Re-Engineering A LEGEND

Manufacturing Engineering, Sep 2005 by Koganti, Rama

Ford has applied low-investment processes and flexible manufacturing to build its GT supercar

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-inwhite 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-inwhite 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 500°C (950 °F). Then a graphite coated blank is introduced into the tooling and also heated to 950 °F. 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.