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.

ADDRESSING HSM IMPLEMENTATION ISSUES

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.

Chatter problems are one of the top areas of research currently underway at NIST. 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.

This has given Davies' team a chance to look closely at the chatter problem and find solutions that might be applicable in a wide range of situations. In its research, NIST has had great success using stability lobe diagrams. These are charts that track the stability behavior of a given machining operation, plotting regions of stable and chattering behavior as a function of practical parameters, such as spindle speed and depth of cut. Stable regions, called lobes, become more pronounced at higher speeds, enabling successful HSM implementation.

THE PROBLEM OF CHATTER

Chatter has always been a problem for machinists. Chatter creates large cutting forces that may accelerate tool wear and can potentially cause catastrophic tool failure, negatively impacting the life of a machine. The resulting workpiece material may also be unacceptable, and require rework or it may be completely unsalvageable.

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.

That's because regenerative machine tool chatter, which is the result of subsequent cuttings on a variable surface, severely limits the material removal rates in high-speed applications. "Chatter can be attributed to this regenerative effect," Davies explains. "This is because a cutter will always cut over a surface that's cut previously." Davies uses the example of a lathe to illustrate his point. "If the cutter is vibrating, it leaves a little wave on the surface. When that wave comes around and hits the cutter again, the cutter responds to the wavy surface, which makes a variable force and potentially leads to even more vibration." To reduce chatter and to ensure a consistent chip thickness, the cutter needs to line up in the same location each time.

RECONSIDERING OVERHANG LENGTHS

A few years back, researchers at the University of Florida conducted simulations from which they were able to determine that sometimes chatter could actually be reduced by lengthening the tool. "This idea was so counterintuitive--that you could actually make a tool longer and remove more material with it than you would if it was shorter," Davies says. And yet, within the context of a high-speed machining environment, the widely held precepts of traditional machining no longer applied.

NIST researchers put theories studied by the University of Florida to practical use in an aerospace machining environment. Recent and ongoing experiments in the tool tuning of end-mill overhang length have had encouraging results. By manipulating overhang lengths to maximize productivity, Davies and his team have found insights that would be of interest to machinists in the aerospace industry. They began by considering the cutter choice.

While it is true that using a three-quarter-inch cutter in aerospace manufacturing can help eliminate the chatter issue in tool tuning, this typically is not enough for most aerospace applications.

Many times chatter creates pockets, and the diameter of the pocket is going to be dictated by the three-quarter inch cutter. Davies' team is in the process of investigating whether a half-inch cutter with a very long overhang could be used to pocket out the corners and further cut down on the mass.

Initial test results are promising. Davies' team has been able to prove that four- and five-inch deep pockets can be further refined with a half-inch cutter, provided the overhang is long enough. But long overhangs have the tendency of making some conventional machinists nervous, since most of them have been taught since apprenticeship that overhangs beyond four or five times the diameter are actually a primary cause of chatter trouble. On the contrary, NIST's tests are relying on eight- to ten-to-one overhangs on the cutter—far greater than traditional theory would recommend.

But in high-speed machining, conventional chatter factors have been replaced with other issues and considerations. The reason for this shift? "In high-speed machining, there are some spindle speeds at which a particular cutter vibrating at a certain frequency will operate optimally, and there are other speeds that will cause the cutter to chatter like crazy," Davies says. To determine the proper spindle speed for a given cutter, some amount of trial and error is usually required. Says Davies, "You can either change the speed of the spindle by adjusting it to operate at a higher or lower speed, or you can change the oscillating frequency of the tool."

These options may indeed seem counter-intuitive to many machinists, who typically are not taught to utilize such trial-and-error methods. 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.

A PROMISING FUTURE FOR NIST FINDINGS

As spindle designers begin to incorporate NIST's findings on chatter dynamics into their designs, the results for manufacturing are sure to be notable. Matthew Davies is enthusiastic about the future. "The idea that you can lengthen a tool and make it dramatically more stable," he says, "opens up a lot of future possibilities for spindle designers and tool designers."

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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