Dangerous curves

Provocative lines and innovative materials are the hallmarks of luxury car manufacturer Jaguar Cars Ltd.

Jaguar has reaped the benefits of aluminum since 1922, when the company, then known as the Swallow Sidecar Co., designed sidecars out of lightweight aluminum panels. Since then the company has realized even more benefits of using aluminum, and for the past 40 years it has incorporated all-aluminum body frames in its high-performance, limited-production vehicles.

Limited-production, until now.

Outpacing Industry Trends

Consumers want fuel-efficient, safe, high-performance vehicles. To exceed customer demands, the company looked to aluminum. "Aluminum delivers real-world benefits such as improving fuel economy, performance, safety, and emissions," said Ken Close, principal engineer, Jaguar Manufacturing Engineering.

Figure 1 The dedicated 9,600-square-meter Castle Bromwich press shop has three press lines consisting of 13 presses with 18 robots and different automation equipment and more than 87 die sets. Photos courtesy of Schuler Inc.

Unusually, the new Jaguar XJ® has a conventional monocoque structure. A monocoque structure is constructed from stamped parts and gains its strength from these sections, unlike an exotic space-frame construction, which is constructed from extrusions and castings. To create a strong structure, Jaguar's engineers adopted construction techniques from the aerospace industry to assemble the body using rivet bonding (self-piercing rivets and epoxy adhesives).

The aluminum XJ body is 40 percent lighter and 60 percent more rigid than its steel predecessor (see introductory picture). Its lightweight architecture represents a major step forward in aluminum-intensive automotive body structures for high-volume production, Close commented.

By carefully screening potential suppliers and creating a team-oriented environment, Jaguar and its partners were able to overcome the challenges of working with aluminum and pioneer a high-volume-production, aluminum-intensive automotive body structure for the 2003 Jaguar XJ.

Figure 2 The presses are fitted with programmable rams and cushions so the material has more time to move into the die, which reduces cracks during deep draws.

Forming Strategic Partnerships

Jaguar benchmarked all the major aluminum stamping operations before drawing up a best-in-class production process to help it find its partners. Various suppliers that provided technical responses were assessed against the predetermined criteria to determine if they would become part of the simultaneous engineering team.

After the lengthy process—about six months—was completed, Jaguar chose Schuler, ABB Ltd., Bilsing Automation, and Alcan Inc. as strategic suppliers. Because aluminum body structures hadn't been used before in mass production, suppliers took on the role of partners to work together with Jaguar to finalize the equipment's technical specifications as well as installation and launch issues. Extensive training also was needed to help operators adapt to new techniques, Close said.

New Production Process

In 2002 a 9,600-square-meter press shop dedicated to production of the XJ's aluminum panels and other components was built at Jaguar's Castle Bromwich (West Midlands, U.K.) site (see Figure 1). The stamping dies that were launched in this facility were sourced from Nothelfer, Laepple, Fagro, Allgaier, and domestically from Dagenham and Koln.

Simultaneous aluminum test builds during the production of the old steel XJ model helped to prove out assembly techniques. To produce approximately 150 cars daily, Jaguar chose hydraulic presses for their control and flexibility, Close commented.

The presses are fitted with programmable rams and cushions to provide flexibility and optimize the drawing of aluminum (see Figure 2). For example, the draw speed for critical parts can be programmed variably. Hydraulic presses also offer smooth contact with the material to reduce die wear, which helps to ensure careful material treatment.

The slow die closing speed gives the material more time to move into the die, which reduces cracks during deep draws. Programmable retraction speed provides smooth retraction during the pad function of the slide cushion or with spring dies to prevent part damage.

The majority of the XJ structure is stamped aluminum panels, which require three press lines. The first line has five production presses and one tryout press—a 2,000-metric-ton lead press, a 600-metric-ton hydraulic press bed cushion (with four-point-controlled cushions to suit different blank holder forces), and four 800-metric-ton line presses. The tryout press is used 90 percent of the time for tryout but also can be used as a production press for parts with up to six operations. This line manufactures larger structural parts such as hoods, body sides, and door panels.

The second and third lines comprise seven—two 600-metric-ton lead presses with 200-ton cushions and five 400-metric-ton line presses. These are used for medium-sized parts such as package trays, cushions, and reinforcements.

To increase flexibility, these presses can be run as two individual lines or combined to run as one line with up to six operations. The automated destacker units are located at each of the line, and the unload conveyors are located in the middle of the line. The unload conveyors can be moved from press to press, depending on the line configuration.

In the press shop, 87 die sets manufacture more than 127 parts. Twenty-eight of these die sets run on the main 2,000-metric-ton line. The others run on the 600-metric-ton line, and local suppliers manufacture all other small stampings.

Working With Aluminum

Changing from steel to aluminum created production challenges at various stages of the project.

"There were several parts that needed to be deep drawn, which required significant development with computer simulations," Close said. "These techniques provide excellent strain predictions in the panels."

The next challenge presented itself during the tool tryout phase. "Aluminum is prone to springback. The tools need to be compensated so that the final shape meets the engineering design," Close said. "This is an iterative process during the tryout stage and is based on experience and judgment."

Figure 3 For protection, the blanks are coated in oil, wax, or paper during transport. Oil creates destacking problems because it acts like glue between the blanks, and wax builds up on dies. To solve the lubrication challenges, the suppliers worked together to create a sophisticated vacuum gripper system with special tooling for blank retrieval.

Traditional destacking systems rely on steel's magnetic properties to separate the blanks before lifting them off a stack of material. However, Jaguar needed a system that could maintain production volume and protect the aluminum panels from damage.

First lubrication and second for protection, the blanks are coated in oil or wax during transport from Alcan's blanking line in Nachterstedt, Germany, which creates two large production issues. Oil creates destacking problems because it acts like glue between the blanks, and wax deposits and builds up on the dies.

To solve the lubrication challenges, the suppliers worked together to create a sophisticated vacuum gripper system with special tooling for blank retrieval (see Figure 3). Peel cylinders lift the corners of the blanks, and air knives jet air into the space created to separate the blanks. Double-blank sensors monitor the destacker for errors.

Because aluminum is a sensitive material, extreme care must be taken to achieve a flawless surface. To do this, Jaguar designed the press shop to minimize the amount of contaminants entering the building. For example, the 2,000-metric-ton press that stamps the car's outer panels has an integrated washing and lubrication machine in the destacking equipment. The washer removes contaminants on the incoming blank and the oiler relubes the part for production.

"We have cleaning processes for both the press equipment and the dies," Close said. "The destacking equipment is the most sensitive area, and there are regular audits of the wash and the oiler, as well as a cleaning procedure for the rollers and the belts. These processes ensure that the amount of surface damage on the parts during a production run is minimized."

Figure 4 Two areas that are continually being assessed and improved are the destacking equipment and the maintenance of a clean environment.

Planning Pays Off

According to Close, the lines are achieving their target strokes of between 170 and 300 hits per hour and, most important, are running efficiently. Die change times are about 20 minutes on the 600-metric-ton line and 30 minutes on the 2,000-metric-ton line.

Two areas the team is continually assessing and improving are the destacking equipment and the maintenance of a clean environment (see Figure 4), which are critical to the efficient operation of an aluminum press shop, Close said.

"The development of the destacking equipment continues as the line runs higher part volumes," Close said. "The technologies to separate the blanks are continually being assessed and upgraded to drive further efficiency improvements. Cleaning methods and frequencies also are improving on the blank washing equipment and the reoiler," Close said.

"The overall project has been a major success," Close said. "The geometry of certain parts proved challenging in aluminum, but by working closely with the design team, toolmakers, and our suppliers, these parts met their targets in full."

By using nontraditional materials, aerospace construction techniques, and creating a team environment for suppliers, Jaguar pioneered a thoroughly modern model of aluminum production efficiency.

Jaguar Cars Ltd., Castle Bromwich Plant, Chester Road, Birmingham B35 7RA, 0121-373-2141, www.jaguar.com.