It will come as no surprise to learn that non-metallic composite components are used increasingly in the manufacture of commercial and military jet engines to reduce engine weight, increase customer payloads, and provide optimum sound suppression. The application of non-metallic materials to jet aircraft engines is relatively new, and there continue to be innovations in the field of composites.

Machining composite materials poses a number of challenges, so there is much to be learned from shops and plants that are now doing it successfully. One of those plants belongs to the General Electric Aircraft Engine (GEAE) group and is located in Albuquerque, New Mexico. One of the most important lessons this plant has learned about machining composites is that every element of the process must be considered carefully. Successful machining of composites requires the right machine, the right fixturing, the right cutting tools, and so on, only the degree of "rightness" required is far higher than typically encountered in most machining applications.

An unusual machine tool installed a few years ago to machine composites provides a case in point. Ultimately, the exact "rightness" of this machine depended on the type of spindle with which it was equipped. With the right spindle, all of the other right choices regarding type of cutting tools and cutting speeds and feeds could be made.

GEAE was attracted to the Albuquerque facility in 1967 because of the large machining capability available there. The plant had previously been operated by American Car & Foundry, Inc. (ACFI) under contract to the Atomic Energy Commission (AEC). It is a state-of-the-art facility and one of two sources for composite components in GEAE.

Management at the Albuquerque plant, in concert with Evendale Engineering (the Evendale, Ohio, facility is the principle assembly point for GEAE) became convinced early on that new methods for machining the very abrasive composite material would be required to meet industry standards. Compared to monolithic structural plastics, reinforced composites including fiber-reinforced matrix, metal matrix, and ceramic matrix, layered up in a multiplicity of configurations, provide attractive strength-to-weight ratios, fatigue resistance, thermal stability, and high temperature operation.

According to a GE spokesperson at Albuquerque, a number of projects were introduced to improve composite manufacturing following GE's purchase of the Albuquerque property from the U. S. Air Force in mid-1984. The result of one such project was the early-1991 installation of a Canadian manufactured Henri Line Gicamill 19; a gantry-type, five-axis high speed machining center. GE specifically looked for a gantry-type machine that allowed fixtures to be arranged on the machine table, while probing for table alignment and fixture orientation to develop JIT processing.

The Gicamill 19 has double way covers and positive air pressure to keep out particles and dust. No metal components are processed on the Line, which is now utilized on two shifts. A key element of the machine tool's successful operation is its spindle. It is a custom-altered, high-speed 24,000-rpm Swiss-produced Fischer Model MFWS 1424 spindle adaptable to several operations. When Line could not locate an acceptable spindle from American and European manufacturers, Fischer Precision Spindles of Maryland Heights, Missouri, the U.S. representative of E. Fischer AG, Switzerland, helped develop a shorter-length spindle to allow machining inside the ID of a composite component. In place of the standard ceramic bearing package, GEAE opted for precision-ground class-seven metallic bearings. Fischer MFW spindles also incorporate an oil scavenge system for contamination-free machining of composite components.

These wide-band, high-frequency Fischer spindles provide both high horsepower and high rpm to power a variety of tools. The job of tool selection becomes particularly critical given the variety of composite structures to be machined. For all practical purposes, tool steel does not provide the life or cutting efficiency required for producing aerospace composite parts, and carbide tools seem most useful for working foam core rather than solid composites. That leaves the big three: PCD (polycrystalline diamond), ceramic (typically aluminum oxide or silicon nitride fortified with strengthening agents), and CVD (chemical vapor deposited) diamond-coated tools.

Cost-wise, ceramics are up to ten times costlier than carbide tools, but PCD tools are more than half again the price of ceramics. The newer CVD diamond coated tools, both thick film and thin film (Norton/Amplex, for example) are somewhat more expensive. Ceramic tools are excellent for finishing and trimming operations, with high cutting speeds and resulting high tool temperatures often suggesting water-soluble coolants and coolant-through spindles.

PCD is good for rough cutting of highly abrasive composites and can last 100 times longer than carbide; PCD may be best for long machining runs with solidly fixtured, non-vibrating parts. The newer CVD diamond tools typically can cut at even higher speeds in semi-finishing and finishing operations on non-ferrous alloys and a range of advanced components. All of these considerations led to the specification of tools for the production of some very difficult composite parts.