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Computers/software help GM Achieva deadline.

GM's Oldsmobile Div needed a set of nine aluminum wheels of a totally new design in less than 29 days to enter its Achieva SCX performance car in Motor Trend's 1992 Car of the Year competition. By applying CAD/CAM technology, WheelTek Div, Amcast Industrial Corp delivered them in 25 days. "Receiving in less than a month nine road-worthy wheels that can meet the rigorous Car of the Year testing program approaches the miraculous," says Ed Welburn, chief designer at Oldsmobile Exterior Studio Two, where the Achieva was styled.

At the time Achieva SCX was nominated to participate in the Motor Trend competition, entry deadline was less than eight weeks away. The car was not scheduled for production until several months later, and its planned introduction to the public was four months after that.

Although prototype components for a basic Achieva had been fabricated, the SCX performance version required additional components, most of which could be pulled together relatively quickly. The wheels, however, did not exist and presented the longest leadtime to produce because they had to be styled, designed, engineered, manufactured, and tested.

Wanting a distinctive design for the new wheel, engineers at GM's Lansing Automotive Div designed and sculpted a new clay model of the wheel face. Using a coordinate measuring machine (CMM), engineers digitized the surface of the model--the mathematical coordinates were recorded on tape as a wireframe model using a CATIA CAD program and then converted to an IGES file.

It was 29 days before entry deadline when WheelTek was advised that they had been chosen to produce the wheels. A WheelTek engineer began designing a CAD model of the rest of the wheel, including the rim, hub, valve hole, pilot bore, snap cap, and lug hole. His design would later be added to Oldsmobile's CAD model of the wheel face.

Conventional production of aluminum wheels are cast. But time was too short to develop the tooling necessary for the casting process. The only choice was to machine the wheels directly out of 6061 aircraft quality aluminum billets. WheelTek prepared drawings of machining fixtures and arranged with an outside supplier to perform the critical profile machining of the wheels from the aluminum billets. Concurrently, a WheelTek engineer was combining the two CAD models into a single model.

Everything was moving along until day four of the program. Subsequent to GM placing the order for the 15" wheels, testing of the Achieva led GM engineers to conclude that 14" wheels provided superior performance. As a result, GM changed the order, necessitating changes to the CAD models and putting the program five days behind schedule.

Regenerating the CAD wireframe wheel model required more than "photographically" scaling down all of the wheel dimensions by the same proportion. Certain dimensions, such as the lug bolt circle, must remain the same. In addition, other design modifications were necessary to provide clearance for the brakes and pilot bore. "A lot of design decisions had to be made in reducing the wheel size just that one inch," said Mr Welburn, "and any one of them could have turned a pretty neat design into something pretty ugly. The WheelTek engineers who made the design modifications demonstrated real creativity with an aesthetic eye in retaining our original styling intent. We've received many compliments on the design of the SCX wheel."

After what seemed like continuous communication between GM and WheelTek, the new CAD wireframe model for the downsized wheel was finalized 18 days before deadline.

The CAD model was then converted to a CAM program to develop CNC machine-tool cutter paths to produce a pattern. Once the cutter paths were verified, the wheels could be machined with the CAM cutter path program or with a duplicating machine tracing the surface of the pattern. Although the latter method was used, either would have been just as productive. Machining began with just 12 days left before deadline and was completed five days before deadline.

All of the wheels were tested for balance and uniformity tested for radial and lateral runout and first and second harmonics. One wheel was rotary fatigue tested to assure cornering integrity. The nine remaining wheels were painted, boxed, and hand-delivered to GM four days early. "The wheels were still warm when WheelTek delivered them," said Vic Ide, Oldsmobile's manager of Specialty Cars, "a real impressive performance."
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Machining Controls & Software
Publication:Tooling & Production
Date:Jun 1, 1993
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