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Troubleshooting · Marks on the workintermediate~3 min read

CNC chatter: causes, diagnosis and fixes

Short answer

Chatter is self-reinforcing vibration: the tool deflects, springs back, cuts harder, deflects again. Reduce stickout, reduce depth of cut, tighten anything loose, and check that the workpiece itself is not what is vibrating.

Symptom

Evenly spaced ripples or wave marks on a cut wall, and a loud rattling or growling sound that is distinctly different from normal cutting. Often the sound arrives before the marks are bad enough to notice.

Check
first
How far is the bit sticking out of the collet? Push it in as far as safely possible, leaving only the length you need to cut, and run the same cut again. Deflection grows with roughly the cube of stickout, so this one change is often dramatic.

Most likely causes

  1. Too much bit stickoutMost common
    The most flexible part of the whole system, and free to fix. See stickout.
  2. Too much engagement: depth or stepoverMost common
    More force means more deflection means more vibration. Reduce depth per pass first.
  3. Workpiece not held rigidlyCommon
    Often it is the part vibrating, not the machine. Thin stock and parts clamped only at the edges are the usual suspects. See workholding.
  4. Something loose on the machineCommon
    Router mount, Z assembly, gantry bolts, V-wheels, a motor coupler. Vibration loosens things, and loose things cause vibration.
  5. Long thin workpiece resonatingCommon
    A tall thin part being profiled will sing. Support it or leave more material until the last pass.
  6. Dull bitOccasional
    More cutting force for the same cut, which is what feeds the vibration.
  7. RunoutOccasional
    An off-centre bit loads one flute harder each revolution, which is a periodic force. See runout.
  8. A resonance at that specific RPMOccasional
    Genuinely real. Try changing RPM by 15 to 20 percent in either direction and listen.

Diagnostic steps

  1. Listen for where it comes from. Put a hand near the workpiece and near the gantry, carefully and not near the tool. You can usually feel which one is buzzing.
  2. Look at the spacing of the marks. Very fine regular ripples usually mean tool vibration. Wider waves usually mean the machine or the workpiece moving.
  3. Run the same cut at half the depth. If the chatter disappears, engagement was the trigger.
  4. Change the RPM and listen. If chatter appears and disappears with RPM, you are on a resonance.
  5. Grab and wiggle, machine off. Check the router in its mount, the Z assembly, the gantry and the workpiece for any play.

Fixes, in order

  1. Reduce stickout. Cheapest, fastest, most effective.
  2. Reduce depth per pass, then stepover.
  3. Improve workholding so the part cannot move or vibrate.
  4. Tighten everything. Do a full fastener check on the machine.
  5. Change RPM by a meaningful step to move off a resonance.
  6. Use a shorter, larger diameter bit if the cut allows it.
  7. Support tall thin parts or leave them attached until the final pass.

Why it happens

The tool deflects under cutting force. As it deflects it takes a lighter cut, so the force drops and it springs back. Springing back means it takes a heavier cut, so it deflects again. If the timing of that cycle lines up with a natural frequency of the tool, the spindle, the machine or the workpiece, the oscillation reinforces itself and grows. That is why chatter appears suddenly rather than gradually: you cross a threshold and the system starts feeding itself.

Prevention

  • Make short stickout a habit rather than a fix.
  • Do a fastener check on the machine periodically. Vibration loosens things over time.
  • Know your machine's working limit through a removal rate test, and stay under it.
  • Hold thin and tall parts properly rather than hoping.
Read next
MachineRigidity, backlash and runoutThe three mechanical properties that set the ceiling on what your machine can cut and how accurately it can cut it.BitsBit anatomy: diameter, shank, flutes and stickoutEvery dimension on a router bit, what it controls, and the two that people confuse most often.CuttingDepth per pass (stepdown) for CNC routersHow deep each pass goes, the rules of thumb based on diameter, and why it is the first thing to reduce when a cut goes wrong.TroubleshootingMachine vibrating, noisy or running hotNew sounds, buzzing, or a router that is too hot to touch. What each one means and which need immediate attention.