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New manufacturing technologies deliver lighter and stiffer 2014 Corvette
7 October 2013
|Aluminum resistance spot welding in Bowling Green. (Photo by Joe Imel for Chevrolet) Click to enlarge.|
General Motors’ $131-million investment in technology at the Bowling Green Assembly Plant, such as the first production use of a GM-patented process allowing aluminum to be spot welded to aluminum (earlier post), is producing the strongest and most precisely built Corvette in its six-decade history.
The new technologies enable more accurate and efficiently produced subassemblies, such as the frame and the components attached to it. Enhanced, laser-based three-dimensional inspection systems verify overall assembly tolerances targeted to be 25% tighter than the previous-generation Corvette.
Approximately $52 million of the investment went to a new body shop that manufactures the car’s all-new, lightweight aluminum frame in-house for the first time. The frame is not only the foundation for the car’s greater driving capabilities, but the platform on which the 2014 Corvette Stingray is more precisely constructed. It is 99 pounds (45 kg) lighter and is 57% stiffer than the previous-generation frame, resulting in a chassis so strong that the convertible model needs no structural reinforcements.
It is also the most complex frame design in the Corvette’s history, featuring main rails composed of five customized aluminum segments, including aluminum extrusions at each end, a center main rail section and hollow-cast nodes at the suspension interface points—all with varied thicknesses that make the most of the strength and mass requirements of each respective section.
Assembling the frame requires more advanced joining processes and more precise inspection methods to ensure strength and dimensional accuracy. That is where aluminum welding, Flowdrill-type fastening and laser welding help ensure the high-quality targets for the frame.
Flowdrill fastening. The Corvette Stingray’s frame features 188 Flowdrill-machined fasteners with structural adhesive. The fasteners are installed by a high-speed drill that extrudes the frame material to create a strong, integral collar that is then tapped for screw-type fasteners. It is a GM first for body structure joining.
Thermal friction drilling, showing Flowdrill and Flowtap tools. Source: Flowdrill.
Flowdrill fastening joins closed sections, where only one side has open access and where arc welding could cause heat distortion or weaken material. Dimensional quality is also maintained, eliminating the need for post-assembly machining.
Aluminum resistance spot welding in the Body Shop. (Photo by Joe Imel for Chevrolet) Click to enlarge.
Aluminum resistance spot welding. Pioneered by GM, aluminum resistance spot-welding process is an efficient method for joining aluminum to aluminum where there is two-sided joint access. It is particularly effective with the thicker materials—up to 4mm—used on the new Corvette’s frame. It is also used for welding aluminum extrusions, die castings and aluminum sheet metal. There are 439 aluminum resistance spot welds on the Corvette Stingray coupé.
The process is used in the Corvette’s aluminum structure tunnel subassembly and in mainline attachments of various components. Additionally, licensed suppliers use the process to produce a few subassemblies for the car.
GM’s new resistance spot welding process uses a patented multi-ring domed electrode that does what smooth electrodes are unreliable at doing: welding aluminum to aluminum—and it does it more cost-effectively than other methods of joining aluminum. The multi-ring domed electrode head disrupts the oxide on aluminum’s surface to enable a stronger weld.
Laser welding. Laser welding is used in the frame’s tunnel subassembly to attach sheet aluminum closeout panels to the tunnel structure. The process enables continuous welding quickly when only single-sided access is available.
Laser booth turntables prepare for welding. (Photo by Joe Imel for Chevrolet) Click to enlarge.
Additionally, the precise beam of high energy used in the welding process minimizes heat beyond the weld area for improved structural accuracy, and the laser creates a leak-free joint that does not require additional sealing, which could add weight to the frame structure.
There are two robotic laser-welding stations in the plant—one with a pair of robots and another with a single robot. Each robot has a dedicated laser power generator and together, they lay down 71 segments for a total of 37 welded feet on every frame.
Laser vision inspection. Laser-based vision inspection for quality assurance now includes Perceptron-supplied tools to monitor critical points on every Corvette body that comes down the line.
By checking every car and major assembly in the plant, process variation can be seen—and addressed—immediately. Tighter tolerances on parts and new, improved tooling for the Corvette’s various assembly procedures are helping the plant achieve approximately 25% or greater improvement in meeting tolerance targets.
Perceptron is essentially a three-dimensional measurement system that uses fixed and robotic-mounted Helix laser sensors, or cameras, to monitor critical build features. It is a three-part “in process” quality inspection for frame rail assemblies, uniframe bodies and composite bodies with a fully dressed cockpit.
On the composite body alone, 39 specific points are measured. It takes approximately 2.5 minutes to measure each car in station to within 0.25 mm. One hundred dimensional measurements are taken with each frame assembly.
The 2014 Corvette Stingray will deliver up to an EPA-estimated 17 miles per gallon in the city (13.8 l/100 km), and 29 mpg (8.1 l/100 km) on the highway, making the new Stingray the most fuel efficient sports car of its type on the market.
The EPA estimate of 17 city and 29 highway is for the Corvette Stingray equipped with an all-new, seven-speed manual transmission. The estimate reflects an average of fuel economy in both the default “Tour” mode, which delivers 28 mpg highway, and driver-selectable “Eco” mode, which delivers 30 mpg highway. For Stingrays equipped with the seven-speed manual transmission, Eco mode enables Active Fuel Management, which disables four of the cylinders for improved fuel economy during light engine loads.
The highway rating represents an 11% increase in fuel economy over the previous Corvette, while the 455 hp of the 6.2L LT1 V-8 represents a 6% increase over the previous Corvette. The LT1 delivers 460 horsepower with the available dual-mode exhaust.
The 2014 Corvette Stingray is the seventh generation of the sports car, which has been built exclusively at the Bowling Green plant since June 1981. Shipping of the Corvette Stingray Coupe to dealers is underway. It starts at $51,995 including destination. The Corvette Stingray Convertible, which goes into production later, is priced at $56,995 including destination.
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