When General Motors Corp.'s Allison Transmission Div. (Indianapolis) began pilot production at a new plant in Baltimore early last year, the company encountered big problems with a coordinate measuring machine (CMM) used in its main housing and fabrication cell.

"We had an inaccurate and unreliable CMM bottlenecking our start-up," laments John Russell, senior quality engineer at the plant, which builds transmissions used in Chevrolet Silverado and GMC Sierra pickup trucks. "We were experiencing over 30% downtime, and even when the CMM was running, the results it reported were highly suspect."

Russell spent the first six months of start-up trying to get the touch-trigger CMM to work correctly. Sixteen machine tools were waiting for release into production, and the quality department couldn't tell the manufacturing department if the tools were truly ready. A single CMM was jeopardizing the company's entire factory launch. The Baltimore plant was designed on a lean manufacturing concept, and transmissions produced are shipped directly to the GM assembly line without further checks, making quality and process reliability especially critical.

"We needed a better solution," says Russell. "The CMM sits right in the middle of the machining cell. There is no redundancy and a relatively small work-in-process buffer. If the CMM goes down, we don't make main housings and we have less than 10 hours before we starve the assembly line."

Fortunately, in another part of its facility, Allison had recently installed a scanning CMM--supplied by Carl Zeiss IMT Corp. (Minneapolis)--that Russell knew had been working well. When Russell asked Zeiss if it could solve his problem, the vendor "really stepped up to the plate and delivered a home run solution," as Russell puts it, recommending a complete system built around the Zeiss Prismo scanning CMM.

The Prismo is equipped with a climate-controlled enclosure that protects the CMM against contaminants on the shop floor, supporting measurement accuracy of I0.0002 inch. With hundreds of features to measure on the main transmission housings, Allison relies heavily on CMM automation. While the original problem CMM measured 227 features in a 50-minute cycle time, the Prismo measures approximately 270 features in less than 46 minutes, says Russell.

The Zeiss system includes an automated two-pallet shuttle to the CMM. While one part is being measured, another is queued on the load shuttle. Climate controlled at 68 F, the load shuttle normalizes the queued part to the correct temperature, so no time is lost waiting for parts to cool. "We have virtually eliminated CMM idle time," says Russell. "If the CMM completes its measurement cycle over a break period, the queued piece is automatically loaded and the next cycle commences. We have increased our verification capacity by 20% while decreasing our unplanned downtime to less than 1%."

The Prismo's operating UMESS software supports true geometric tolerancing, enabling the CMM to accurately measure size and position of bore holes to within 0.0002 inch. Allison also uses Zeiss' Strata SPC analysis software in the Prismo CMM to determine machining trends and create graphics that enable operators to make needed changes before out-of-tolerance conditions are reached, says Russell. "For example, if we're measuring an inside diameter and we see it getting smaller over time, we know that the tool is wearing and needs to be changed," he explains.

Since its installation in October 2001, the Zeiss Prismo scanning CMM has significantly reduced the process variation in main housing manufacturing. Allison was having difficulty maintaining a consistent internal diameter. When the plant switched to the Prismo CMM, Allison discovered its measuring variation dropped from 15% to less than 5%. "We had been fighting this problem for months," Russell says. "Once we changed over to the Zeiss Prismo, the problem disappeared. It turns out that we weren't fighting machine variation at all. We were chasing measuring variation."

Allison has gone from using CMMs as tools for parts inspection to relying on CMMs as means for improving its overall manufacturing process. By providing Allison with more information for understanding what is happening with its pro-cesses and why, the Prismo allows operators to be proactive, Russell says. And that, he observes "is the biggest engineering benefit of all."

Carl Zeiss IMT Corp.
(763) 744-2400 Reply 10
http://www.zeiss.com

BENEFITS

• A new CMM enabled Allison Transmission to boost measurement throughput by 20%, while reducing downtime from more than 30% to less than 1%.
  
• Measuring variation dropped from 15% to less than 5%.
  
• Allison operators can now act proactively, making process changes and adjustments before processes reach out-of-tolerance conditions.