Casting Conversion Fly to the Forefront at Boeing.

Following a directive from its top executive, Boeing is examining conversions to castings as a means of part consolidation to reduce airplane costs.

It is rare for the top executive officer of a company as large as Boeing to become involved with the design of the individual components that make up his firm's products. It probably is even more rare for one to recommend to his engineers the manufacturing method for them to source their metal components. But for Boeing and its Commercial Aircraft Group (BCAG), the time for "thinking outside the box" had arrived and its president and COO, Harry C. Stonecipher, knew it.

In 1998, Seattle-based BCAG reportedly earned less than $0.01 on every dollar from airliner sales. Although the firm had more than a 70% share in the global commercial airplane market, the increased pressure from airlines to reduce costs made this revenue figure unacceptable to the firm's management and shareholders. In addition, BCAG had set before itself the goal to "achieve the industry's lowest cost structure" as one of the four elements in its long-term business strategy.

In September 1998, Stonecipher believed he knew part of the solution. He participated in a videotaped interview, which was made available to his engineers, encouraging them to use more cast components, instead of forgings, assemblies or fabricated parts, in the design of the company's airplane frames and supporting structure. (Its airplane engines, which utilize castings to a large degree, are manufactured by outside suppliers and shipped ready to install.)

The impetus behind Stonecipher's message was that cast components' quality levels had dramatically improved since the '60s and '70s and that, on average, Boeing could achieve at least a 20-35% cost reduction in tooling alone by converting an assembly to a cast component. With more than 3 million components (excluding rivets and fasteners) typically manufactured into one "seamless" airplane, every part consolidation achieved with castings would provide Boeing a cost savings in tooling, inventory, labor, rework, materials, design, testing and manufacturing.

For Tim Scoville, principal engineer, new technology/plane development, BCAG, Stonecipher's message was music to his ears. Scoville's responsibility within BCAG is to develop casting conversion business cases for airplanes in production and under development.

"As airframe producers focus on affordable structures, casting technology has enjoyed greater attention while igniting the imagination of the design engineer," said Scoville. "Design simplification that employs fewer components and fasteners has become more practical with the availability of new and improved casting manufacturing processes.

A Look at the Past

The reason Stonecipher even had to send a message about the opportunities with cast components can be traced to the 1960s when the foundry industry developed a sub-standard reputation among U.S. commercial airplane engineers. Foundries were producing components containing microscopic cracks and holes, which reduced the component's strength and increased its vulnerability to shock and fatigue. To compensate, airplane design standards for castings were developed that forced engineers to add a "fudge factor" to casting design. Termed "The Casting Factor," this design guideline results in a cast component 25-50% heavier than is actually necessary, virtually eliminating the possibility of using castings for structural components. Even more so than in today's automobile industry, weight is a killer in aircraft design.

As a result, cast components were first introduced to the early generations of commercial aircraft engineers with a "defective, design-with-caution" label attached to them. As a result, a design engineer striving to reduce aircraft weight must add weight to produce a casting design.

Another factor that emerged 30 years ago to dissuade casting use in aircraft design was the mindset that itwas easier to design and manufacture simple easy-to-tool components made from readily available wrought metals. These components could be fabricated by a plentiful, low- to medium-skilled workforce with matching pay scales and then subsequently could be assembled into larger assemblies. Despite the multiple tools per assembly (and staggering accumulated tooling costs), this became the recommended path.

Both approaches served the Boeing designers who, during these times, weren't as focused on cost as they were on time-to-market.

But times have changed. Casting quality has changed.

The merger of Boeing and McDonnell Douglas in 1997 and the entry of Europe's Airbus into the commercial airplane market have reshaped the global airplane industry and the two players competing in it. And, as commercial air travel has become more competitive, the cost of new airplanes has been squeezed as well. The result for Boeing has been a renewed statement of its goal of achieving "the industry's lowest cost structure."

Central to this goal, as stated by Stonecipher, is the reduction of components used to manufacture its aircraft. Central to this is BCAG engineers' re-acceptance of castings as a viable alternative for frame and structural components. Currently, according to BCAG, it purchases $58 million of castings/year directly from suppliers, however, far more than that reaches an airplane each year through systems (such as engines) purchased by the firm. If Stonecipher achieves his goal, this figure will rise dramatically.

"Advances in foundry practices over the last 20 years have shown a consistency of properties that rival wrought alloys and forged components," said Scoville. "It now can be said that 'casting factors' are no longer required."

In fact, according to Scoville, an agreement was reached last year between Europe's Joint Aviation Authority and the U.S. Federal Aviation Administration that will allow castings to be designed without a casting factor. (BCAG, however, has not determined how this will affect its approach to casting design.)

"As foundry processes have matured to offer consistently higher properties, thin gages and increased tooling sophistication to provide an avenue for larger, more complex and repeatable products, engineers have begun to look at castings for structural components," said Scoville.

Cockpit Main Instrument Panel

One of the shining examples of Boeing's conversion success is the main instrument panel for the 767-40ER airplane, which houses all the monitors and instruments the pilots use to control the airplane. With Boeing's initiative of decreasing build time cycles and component tooling costs, this sheet metal assembly of more than 296 part numbers used in previous cockpit designs became a target for consolidation and conversion for the 767-40ER. Using investment casting this assembly was redesigned to 11 A357 cast components (Fig. 1) (53 part numbers). Produced by both Citation Precision (Citation Co.), Rancho Cucamonga, California, and Tital, Bestwig, Germany, for up to 20 airplanes/month, the castings are T6 heat-treated, machined, alodined, have zeus rails attached, and are 100% visual and penetrant inspected by the foundry before delivery within 16 weeks.

This conversion resulted in an overall assembly weight savings of 6 lb and a 50% overall cost savings. Boeing reduced its build cycle time on the assembly from 180 hr to 20 hr (89%), its tool count by 90% and its fastener count by more than 600. In addition, Boeing can use this same design for the 777 airplane. Similar conversions are under consideration for other Boeing commercial and military aircrafts.

Investment casting became the process of choice for the converted assembly's 11 cast components for three reasons:

* the required high-quality surface finish (120 micro in.) because portions of the castings are visible in the cockpit;

* the design required the incorporation of several supply air ducts into the castings that could be achieved in investment casting without the use of sand cores;

* wall thickness to 0.070-in.

"The concurrent engineering we performed with Boeing allowed us to eliminate a lot of the fabrication, cutting and fitting that was previously done to make up the previous instrument panel assembly," said Richard Farina, Citation Precision general manager.

According to Farina, one of the other challenges faced by the foundry with these components was maintaining straight walls. Since each piece had to fit together like a puzzle, tolerances were tight. He also credits the success of this casting development to the use of rapid prototypes to reduce the lead time to produce prototype castings. By building prototype waxes of the components and then making molds and pouring, the foundry verified the designs faster and hit the tight time-to-market requirements of Boeing.

Improving Lead Times

Another knock against cast components that reduced their utilization by airplane engineers was lead time. Other simple components could be machined easily without tooling and then assembled together in weeks, while foundries often quoted lead times of up to a year. As the times have changed, however, lead times have shrunk.

"Tools are available today that offer large reductions in lead time for castings and reduce technical risks," said Scoville. "Rapid prototyping allows the engineer to hold a plastic replica within days of CAD solid model generation and a casting within another week or two. Strides also are being made with patternless sand castings that could significantly shrink time-to-market and enhance their use in aerospace."

Further education of engineers as to the rapid prototyping opportunities available to reduce cast component's time-to-market is the first step, according to Scoville.

But Scoville and other casting proponents within Boeing also have decided to take the "back door" approach to casting conversions. Time-to-market pressures often force Boeing's engineers to source a component as an assembly or hog-out because it is the simplest approach to design, source and then subsequently install into the existing design framework. As a result, instead of fighting the time pressures, which could result in a difficult timeline for a foundry to meet, casting proponents look for conversion opportunities in the airplane after it is manufactured and then develop a business case.

"In this scenario, we aren't fighting resistance from engineers under time constraints," said Scoville. "The business case can be developed and presented as a tool to reduce costs. Aside benefit is that the more engineers that have designed castings successfully, the more likely they will look for opportunities in the future."

BCAG's business cases for conversions require thorough analysis, including: cost savings per airplane for conversion; an estimate of man-hours to redesign; an estimate of testing costs; an estimate of the payback period (which must meet the minimum threshold); and sourcing options. As a rule, the firm currently focuses on conversions with at least a 30% cost savings because those are the most obvious candidates for which to champion the cause.

Boeing's Business Case Conversion Successes

One business case conversion success is the main landing gear door uplock support (Fig. 2) for the 767. This component holds the main landing gear door closed during flight. Currently produced as a one-piece, 34 lb, D357 aluminum nobake sand casting by Hitchcock Industries, Inc., Minneapolis, this component previously was a sheet metal design that was made up of 35 different part numbers.

Nobake sand casting was selected to cast this component because of the process' ability to hold tolerances and maintain dimensional stability. In addition, the foundry uses 20 nobake cores within the mold cavity to form the component's shape and help achieve a minimum wall thickness of 0.08 in. in specific areas. The foundry also machines the attach points and installs bushings on the component.

"In this project, we started with 19 drawings that illustrated the geometry of the component," said Tim McKoewn, Hitchcock engineering manager. "The conversion to one casting and one drawing reduced the component's cost by 50% and eliminated 35 parts, 25 shims and 2 assembly jigs."

This component also was a design collaboration between the OEM and foundry in that Boeing went to Hitchcock looking to utilize their design expertise in airplane castings. The D357 aluminum alloy selected by the foundry for this component has tighter tolerances (in regard to purity of alloying) than the more common A357, ensuring the component meets mechanical property specifications.

Another business case cast component conversion for Boeing is the outboard overhead stow bin end frame (Fig. 3) for the 777. This component is attached to either side of the stow bin and supports it by securing it to the overhead framework of the airplane's passenger cabin. Cast in A357 via gravity tilt-pour semi-permanent mold casting by Progress Casting Group, Inc., Plymouth, Minnesota, this component replaced a 17 component stamping assembly. Semi-permanent mold casting was chosen for the process due to the volume of production (1000 pieces/month) and the required surface finish (C40 specification) of the component. Although the component is not visual, the surface finish is required for Boeing's computer qualification.

The stow bin end frame has been designed to withstand a 9G impact load (with the stow bin fully loaded). In fact, in testing, the component has withstood up to 300% of that impact load without breaking. In addition, the component is cast with 0.085-in. nominal walls.

"During the design stages, we met with Boeing engineers once a month to develop components," said Duane Olson, Progress' Director of International Sales and Boeing account manager. "It was a give and take process between our engineers and Boeing engineers to determine what was manufacturable and what was required to withstand the necessary loads."

The foundry supplies a finished casting to Boeing that has been liquid penetrant inspected, 100% X-rayed in the critical areas, solution heat treated, straightened, artificially aged, black annodized and sub-assembled with bushings and nut plates.

The path to more widespread casting use at BCAG still is clouded in uncertainty. Despite the proclamation from above and the growing "excitement" of Boeing engineers, continued education on cast component use and design will be critical. In addition, with every successful casting conversion business case a new engineer is enlightened to the high-flying opportunities made possible with cast metal components.

This ankle originally appeared in the Fall 2000 issue of Engineered casting Solutions.

How to Identify a Conversion Candidate-the Boeing Way

The renewed emphasis on converting assemblies, forgings and machined components to castings has Boeing engineers looking for part consolidation opportunities. Following is a checklist of qualities these engineers look for in a possible conversion candidate;

* a 3-D part that can't conveniently be machined from plate stock;

* an assembly requiring lots of assembly labor and tooling;

* a "basic and stable" part configuration, which does not change due to customer tastes (unless a generic cast part can be simply machined to achieve peculiar differences);

* assemblies requiring lots of shims and rework;

* expensive machined parts requiring 4 and 5-axis capability and/or numerous workpiece repositioning for cutter access to all features.