Dr. Matthew Davies is a mechanical engineer the National Institute for Standards and Technology (NIST). He leads several projects pertaining to high-speed machining. In this article, he reviews some of the relevant findings of his team as they pertain to the common machining problem of chatter.

The successful implementation of high-speed machining (HSM) requires the most accurate knowledge of the machine dynamics that are in play in any given application. By conducting studies and recording and publishing their results, NIST researchers are able to eliminate a great deal of measurement time on the shop floor.

Dr. Matthew Davies and his colleagues have done exhaustive testing in the machining of complex components with intricate geometry, deep pockets, and wall and floor sections with thicknesses measuring only fractions of a millimeter. Such geometric complexities often lead to chatter.

In its research, NIST has had great success using stability lobe diagrams. Stable regions, called lobes, become more pronounced at higher speeds, enabling successful HSM implementation.

Chatter creates large cutting forces that may accelerate tool wear and can potentially cause catastrophic tool failure, negatively impacting the life of a machine. For many years, the traditional solution to chatter was to decrease surface speed while increasing feedrates.

Unfortunately, this approach, which works measurably well in traditional machining environments, is often counterproductive in a high-speed machining environment. In fact, increasing feedrates and decreasing surface speed can often have the opposite result than what was desired.

NIST researchers put theories studied by the University of Florida to practical use in an aerospace machining environment. By manipulating overhang lengths to maximize productivity, which he terms counterintuitive, Davies and his team have found beneficial insights to machinists in the aerospace industry.

Davies' team has been able to prove that pockets created in the high-speed machining process can be further refined with a three-quarter inch cutter, provided the overhang is long enough. And work with a half-inch cutter with longer overhangs is underway. But long overhangs have the tendency of making some conventional machinists nervous. NIST's tests are relying on eight to ten-to-one overhangs on the cutter—far greater than traditional theory would recommend.

Davies' team chose to conduct their tests using the second option, that is, changing the oscillating frequency of the tool. As such, they were able to show that a ten to one overhang on a half-inch cutter was significantly more stable than a nine to one overhang on the same size cutter, both operating on the Makino A55. As spindle designers begin to incorporate NIST's findings on chatter dynamics into their designs, the results for manufacturing are sure to be notable.

For additional information about NIST, the National Institute of Standards and Research, contact them by phone at 301-975 NIST (6478). Or, visit their Website at www.nist.gov.

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