Chatter Control In A Box

This company learned how chatter control may be attained through an analyzer that 'listens' to an unstable cut, and isolates the distinct frequencies in this sound in relation to the number of cutting edges on the tool, and the analyzer then calculates the optimum speeds for smooth cutting in that particular operation.

As shops everywhere run at higher speeds, more and more of them confront problems with chatter. The interaction of machine, tool, and setup creates a complex vibrational system with its own peculiar resonant frequencies. And when the spindle speed is increased, this can raise the frequency of the cut itself into the range of one of these resonant values. Chatter is the result, as vibrations feed on themselves to make the tool oscillate. The effects can include shorter tool life and a terrible surface finish.

Even so, reducing chatter is not necessarily a matter of running slower. For any spindle rpm at which chatter is a problem, there may be "sweet spots" at both higher and lower speeds where the cut proceeds smoothly. That's why machinists sometimes dial the spindle speed up in the hope that chatter will go away. They know that chatter control doesn't demand less speed, but just the right speed for the process.

Then how can a shop find that particular speed? Playing with speed override provides only a hit-or-miss approach. Now, Kennametal (Latrobe, Pennsylvania) offers an alternative—a chatter-control solution that is less time-consuming, and not much more difficult to use than the speed override knob.

The company calls the system "BestSpeed." In essence, it delivers vibration analysis in a box. By means of a built-in microphone, the hand-held BestSpeed analyzer "listens" to an unstable cut in milling, boring, or (sometimes) turning, and isolates the distinct frequencies in this sound. The user inputs the number of cutting edges on the tool. Equipped with only this information, the analyzer then calculates the optimum speeds for smooth cutting in that particular operation. For example, the first cut may be at 5,000 rpm and may produce a poor surface. But after listening to the chatter, the analyzer will recommend a very specific speed like 6,274 rpm—a point at which the vibrational characteristics of the system will favor a smoother cut.

Armed with such precise knowledge of the sweet spot, a shop may be able to cut to greater depths—and to higher metal removal rates—than it could have achieved effectively where chatter was a danger.

The analyzer is not a universal solution for chatter problems, Kennametal says. Sometimes it can't work with the signal it receives. For example, some interrupted cuts generate free vibrations the analyzer cannot interpret. Also, tools with staggered cutting edges will fail to produce a steady frequency. Said one Kennametal representative, "It can't always give the answer, but when it works, it works great."

It has even worked over the phone. Kennametal engineers have used the analyzer to diagnose a remote shop's chatter problem by placing a phone call. On one end, the machine operator held the phone up to the cut. On the other end, the listener pointed the analyzer at the phone receiver. In this way, the analyzer determined the best speed for the machining process at the other end of the call.