FOR IMMEDIATE RELEASE:             NIST 96-06
Feb. 27, 1996

Contact: Mark Bello                 'SUPERFIT' GREATLY REDUCES
     (301) 975-3776                 MAJOR SOURCE OF CMM ERRORS
     mark.bello@nist.gov


     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 tradeoff 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 needlelike 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.

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