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Machining chatter: find a stable spindle speed

Chatter is a self-feeding vibration. Each tooth cuts the wavy surface the last one left, and the waves grow. Often the fix is not to slow down but to move to a spindle speed where the teeth arrive in step with the vibration.

Chatter tone vs. tooth rhythm

Every cut makes a tone at the tooth-passing frequency, and that tone is normal:

Chatter is a separate, louder tone near a natural frequency of the tool, holder, spindle or part. If the loudest tone matches the tooth-passing frequency or one of its multiples, the cut is stable and that is just the teeth hitting the part.

Stable speeds from the chatter frequency

fc is the chatter frequency and k is the lobe number (1, 2, 3 and so on). Example: chatter at 1,800 Hz with a 3-flute end mill gives 36,000 RPM for k = 1, 18,000 for k = 2 and 12,000 for k = 3. Pick the one your spindle can run, and prefer the higher ones because their stable pockets are wider.

Go straight to the new speed. Small changes of 5 to 10% usually land on the unstable side and often make it worse. Change one thing at a time. If the speed doesn't help, cut depth, width of cut, tool stick-out or workholding is next.

The Chatter Listener

Hold your phone near the cut and tap Listen. The app finds the loudest tone and, given the spindle speed and flutes, tells chatter apart from the tooth rhythm. It then suggests the nearest stable spindle speed and the next one up or down. Mill and lathe modes are included. Sound is analyzed on the device and never saved or sent.