Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of chatter. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
Chatter is regenerative: the tool leaves a wavy surface, which varies the chip thickness on the next pass, which makes the vibration worse. Once started it rarely stops on its own.
The cures target rigidity, not speed alone: shorten the tool overhang, support the workpiece, reduce radial engagement, or shift the spindle speed off the resonant frequency. Variable-helix cutters break the regeneration by making the tooth spacing irregular.
Common misuse
Trying to cure chatter by reducing the feed. It usually makes it worse — the tool rubs instead of cutting, which sustains the vibration.