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Troubleshooting · Toolingintermediate~3 min read

Why CNC router bits keep breaking

Short answer

Most broken bits are broken on plunge entry, in a full width slot, or from too much stickout. Turn on ramping, reduce the depth per pass, and push the bit deeper into the collet. Those three changes stop most breakages.

Symptom

A bit snapping during a cut, usually at or near the collet, or at the base of the flutes. Most often a small diameter cutter, and most often at a specific moment rather than randomly.

Check
first
Where in the job does it break? At the start of a cut means plunge entry. In a slot means engagement and chip packing. At a corner means deceleration and a direction change. Randomly means load or a damaged bit.

Most likely causes

  1. Plunging straight down at feed rateMost common
    The centre of most end mills barely cuts, so plunging pushes rather than cuts. Turn on ramping. See plunge rate.
  2. Too much engagement, especially a full width slotMost common
    Slotting is the hardest thing a router bit does. Halve the depth per pass for slots.
  3. Excessive stickoutCommon
    A long bit deflects, and deflection at the collet is where it snaps. See stickout.
  4. Chips packed in the cutCommon
    Packed flutes can seize a bit in a slot. See chip evacuation.
  5. Dull bitCommon
    More force for the same cut, and force is what breaks carbide.
  6. Workpiece movedCommon
    A part that shifts drives the bit into material sideways. See workpiece moved.
  7. Hitting a screw or a hidden nailOccasional
    Instant and unmistakable. Know where your fasteners are, and check reclaimed wood for metal.
  8. Damaged bitOccasional
    Carbide chips when dropped. A bit with an invisible chip in the edge fails under normal load.
  9. Machine losing position mid-cutOccasional
    A sudden jump drives the tool sideways into solid material. See lost steps.

Diagnostic steps

  1. Find the break. At the collet: deflection and stickout. At the base of the flutes: side load in the cut. Chipped edges rather than a clean snap: a damaged or dull bit, or it hit something.
  2. Find the moment. Watch the next one, or read the G-code line the controller was on. Plunge, slot, corner or random each have a different cause above.
  3. Look in the cut. Packed chips, a shiny burnished wall or a scorched floor all say the bit was rubbing or seized before it broke.
  4. Check the workpiece and the zero. A part that moved, or a Z zero that was set on the spoilboard, both drive a bit into far more material than the toolpath expected.

Fixes: the three changes that stop most breakages

  1. Turn on ramping or helical entry. Plunge entry is where small bits die, and it is a checkbox in almost every CAM package.
  2. Reduce depth per pass, and reduce it further for full width slots. A 1/8 inch bit slotting at 1/8 inch per pass in hardwood is asking a lot.
  3. Push the bit further into the collet. Free, instant, and it removes the mechanism by which most bits actually snap.

Small bits are different

A 1/8 inch bit is not a smaller version of a 1/4 inch bit in terms of what it can take. Stiffness falls off very fast with diameter, so a 1/8 inch cutter is far more than twice as easy to break as a 1/4. Practical implications:

  • Rule of thumb Much lighter depth per pass, often a quarter of the diameter or less.
  • Chipload scales down with diameter too. Do not use 1/4 inch numbers.
  • Never plunge them. Always ramp.
  • Use them only where a larger tool cannot reach, via rest machining.
  • Minimum possible stickout, every time.

Why it happens: carbide snaps rather than bends

Carbide is very hard and very brittle. It holds an edge far longer than steel in wood, and it has almost no ability to deform before it fails. So a bit under too much load does not bend and warn you: it goes from fine to in two pieces. That is also why a dropped bit can be invisibly compromised, and why a bit that has been overheated by rubbing is both dull and weakened.

Prevention

  • Ramp entry on by default in every CAM tool definition, so you never have to remember it.
  • Keep a separate, lighter set of settings for slots and for anything under 1/8 inch.
  • Store bits in a case, not a drawer. A chipped edge from a dropped bit is invisible and fatal.
  • Check reclaimed wood with a magnet or a metal detector before it goes on the machine.
  • Watch the first minute of every job, which is when plunge and zero mistakes show.
Read next
CuttingFeed rate and plunge rate for CNC routersThe speed the tool travels through material, why plunge is separate and slower, and what ramping solves.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.BitsBit anatomy: diameter, shank, flutes and stickoutEvery dimension on a router bit, what it controls, and the two that people confuse most often.CuttingChip evacuationWhy getting chips out of the cut matters as much as making them, and what happens when they stay.