"We want to modify the traditional processes in an aim to reduce cost, waste and cycle time while maintaining high standards of accuracy. Waste control and accuracy are the two major concerns of the present-day industry," said Jerry Halley, an associate technical fellow of Boeing's Aircraft & Missile Division in St. Louis. "An emphasis remains on part accuracy because of the recent technological advances in the aerospace field that can affect assembly cost and time."

Highly accurate monolithic parts have significantly reduced assembly costs. However, they have also led to high levels of waste according to Halley, considered by many to be an innovator of high-speed machining in aerospace. "Aerospace manufacturers incur a significant loss in the transfer between raw material-to-finished product, and hopefully advances in our newest technology, near-net machining, will remedy the waste control problem while maintaining accuracy standards," said Halley.

Near-net machining is the next era of aerospace manufacturing, and the offerings of this developing technology are numerous and desirable. The goal of this technology is to provide manufacturers the opportunity to create accurate parts from billets that are closest to the actual size of the finished product.

"The very aim of near-net technology is to eliminate unnecessary use of raw materials by creating a closer-to-actual-size part production," states Halley. "The associated obstacles of high-speed and thin-to-thin machining processes must be addressed to further advance the near-net machining concept."

HIGH-SPEED MACHINING

Aerospace manufacturers need high-speed machining to meet the demands of the industry for more products in less time and with less initial material. The modern aerospace machinist is well practiced in the technique of high-speed machining. However, merging that with the near-net process still presents a unique set of advantages and disadvantages.

"Today's high speed machinist is concerned with two fundamental points of concern with the actual machining of the material," said Halley. "One being the placement of the tool tip on the material to ensure stability of both the tool tip and the part material itself. How the tool is being used is also very important relative to the cutting parameters and the toolpath. The dimension of the billet has to become the third factor."

Together, aerospace machinists and manufacturers have determined techniques for optimal production. The procedures differ from material to material.

Chatter, part deflection and residual stresses are the common obstacles faced by the machinist. Many provisions, such as tool speed and cut depth, are made to reduce their effects on production. However, these effects are the limiting factors affecting the expansion of near-net technology within the aerospace field.

THIN-TO-THIN MACHINING

Aerospace technology requires intricate parts both large and small. The production of intricate parts presents a host of obstacles for aerospace manufacturers. "Presently, large billets of raw materials are required to create an accurate finished product," said Halley. "The trade off between the part weight and the raw material weight can be as low as 2 percent."

Thin-to-thin machining is a very delicate process. Using today's popular machining techniques, production of parts from dimensionalized billets of raw material is quite troublesome.

"Ultra thin, lightweight parts are very difficult to make from things that are already thin and lightweight," said Halley. "Thin-to-thin machining, as it is referred, is a very difficult process in which to maintain accuracy."

A machined part can be very flexible, and during the machining process it deflects and vibrates, tremendously affecting the accuracy of the part. Halley says that a larger billet provides stability with the additional bulk present to provide more material to grasp in order to stiffen the material while the part is actually being cut.

Chatter is then avoided by proper process parameter selection, greatly reducing the chance of inaccuracy. The very nature of aerospace equipment demands extreme accuracy due to the volume of stress placed upon the equipment in flight.

"We like stock plate, since it is uniformly heat treated, aged and then mechanically stretched," says Halley. "This causes the material to yield a small percentage, which results in a more uniform stress state in the stock. The part cools more quickly closer to the surface, which causes a temperature gradient and thus a stress gradient, which means the stress is not constant across the thickness. While the stretching improves this, It does not completely resolve the situation.

"The reason we like plate is that the range of residual stress is smaller, which results in less part movement. We can use the stock that has not been cut to stiffen the area where we are cutting, and tool the part so we do not need to use fixtures or other tooling."

NEAR-NET MACHINING

Machinists, scientists and manufacturers have worked to develop a more cost-effective machining technique. Incorporating innovative thin-to-thin machining technology with traditional high-speed machining processes leads to near-net machining. Still in its infancy stage, Halley notes that near-net possibilities have become more realistic for the aerospace industry-in terms of cost efficiency and capability, over the recent years.

The near-net machining concept builds upon the concept of monolithic part production and less assemblies. "Monolithic machining is producing very accurate parts for airplanes," says Halley. "A forward fuselage on an F-18 that once required over 40 components is being worked down to one component, and done more accurately."

Monolithic production allows machinists to work more exclusively with the core of the billets, greatly reducing impurities because of the core material traits. As these structures do not require plate material, they could be made from near-net practices. Also, the obvious advantages of fewer assemblies are reductions in machining time spent on a piece of equipment and less wasted material.

"We always knew that we could make these really nice parts from a single billet, but making all parts from near-net is technically beyond us right now. This is because of the problems incurred by excess vibration and chatter," says Halley.

But he adds that advancement in monolithic production is the first step toward incorporating near-net machining into the process of manufacturing aerospace equipment. The idea of creating a complete component from a billet that is closest to the size of the actual part is not obtainable yet in many cases.

"Don't confuse our limitations with near-net machining with a disinterest in this new technology," adds Halley. "The aerospace field views near-net machining as one of the next steps toward the future of airplane manufacturing in a more accurate and cost-effective manner."

Near-net machining is the next era in aerospace manufacturing, and the offerings of this developing technology are numerous and desirable. Providing manufacturers the opportunity to create accurate parts from billets that are closest to the actual size of the finished product, near-net machining could prove to be the solution to even more advancement in reducing costs, saving time and reducing material waste.

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