Collaboration on Target, NIST Invention Heads for the Factory

A computer model developed by National Institute of Standards and Technology researchers can substantially improve the cutting accuracy of a piston-turning machine used by U.S. automakers, according to results of recent tests conducted at NIST for Giddings & Lewis—maker of the high-precision machine tool—General Motors Corp., Ford Motor Co., and the National Center for Manufacturing Sciences, which managed and funded the collaboration.

Improved accuracy will translate into higher productivity and higher precision in the manufacture of automobile-engine pistons. Because it eliminates the need to warm up machines before production begins, the technology also is expected to generate energy savings for engine makers.

The successfully completed tests conclude a 13-month phase of the NCMS-organized collaboration. The industrial partners now will focus on steps to incorporate the NIST-developed technology into new Giddings & Lewis machines for GM and Ford.

"This project perfectly fits our mission of improving U.S. manufacturing productivity as well as enhancing the competitive position of the U.S. machine-tool industry," says Jay Nilsson, NCMS project manager.

During several days of testing at the NIST facility, aluminum piston billets were machined over six-hour periods. In an attempt to simulate actual production conditions, certain environmental conditions were varied and production stoppages corresponding to plant coffee breaks and lunch hours were incorporated into the testing periods. Results showed significant improvements in the billets machined with the NIST temperature-compensation technology as compared with those machined without it. On the basis of these results, NIST experts believe that as much as an 80 percent reduction in the variation of piston size could be achieved.

In practical terms, this means that the unmodified machine, which Giddings & Lewis introduced commercially in mid-1992, already can cut automobile-engine pistons so precisely that the dimensions vary by less than half the diameter of a human hair. With the NIST software, which compensates for temperature-caused changes and other sources of "errors" that limit accuracy, dimensional variation in the slightly oval parts can be reduced to far less than one-tenth the diameter of a human hair.

For engine builders, hair-splitting differences are not trivial. "Now, a manufacturer may make one size of pistons for an hour or so and then switch to another size class," explains Ken Drew, Giddings & Lewis project engineer. "With the computer model you can go a step farther and make any size class on demand."

Martin Suchoski, a senior project engineer at the GM Technical Center in Warren, Mich., expects additional payoffs: reduced machine "dead" time, less scrap and higher productivity.

Because the software corrects for expansion and contraction as the machine warms and cools, he says, the equipment need not run idle for about an hour until attaining a steady operating temperature for production. It also eliminates the need for relief help when operators take a break because the machine can be shut off and restarted without harming its performance.

Although logistics and details are now being worked out, says Giddings & Lewis's Drew, "the model will definitely be put into the machine." He adds that Giddings & Lewis intends to pursue other potential performance enhancements identified during the collaboration.

"This is a well-focused project," says GM's Suchoski. "We have usable results and they are going to be applied."

Earlier consultations between NIST and Giddings & Lewis researchers led to modifications in the machine's final design. Those changes helped to create stable temperature patterns inside the machine, reducing irregularities that can distort the geometry of machine components and, as a result, impair performance.

The idea of using software-based methods to improve the performance of machine tools and coordinate measuring machines was pioneered at NIST's Manufacturing Engineering Laboratory.

"The underlying concept is that the accuracy of machine tools is limited—to a significant extent—by geometric imperfections and non-random irregularities, or errors such as thermal deformations," explains Alkan Donmez, who led the NIST side of the collaboration with Giddings & Lewis, GM and Ford. "Because these errors are often very repeatable, they also are predictable, which means you can build predictive computer models for real-time error compensation."

Brown & Sharpe and Sheffield Measurement (now part of Giddings & Lewis) were among the first to adopt the approach, applying it commercially to coordinate measuring machines, which are used to inspect the dimensions of parts. Donmez and his colleagues are continuing to refine approaches to enhancing machine-tool accuracy and to extend the range of applications.

"This successful project is another example of the laboratory's mission, namely to apply our expertise in collaborating with industry to solve meaningful technical problems that affect U.S. competitiveness," says Dr. Michael J. Wozny, the director of the Manufacturing Engineering Laboratory.

A non-regulatory agency in the Commerce Department's Technology Administration, NIST promotes economic growth by working with industry to develop and apply technology, measurements and standards.