'Superfit' Greatly Reduces Major Source Of CMM Errors

Researchers at the Commerce Department's National Institute of Standards and Technology have developed a computer model that greatly increases the accuracy of the most commonly used type of coordinate measuring machine (or CMM) probe, potentially benefiting makers of the automated inspection equipment and the thousands of manufacturers that use CMMs to check the shape and dimensions of their products.

The new "SuperFit" software contains a mathematical representation of the behavior of touch-trigger probes used on about 98 percent of the some 30,000 CMMs in U.S. factories and laboratories. When programmed with data gathered during a simple test, the model anticipates and then compensates for so-called probe-lobing errors, a chronic and typically large source of measurement uncertainty in touch-trigger-probe CMMs.

The new NIST software can reduce probe-lobing errors by as much as 90 percent. "For a typical measurement, the improvement reduces probe-related measurement error from a level of more than 6 micrometers to less than 1 micrometer," explains W. Tyler Estler, physicist in the NIST Precision Engineering Division.

The advance comes just as major patents on the touch-trigger technology, held by foreign organizations, are about to expire. When patent protection ends later this year, new suppliers are expected to enter the market and drive down the price of touch-trigger probes. With the NIST innovation and its anticipated adoption by manufacturers of CMM probes, significantly improved performance looms as another prospective benefit.

Several U.S. CMM manufacturers are now evaluating the software. Working with the NIST team, they must determine whether the model meets the specialized requirements of their particular products and whether subsequent refinements and adaptations are cost effective.

"This is the most technically advanced model of probe errors developed to date," says Thomas Charlton, director of engineering at Brown and Sharpe Manufacturing Co., the Rhode Island-based maker of CMMs. "It's not yet ready for prime time, but it shows tremendous potential. As the accuracy of CMMs continues to improve, this model has the potential to largely remove one of the major remaining sources of measurement uncertainty."

Today, CMM users must pay a premium to eliminate the measurement uncertainty caused by probe-lobing errors. They can purchase CMMs equipped with probes that are not prone to the problem. But these specialized probes are as much as 10 times more expensive, according to Charlton, than the touch-trigger variety, placing them beyond the budgets of many CMM users.

The NIST model could erase much of the trade-off between price and accuracy, says Steven Phillips, head of NIST's large-scale coordinate metrology group.

Probe-lobing errors are a long-recognized limitation of touch-trigger probes. They result from variations in the small, unaccounted-for displacement that occurs during the instant between when the probe's needle-like stylus first touches the surface of the part and when an electronic switch inside the probe is triggered to record the stylus's position. During that brief span, force exerted on the probe bends the stylus. Because the magnitude of the force depends on the direction of the probe's approach to the part, the amount of bending, displacement and, therefore, error varies accordingly.

In addition to the so-called non-isotropic probe-lobing response, the NIST model accounts for friction and other non-uniform influences on probe performance, Phillips explains.

Working with two George Washington University scientists, the NIST team characterized the mechanical workings of touch-trigger probes and the factors involved in displacing the probe stylus. Phillips says the evaluation revealed that lobing errors are the result of regularly repeating mechanical interactions, which are strongly influenced by the direction from which the probe approaches a part. In turn, these interactions generate "highly repeatable systematic errors," he says. These findings were presented at a recent conference of the American Society for Precision Engineering.

Using the results of the evaluation, the researchers developed a detailed mathematical model of touch-trigger probe mechanics. The model enabled them to determine the probe's variable movements and to compensate for the resultant measurement errors.

Using a standardized sphere, a CMM maker or user can generate the data needed to tailor the model to a particular machine and probe set-up. Phillips says the entire process should take less than 10 minutes. Only when the probe setup is changed by installing a new stylus, for example does the software need to be updated with new data.

NIST and industry representatives are now discussing how best to make the model available so that it is adopted and applied on the floors of U.S. factories. One CMM manufacturer already has decided to refine and integrate the NIST error-compensation technology into future products.

NIST research leading to the error-compensation software was partially supported by the U.S. Air Force and NIST's computational metrology project. As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.