Re-Engineering a Legend
Ford has applied low-investment processes and flexible manufacturing to build its GT supercar
By Rama Koganti
Body Manufacturing Supervisor
Manufacturing and Vehicle
Design Research and Advanced Engineering Laboratory
Ford Motor Company
The original Ford GT 40 was a racing machine developed during the 1960s to do one thing: end Ferrari's dominance of the famous 24-hour race at Le Mans. The car became a legend when it swept the podium in its 1966 Le Mans debut, then followed up with victories in the next three consecutive years.
The new Ford GT was unveiled as a concept car in January 2002 at the North American International Auto Show in Detroit. The concept vehicle was well received and Ford committed to production. An engineering team was established by May 2002 to deliver the production vehicle, which went from clay model to production in just 24 months.
The main difference between the original GT40 and the current GT, of course, is that the former was a purebred race car and the latter is destined for the street. Another difference is the materials used for body construction. The GT40 was built using aluminum honeycomb composite materials, and the current GT is constructed using lightweight aluminum alloy materials. It's an aluminum-intensive vehicle.
The 2005 GT features a spaceframe architecture very different from that of the stamping-intensive unibody or body-on-frame construction used for most production vehicles. This spaceframe consists of aluminum extrusions, stampings, and castings joined together using MIG welding, adhesive bonding, and mechanical fasteners.
In developing the design, Ford engineers looked at several technologies that would favor the practical manufacture of a low-volume, high-performance vehicle.
The GT is manufactured at several supplier locations. The spaceframe is produced by TK Budd-Milfab (Detroit), then shipped to Mayflower Vehicle Systems (Norwalk, OH) for e-coating and assembly of the body-in-white panels using using adhesives and fasteners. Next, the body-in-white is transported to Saleen Speciality Vehicles (SSV; Troy, MI) for trim and paint operations before shipping to Ford's Wixom (MI) Assembly Plant for final assembly.
The GT's spaceframe contains 35 detailed extrusions and four large, complex castings for the front and rear shock towers. This design offers an opportunity for part consolidation; for example the rear shock tower casting has mounting brackets and several key attachment points for the side rail, rear crash box, and cross car beam reinforcements.
Castings were made in permanent molds to give a better finish, better dimensional control, and eliminate the porosity characteristic of sand castings.
Primary drivers of the spaceframe architecture were:
- Performance and light weight
- Weight distribution (43/57 front/rear)
- Accelerated time frame (production build within 24 months)
- Cost--minimal capital investment
- Styling--Preserve the vintage GT40 look
Several new forming technologies are being used to produce body-in-white panels for the car. They include superplastic forming for body exterior panels such as fenders, roof, rear quarters, engine cover, door outers, and door inners; roll bonding for floor panels for light weight and stiffness; and friction stir welding of critical interior sections. It is expected that the successful application of these manufacturing and assembly technologies will eventually be applied to later Ford vehicle designs. The hood inner and outer panels are fabricated from chopped fiberglass composite. Front and rear bumpers and rocker panels are manufactured using reinforced reaction injection molding (RRIM).
The complex exterior contours of the Ford GT meant conventional aluminum stamping was not an option. The forms needed for the proposed aerodynamic shape could not be achieved by conventional methods.
Instead, superplastic forming was used. Widely applied in the aerospace industry for spacecraft and aircraft structures where complex surfaces are required, this process uses a one-sided die representing the positive surface of the body panel, instead of the usual two-sided type. In operation, the forming die is heated to 500ºC (950 ºF). Then a graphite coated blank is introduced into the tooling and also heated to 950 ºF. High-pressure air is introduced to the opposite side of the blank, forcing the softened metal into the die cavity. Applied on the class B surface, the graphite serves as a lubricant during superplastic forming.
Pressure ramps are defined depending on part geometry. The rate at which pressure increases must be accurately controlled to achieve the precise strain rate needed for superplastic behavior. In developing the process, manufacturing engineers overcame the problems of localized material thinning, dimensional inconsistencies caused by the cooling cycle, and problems with lubricant imperfections that can damage panel surface finishes.
The aluminum alloy selected for superplastic forming is 5083, a blend of aluminum, magnesium, and manganese with a grain size of 5 - 10 µm. The material has good strength, dent resistance, good formability, and produces good surfaces. A fine grain structure is the main prerequisite for alloys to be used in superplastic forming.
Advantages of the super-plastic forming process include an ability to produce complex shapes and smaller radii than conventional stamping; forming of panels up to 3 X 2 X 0.6 m in one piece; ability to achieve deeper draws than conventional processes; elimination of spring-back; and lower investment costs than conventional stamping.
Challenges include longer cycle times than conventional stamping and the need for secondary operations to produce features such as holes and slots, and to trim the parts.
The GT's composite deck inner is mated with the aluminum outer panel using a patented process. E-glass prevents galvanic corrosion at the aluminum/composite interface.
The GT's deck lid inner panel is produced from composite material for weight savings, parts consolidation, and cost reasons. The composite prepreg is first hand laid-up in a mold, then transferred to an autoclave for curing. When finished, the part has the same thickness and contour as aluminum, but 60% of the weight. The composite deck lid helps maintain the necessary weight distribution, has high stiffness for lower deck closing effort, and provides a good appearance when the decklid is open.
Durability is another key performance attribute. The composite panel is durable enough to withstand under-hood temperatures to 120ºC. To prevent galvanic corrosion where the composite panels are hemmed to the aluminum outer, the engineering team developed and patented a design that added e-glass to the composite panel at the aluminum interface area.
In terms of cost, production of the composite panel is relatively inexpensive for the low-volume and niche specialty vehicle segment. Some savings derive from part consolidation; the initial design called for an assembly of four aluminum panels. Tooling cost savings result from development of one single-sided tool (versus four double-sided tools for the aluminum design). Tooling lead time was also shorter.
Compared with the use of four aluminum panels, total cost savings from use of the composite panel over the life of the GT program are expected to be $2.92 million. The design also provides environmental benefits by eliminating fasteners and adhesives, and allows recycling of scrap.
Roll bonding is another unique process used to produce body panels for the GT. It enables use of 0.20" (5-mm) sheet for stiffness, but panels weigh only as much as sheets 1.4 mm thick.
Roll-bonded aluminum is a sandwich consisting of two sheets bonded together. A coating of graphite or titanium dioxide is first applied to the joining faces in a defined pattern to mask areas where bonding is not necessary or desired. Areas without the coating become joining points when the two sheets are subjected to the heat and pressure of a rolling mill. After rolling, air pressure is applied to expand the clearance between the two sheets--except where a joint exists--yielding the desired panel stiffness. The process is used to produce the vehicle's floorpan.
Welded in predetermined locations then machined in place to establish panel mounting surfaces, Rivnuts are used with an adhesive to join outer panels to the space frame.
To join inner and outer panels in automotive body structures such as doors and deck lid assemblies, and also to form a flange for front fenders and rear quarters, robotic roller hemming is used. The process is well suited for low-volume automotive applications, providing a distinct advantage in manufacturing closures and subassemblies with developed flanges. The primary benefit of roller hemming is the flexibility to handle multiple product variations.
In operation, the outer panels have open flanges between 90 - 130º that are trimmed longer than the inner panel. Open flanges allow the parts to nest correctly within themselves. The panel is broken from a preformed break line, then formed around the inner panel.
This process is well proven for hemming both similar and dissimilar materials. Depending on part complexity, the robot makes two or three passes to complete the hem, and carries a small roller that traverses the periphery of a panel. Pressure is exerted on the flange from robot pressure controls in the head of the roller tool. Parts are nested within an anvil and are fixed in position using an overhead spider frame. The anvil is positioned on a turntable that works as a seventh axis and allows the robot to access all areas of the parts being hemmed. Fully programmable, it enables production of features such as character lines and corners, and has a cycle time of 60 - 90 sec.
Robotic roller hemming can replace some flanging operations, reducing overall investment cost. Tool changeover requires approximately 30 min, and the process can handle 5000 and 6000-series aluminum alloys. On the GT, all closures are adhesively bonded and roll hemmed.
Friction stir welding contributes to the rigidity of the GT spaceframe. In the process, a tool rotating at 10,000 rpm applies pressure to a seam and blends the metal surface, forming a smooth, continuous seam.
Friction stir welding improves dimensional accuracy and creates a joint with strength 30% greater than a conventionally welded joint. It does not use filler material, and adds no distortion-causing heat.
On the GT, friction stir welding is used in construction of the multipiece aluminum housing for the fuel tank. The process attaches the stylized structural members at the bottom corners of the tunnel without distortion or the need for additional sealing.
Rivets are also used to assemble the GT body-in-white. Assembly includes both Rivnuts and pop rivets. Rivnuts are welded in predetermined locations, then machined in place to establish a nominal surface on which to mount the body panels and assure that panels are properly positioned. Some earlier niche-speciality vehicles, for example, the Ferrari Modena and BMW Z8, used shims and washers for panel positioning with less consistent results.
Pop rivets are used, along with an adhesive, to more securely join some panels. The adhesive is a two-part, room-temperature-cure epoxy. It was selected for durability under diverse conditions, cohesive strength, impact performance, and ability to withstand operating temperatures to 120ºC at the engine cover area.
In addition to providing stronger bonds, the adhesive acts as a sealant, corrosion inhibitor, and sound insulator. Adhesive also increases body torsional and bending stiffness and provides excellent impact performance.
The New Ford GT by the Numbers
Ford's 2005 re-interpretation of the GT40 is faithful to the original yet takes full advantage of 21st-century technology. It does not share a single dimension with the original--the new car is more than 18" (455 mm) longer and sits nearly 4" (102 mm) taller.
And it incorporates modern engineering such as using wind tunnel tests to eliminate the aerodynamic lift inherent in the GT40's design--a benefit the original car could have used at 200+ mph (320 kph) on the front straight at Le Mans. The new Ford GT includes racing-inspired ground effects ducting under the rear fascia.
Based on Ford's 5.4-L modular V-8, the GT's engine produces 550 horsepower (410 kW) and 500 lb-ft (678 N*m) of torque. Both figures are comparable to those of the 7-L engine that powered the original car to its first wins in the 24 Hours of Le Mans.
The all-aluminum power plant is fed by an Eaton screw-type supercharger. It features four-valve cylinder heads and forged components, including the crankshaft, H-beam connecting rods, and aluminum pistons. Power is put to the road through a Ricardo six-speed manual transaxle featuring a helical limited-slip differential.
Here's a quick rundown of more GT specs:
- Overall length: 182.3" (4.63 m)
- Wheelbase: 106.7" (2.71 m)
- Height: 44.3" (1125 mm)
- Width: 76.9" (1.95 m)
- Curb weight: 3199 lb (1440 kg)
- Engine: DOHC supercharged V-8
- Top speed: 205 mph (330 km/hr)
- Acceleration: 0 - 60 mph (0 - 97 km/hr) in 3.3 sec
- Braking: 60-0 mph: 110' (33.5 m)
- Approximate cost: $150,000.n
This article was first published in the September 2005 edition of Manufacturing Engineering magazine.