CNC chatter leaves periodic waves, scallops, noise, and poor dimensional control along a routed edge. It is a vibration problem involving the cutter, spindle, fixture, part, and toolpath.
Thin thermoformed shells are difficult to support. A vacuum fixture may hold broad areas while allowing a flange or deep wall to vibrate. Long tool projection, worn bearings, excessive runout, dull cutters, and an unfavorable feed-speed combination increase instability.
Correction
The part should be supported close to the cut and located without preload. Fixture vacuum, seals, and contact pads should be checked. Cutter projection should be minimized, and spindle runout measured rather than assumed.
Feed, speed, depth, and cutting direction can then be optimized. Slowing the feed is not always helpful; it may increase rubbing and heat while leaving the vibration source unchanged.