Code One Magazine: Design for Manufacture

Design for Manufacture photo The structures, wing carry-through sections of two variants of the JSF, are the product of the Airframe Affordability Demonstration, or AAD.

As workers in California put the finishing touches on the two flying JSF prototypes (called the concept demonstrator aircraft), results from AAD and several related projects will be used in Fort Worth to refine the design and full-scale production methods of the JSF for the next phase of the program.

“While the concept demonstrator aircraft are designed and constructed to address key requirements of propulsion and flying qualities, the structures built in AAD and in other affordability initiatives are designed to verify that our production methods are sound and cost effective,” explains Bill Saathoff, the lead engineer for the AAD integrated product team.

At $30 million, AAD is the largest of four projects related to JSF manufacturing. The prime motivation for these projects, which total more than $125 million, was to investigate innovative ways to reduce manufacturing costs. Teams of engineers and technicians from many companies shared their best ideas and invited ideas from manufacturing experts in other industries. They applied their findings — a combination of manufacturing advancements, best practices in industry, unique approaches, and a healthy dose of common sense — to representative structures of the Lockheed Martin JSF design. In total, production-relevant structure that addresses over eighty percent (by weight) of the airframe has been designed and built through demonstrations like AAD.

Assembly Times Reduced From Days To Minutes

Design for Manufacture photo The results from AAD quieted vocal skeptics and even surprised AAD team members themselves.

One assembly process that normally takes days to complete was reduced to minutes. “We loaded all the parts in the upper wing in a half-hour,” Saathoff says. “That process typically takes about two weeks.

The results are better than anything we expected. We didn’t use any magic — just a lot of hard work in consistently applying some innovative principles.”

Some may question why fighters haven’t been built this efficiently all along. The answer lies in recent technology advances that collapse the gap between computer-aided design and computer-aided manufacturing. Today’s software can weave a digital thread from design through assembly—and beyond. In the past, each step along the way from design to fabrication to assembly presented its own limitations. In the pre-computer age, each step may have required iterations of blueprints. In the early post-computer age, the steps required iterations of basic wire-frame digital models. Both methods duplicated effort, with chances for confusion and error to creep into the process at every iteration.

“We turned everything upside down in AAD,” explains Martin McLaughlin, the integrated product team leader for the JSF airframe and a Northrop Grumman engineer. “In the past, we had a confusion of databases that contained engineering data, tooling data, fabrication data, and assembly data. None of them were connected. Tooling and fabrication assembly approaches were developed from physical masters—plaster mockups or jig masters of various types. We have done away with all of those physical models. We start with a digital three-dimensional solid model and use it throughout all aspects of the design, build, and checkout phases.

Design for Manufacture photo “Today’s design tools are more matured,” McLaughlin says. “We have three-dimensional solid models of every part and every tool. Before we ever assemble a single part, we can simulate that assembly process on a computer. Such a digital environment was not possible until the last few years. Now we can ship a digital model of a part to a supplier electronically. The supplier cuts the part to that digital model and then checks the part against that model. When we receive the part, it fits.”

A digital environment also provides a foundation for further improvements. Design features can eliminate specialized tooling in the assembly process. Fabrication methods can create large complex shapes as a single part. Automated drilling processes can significantly reduce scrap and increase precision. And “lean” production approaches can address everything from how a part moves through a production line to how the tools used for assembling an airframe are organized and located.

“Intuitively, we know that the assembly span time accounts for a lot of the costs associated with the manufacture of fighter aircraft,” says Saathoff. “A shorter assembly span is more efficient. Shrinking that span requires us to pay greater attention to how we fabricate all the detailed parts that arrive at the assembly line. We have to make these parts quicker, more accurately, and more cheaply. We have to package them more sensibly. We have to deliver them exactly to the point they are needed on the line.”

Determinant Assembly And Self-Locating Parts Improve Efficiency
Assembling a major component of a fighter aircraft can become a daunting task—even for seasoned veteran mechanics. As the AAD designs evolved, engineers made the design requirements more straightforward with a technique called determinant assembly.

With this technique, parts were configured to assemble one way and one way only. To prevent a part from being loaded incorrectly, for example, critical tooling holes were asymmetrically located across the center of a part. Precision tooling holes and offset pads were designed into jig-located parts. Moreover, similar parts from different versions of aircraft can now be assembled in the same fixture without unique locating details. These key concepts helped make fixture loading simpler and faster. It also reduced the complexity of the assembly jig and, thereby, lowered the cost and delivery time of these critical tools.

AAD reduced time on the line because of the small features designed into the structural components. The features — small tabs, pre-drilled pilot holes, and raised locating surfaces — do away with most of the tooling normally used to position and hold structural pieces before they can be drilled and fastened together. In essence, the tooling becomes part of the structure itself. The technique (called designing for assembly) was borrowed from the automotive industry, where it is used in some areas of the production line. The AAD team applied the technique across the entire assembly process for the airframe structure.

“We can eliminate more than ninety percent of the tooling with this technique,” says Saathoff. “We also eliminate costs associated with building those specialized tools, storing them, maintaining them, and using them repetitively.” The tradeoffs are minimal. The self-locating features add only a little weight, a few grams per part. The cost savings are substantial.

Module Mating Process Simplified
These self-locating parts come together to form large structural pieces of the airframe called modules. How these modules join to form an entire airframe provides another area ripe for improvement. “Traditionally, modules are attached in a complex process that involves a lot of drilling, redrilling, and countersinking,” Saathoff explains. “We then route a wide variety of hydraulic and electrical lines and high-pressure tubes through the structure. The upper surface or skin of one module overlaps the structure of the adjoining module. The overlapping skin is positioned, drilled, and fastened. Then workers run the lines and tubes through the entire structure.

We want to minimize drilling in the module mating process and simplify the tube and line routing.”

Instead of drilling and fastening overlapping skins for the JSF, the bulkhead on one module forms the interface between modules. The bulkhead has integral tubing fittings to simplify the routing of larger high-pressure tubes. Those tubes are joined at the bulkhead itself. Lines and tubes are routed at the module level and joined with “quick” connectors during the module mating process. The approach can reduce the time for mating modules from two weeks or more to less than five days.

Fighter airframes by their very nature are complex structures designed to handle high g forces, carry heavy payloads, withstand battle damage, deal with engine heat and vibrations, minimize radar returns, provide room for a host of electronic equipment and sensors, and allow easy access for maintenance personnel and weapon loaders. And those are just a few design demands. These demands not only influence the design, but they also affect material selection.

Advances in computer design tools allow engineers to consider these demands as well as manufacturing requirements earlier in the design process. Com-plexity is transferred from the production line to the computer chip. Instead of addressing production issues as they arise, engineers can anticipate problems and work out the entire production process before any metal is cut or any resin is cured.

Fabrication Advances Reduce Time And Cost
Assembling parts into modules and mating modules together to form airframes define only a portion of the manufacturing picture. How those parts are fabricated is just as critical. “Our approach requires more sophisticated communication between the company and its suppliers,” says McLaughlin. “Getting there takes an attitude change on both sides. We have to be willing to work with suppliers and elevate them to a higher level. They have to deliver parts that are ready for major assembly. And that requires a clear understanding of the design and the assembly process.”

Suppliers will use the digital description of the JSF to apply some of the latest fabrication methods to materials selected for the design. These methods include high-speed machining, resin transfer molding, and fiber placement.

The rotational speed of the cutter bit and the feed rates are the primary differences between high- speed machining and traditional numerically controlled machines. While the bit on a traditional cutter spins at 2,000 to 5,000 revolutions per minute, the bit on high-speed cutters spins as fast as 30,000 rpm. The faster cutter speeds allow high-speed machining to form metal parts faster than standard machining techniques. The higher speeds also produce cleaner cuts, are less likely to warp the metal, and cut to higher tolerances. The machines are used to produce large intricate parts, like bulkheads, out of aluminum and aluminum alloy.

Resin transfer molding is a fast process for making smaller composite parts to very precise dimensions. The process is ideal for some of the JSF substructure and for several access panel doors that require super high tolerances. Resin transfer molding involves injecting resin into a two-part mold that contains carbon fiber material. The material is placed in the bottom half of the mold. After the mold is closed and clamped, resin is pumped under pressure into the mold cavity. The resin wets the reinforcement and cures to form the composite part. “We design the molds so we can make more than one part at a time,” Saathoff says. “Stiffeners within a door can be molded in one shot with resin transfer molding. Other methods would require a secondary bonding operation to add the stiffeners. So the process can save some additional steps as well.”

Design for Manufacture photo Fiber placement uses computer control to automate the production of complex composite parts that conventionally require extensive hand lay-up. The AAD team used the process to create complex skin structures, like ducts, wing skins, and larger doors. Fiber placement machines start with small strands of resin-impregnated carbon fibers called tows. A collection of tows is fed into a computer-controlled placement head and come together to create a larger carbon fiber band. A heated roller in the placement head laminates the band over the surface of a form called a mandrel. After applying all the layers of composites needed to form the part, the part is cured in an autoclave at precisely controlled temperatures and pressures.

Still, the best way to build some parts is to lay them up by hand. But even this more traditional method has been improved with modern technology. Instead of following paper-based work instructions and large templates, workers place the carbon fiber material corresponding to laser-projected three-dimensional images of the proper size and ply direction. The trimming of large composite parts, such as wing skins, has been improved as well. Instead of securing a part in a large fixture designed specifically for trimming it, the part is placed on a generic table with a flexible grid of suction cups that act like a vacuum clamp. A computer-controlled router then trims the part according to the digital model of the design. The method reduces the amount of specialized tooling and improves accuracy.

New Technology Revolutionizes Inspection Techniques
Inspection techniques for large complex composite parts have been vastly improved as well. Since composite parts are not formed from a single stock of raw material, like aluminum or titanium, that can be tested for integrity before the part is made, they require sophisticated forms of non-destructive inspection. The preferred inspection technique involves ultrasonic frequencies — basically, inducing a known frequency at the surface of a part and measuring the returned signal to gauge the part’s structural integrity.

Conventional methods for ultrasonic testing are water-based. A jet of water is sprayed at an angle perpendicular to the surface of a composite part and a transducer in the spray head induces and measures the resulting wave action. Internal delaminating or other flaws show up as irregular waves. The process, though most often computer-controlled, is slow and requires specialized fixtures and a lot of setup time. A newly developed laser-based technique offers a vast improvement. The technique uses pulsed lasers to generate and detect ultrasonic signals in the parts. An interferometer and sophisticated computational system then measures the resulting surface vibration. This laser-based inspection method requires no specialized tooling and a fraction of the setup time of water-based testing. As a result, laser ultrasonic testing can reduce inspection times by ninety percent.

Improved inspection methods are also coming to the assembly line, where tolerances within two-hundredths of an inch are often needed. A portable inspection device called a Metronor uses infrared-sensitive digital cameras and a hand-held pointer that contains light-emitting diodes. The pointer is placed on the surface of the assembly to be inspected. A computer analyzes the digital image and triangulates the exact position of the tip of the pointer.

“The system can be used to check hole positions after drilling, the thickness of any material, and the proper placement of design features like webs and flanges,” Saathoff explains. “Historically, such inspections have relied on elaborate coordinate measuring machines positioned on large granite slabs in special temperature-controlled rooms. The Metronor is fully portable and about one-fourth the cost of a large coordinate measuring machine. We can take it to the part and set it up in a few minutes.”

Vastly Fewer Holes And Automated Drilling
A lot of time and effort on any aircraft production line is spent drilling and filling holes. For JSF, engineers have investigated ways to reduce the number of holes, automate the drilling of most of those that remain, and standardize the fasteners used to fill them.

“We reduced the number of holes in the upper wing surface alone by two-thirds,” notes McLaughlin. “Our JSF design requires fewer fasteners because more of the structural load is carried through the surface of the airplane than through its substructure. This design approach requires stronger and larger wing skins made possible by high-strength composites. Our wing skin uses a laminate of a syntactic film sandwiched between advanced composites. The material is very strong, light, and thin.”

“Spacing the substructure farther apart and carrying more load through the skin got rid of one-third of the fasteners,” McLaughlin continues. “We eliminated another third by integrally stiffening the skins rather than adding detail stiffeners.”

Automated drilling takes care of about ninety percent of the remaining holes. The automated drilling system used in AAD can drill holes at a rate of six per minute. A computer-controlled laser-positioning system drills and countersinks fastening holes precisely according to hole locations prescribed by the digital description of the design. The system consists of a gantry that positions the drill head in two dimensions over a large surface. (The wing carry-through structures in AAD measure twelve by fourteen feet.) The drill head is mounted on a swiveling and extending arm that provides movement in three more axes (for a total of five). The system can position the bit at the correct angle even on curved and complex surfaces and drill and countersink holes at speeds programmed for the material type and thickness. The system is easy to maintain and requires no special foundation, which further reduces tooling costs.

“The automated system can drill, ream, and countersink in the same operation,” says McLaughlin. “Hand drilling requires an initial undersized hole that is stepped up to the final size. Then the hole is reamed and countersunk. That’s four steps with four separate drill bits. Furthermore, with digital control, we have drilled as many as 1,500 holes with one drill bit. By hand, we can typically drill about twenty holes per bit before the bit has to be replaced. Now that saves a lot of drill bits, but that’s not my point. When we can control the feed and speed of the bit precisely, we get higher quality holes, which make fasteners much easier to install. Every hole is the right diameter and the fastener slides right in.”

The drilling and filling process is further simplified by standardizing the fasteners. “Normally we use thousands of different fasteners on an airplane,” says McLaughlin. “Computerized databases now give us an electronic bill of material, so we can more easily monitor the total number and types of fasteners used. We have implemented a fastener usage policy for JSF. We have about ten standard fasteners, and engineers must go through a special approval process to use a nonstandardized fastener in the design.”

Visualizing Future Fighter Production Lines
These advances, innovations, and new techniques come together on much different production lines. “The assembly line will be very clean and very flexible,” says McLaughlin. “By flexible, I mean the line will always be evolving to accommodate changes in production rates and manufacturing improvements. A one-meter square grid pattern on the floor will make it easier to plan and implement these changes. Utilities, like shop air, vacuum, and wiring for test equipment, will be routed through a false floor. We will use air bearings to move the assemblies around.”

“The work cells will be more self-contained,” adds Saathoff. “We won’t have much paper lying around because work instructions will be in an electronic format. Instead of separate inspection and assembly processes, laser-based inspection systems will provide realtime assembly checks. Instead of having a subassembly area located next to the production line, subassemblies will show up at the line ready to go into the aircraft. Parts and fasteners will be assembled in kits before they are brought to the assembly line, and the kits will be sequenced according to the order they are to be installed. But the biggest visual difference between current and future production lines will be the absence of specialized tooling and the layout of the factory to facilitate product flow.”

The workers on the JSF production line will be more involved in configuring their own work space and in controlling how parts flow through it. The digital thread will provide more direct contact with the entire design. Production will be less strenuous since the whole assembly process can be analyzed ergonomically on computers beforehand. Shrinking the amount of time needed to produce a fighter will allow improvements to be implemented much faster.

Setting And Beating The Standard
The F-16, widely recognized as the most cost-effective and most capable fighter on the market, sets a tough standard for any new fighter to beat. The Fighting Falcon’s low cost can be largely attributed to its lightweight fighter legacy—paradoxically, its original designers addressed cost by maximizing air-to-air performance. To them, maximizing performance meant minimizing size and weight. Since then, Lockheed Martin has maintained the cost of the F-16 even with vast improvements in capability and reductions in production rate (from 165 aircraft in 1993 to twenty-seven aircraft in 2002). While many of the practices that made these unique achievements possible for the F-16 are being applied to the JSF, all fighters will benefit even more from advances made possible by the digital revolution.

Engineers are taking the results from AAD and from other projects to refine Configuration 230-5, the Lockheed Martin design for the next phase of the JSF program. “A lot of what we do on AAD is a matter of common sense,” notes Saathoff. “The JSF is the first fighter program that, from its outset, is focused on cost to the same level as engineering. We negotiate cost with engineering. We negotiate the tolerances needed. We negotiate materials and even performance. We find dollar savings wherever we can. This focus on cost represents a fundamental shift in design philosophy. The JSF is a revolutionary approach to fighter design.”

Eric Hehs is the editor of Code One.