The use of multi-axis CNC machining on the Pratt & Whitney (P&W) alternate fuel turbopump shrouded impellers has greatly improved manufacturability and quality of the part. In particular, the machining of the high pressure fuel turbopump (HPFT) at Metalex Manufacturing, Inc., represents a quantum leap in complex contour machining technology.

Previously, complex contours were created by using templates which represent the cross section of a given part. The template is linked to a cutting tool, and the desired shape is created by essentially tracing the outline of the template with a cutter. This pantograph process, as it is known, is fairly accurate for two dimensional shapes. However, as the complexity of a shape increases, the machining accuracy drops dramatically. Multi-axis milling is better for creating complex shapes like the HPFT. Upon P&W's request, Metalex, a machining supplier in Cincinnati, Ohio, put together a cross-functional team to develop the capability to produce this part on a CNC machine.

Initially, Metalex worked with Makino, who provided a custom made horizontal machining center with high-speed accuracy in all five axes. The next challenge was finding cutting tools that were capable of machining such an intricate part. These cutters are required to control the dimensions of the part as well as to maintain rigidity over relatively long distances without interfering with the geometry of the impeller.

Ultimately, Metalex designed and fabricated their own tools which were able to take full advantage of the 5-axis milling system. Concurrently, Metalex began the mammoth task of writing the CNC programs. Utilizing CAD/CAM technology, a team of programmers worked full-time for three months writing the CNC code. Taking advantage of the increased degrees of freedom available from the multi-axis cutting tables, the programmers were successful in accessing the hard-to-reach areas of the internal flow paths. This has eliminated all need for hand blending on the impeller.

The teamwork between Metalex, P&W and Makino appears to have paid off with improved manufacturabiltiy of the shrouded turbopump. The use of the high-speed, 5-axis milling machine to make finer cutter passes has resulted in a dimensionally accurate part with exceptional surface finish. Part set-ups were reduced to two and tool cutter changes are now automatic. The equivalent milling time, representing almost 95 percent of the total part work, has been reduced from more than six months to less than one and a half months.

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