out the excess material in between the blades. Multiple depth cuts are needed, but roughly 3x more depth cuts are needed near the leading edge than near the trailing edge. The most efficient cutting method is desired—one where the tool spends only a minimum amount of time cutting air. The tool length needs to be kept to the minimum to avoid vibration and to prolong tool life while allowing the most aggressive roughing possible.
The roughed part should look similar to the one shown bellow:
Stay tuned for future blog posts when the machining process will be covered in greater detail . . .