However, like most of the industry, NASA also has budgetary constraints forcing it to look for ways to reduce costs while maintaining or even heightening quality standards. This sounds contradictory, but through a newly developed process, NASA has satisfied all objectives in manufacturing one part in particular—the Pratt & Whitney (P&W) alternate fuel turbopump with shrouded impeller.

Compared to present manufacturing techniques, the use of multi-axis CNC machining on the P&W alternate fuel turbopump shrouded impellers has greatly improved both manufacturability and quality of the part. In particular, the machining of the high pressure fuel turbopump (HPFTP) represents a quantum leap in complex contour machining technology.

The second impeller is a titanium part with six sets of blades, one long, medium and short blade per set, equally spaced around a hub, and covered on both sides with a shroud. The impeller functions in series with two other impellers to increase the pressure of the liquid hydrogen flowing through the turbopump.

Metalex Manufacturing, Cincinnati, Ohio, is machining the second impeller on a custom-configured Makino MC98 horizontal machining center with high-speed accuracy in all five axes. Previously, the complex contours of the second impeller were created through a pantograph process, which was very expensive and required extensive benchwork.

The pantograph method uses 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 is fairly accurate for two-dimensional (2-D) shapes. However, as the complexity of a shape increases, the machining accuracy drops dramatically.

In order to compensate for the use of 2-D templates, hand blending is performed to attain the final complex contour with a continuous surface. Not only is this process time consuming, but substantial tolerance ranges must be accepted, and product irregularity and inconsistency is inherent. Additionally, the pantograph method requires numerous part set-ups and cutter changes.

Upon P&W's request, Metalex approached Makino of Mason, Ohio, and put together a cross-functional team to develop the capability to produce the second impeller on a CNC machine. Starting from a clean slate and not taking into consideration how the second impeller had been manufactured in the past, Makino and Metalex set out to develop a better solution, each taking on separate developmental responsibilities.

Makino's application team studied the part and came up with the current process utilizing their experience with titanium and Flush Fine machining. Flush Fine machining is a high-speed, high-definition and low-heat machining process pioneered by Makino. Like nickel and cobalt alloys, titanium is a difficult metal to cut, because it is a heat resistant material. When titanium is cut, it will not absorb any heat. Instead the heat is directed back into the cutting tool—overheating the tool and forcing it to fail.

"We knew right away we would use our Flush Fine technique," said Steve Colston, Makino Aerospace Group Sales and Marketing Manager. "Flush Fine blasts away chips and prevents heat buildup in the tool for greater thermal stability and chip control by combining high spindle speeds with precisely controlled, high pressure coolant. This provides superior accuracy and finish, as well as high metal removal rates and longer tool life. We had done titanium machining in the past and felt very comfortable applying our process to this application."

While Makino provided the machine, machining process and heat sensitive cutting techniques, Metalex was challenged with finding cutting tools that were capable of machining such an intricate part. Ultimately, Metalex designed and fabricated their own tools. The tools Metalex developed are able to take full advantage of Makino's MC98 five-axis milling system. These cutters are required to control the dimensions of the part while maintaining rigidity over relatively long distances without interfering with the geometry of the impeller.

Concurrently, Metalex began writing the CNC programs. A team of programmers worked full-time for three months writing the CNC code. With Makino personnel on site each segment of the program was proof tested both electronically and on a machining center test piece. Taking advantage of the increased degrees of freedom available from the MC98 multi-axis cutting tables, the programmers were successful in accessing the hard-to-reach areas of the internal flow paths.

This process, specially designed to machine a shrouded impeller, produces more consistent blade airfoil profiles, significantly reduces manual blending and machining operations, provides tighter dimensional control, improves the balance response of the impeller, and results in less part-to-part variation.

The use of Makino's MC98 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. And, while this process is still on the learning curve, the equivalent milling time representing almost 95 percent of the total part work, has been reduced from more than six months using the pantograph to less than a month and a half with the CNC method.

NASA required an out-of-balance specification for the part of five grams. After machining the second impeller on Makino's MC98, it came in at under a gram of unbalance, giving the part a much longer usage cycle. A part that once required extensive benchwork now comes off the machine in balance and practically ready to go. Using the pantograph method, NASA would get only three flights out of the second impeller. Now with Makino and Metalex's CNC multi-axis machining process, NASA expects to get up to ten flights.

The teamwork between Makino and Metalex has paid off with improved manufacturabiltiy of the shrouded turbopump. In fact, the first flight quality impeller was completed at Metalex with zero discrepancies.

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