A bit shears off in the cut, sometimes on the first pass, sometimes after a few minutes. It may happen repeatedly at the same kind of feature - most tellingly, around curves - or seemingly at random. A broken bit is almost always a symptom of a load the shank could not carry, not a bad bit.
Quick answer. The two dominant causes are wrong feeds and speeds - counter-intuitively, too slow is as deadly as too fast, because a rubbing bit overheats, dulls and then snaps - and inadequate hold-down, where the workpiece shifts under side load and side-loads the bit. If bits break around curves specifically, suspect hold-down first.
These are ordered by how often they are the answer, not by severity. Check each in turn and stop at the first one that confirms.
Confirm it's this
The bit runs hot or discolours before it breaks, or you are guessing at settings. Both extremes break bits: too slow makes the bit rub, overheat and dull until it snaps; too fast overloads it outright.
The fix
Set a real chip load for the material and cutter with the calculator instead of guessing. Aim for a chip thick enough to carry heat away but within the cutter's load limit. This alone resolves a large share of breakages.
Confirm it's this
Bits break mid-curve or when the cutter re-enters the material - the classic 'breaks around curves' tell. The part has shifted or lifted a fraction under the changing side load and pinched or slammed the cutter.
The fix
Improve workholding: more clamps, better tape-and-glue or a vacuum, tabs on cut-through parts, and never rely on a part that can rock. A cutter can take a straight cut and then snap on the curve purely because the part moved.
Confirm it's this
The bit shows a witness mark of having crept down in the collet, or pull-out happens under load. Collets fatigue and stop gripping concentrically well before they look worn.
The fix
Clean the collet on every bit change and replace it every 3-6 months of regular use. A tired collet lets the bit pull down or run out, which spikes the load until it breaks.
Confirm it's this
Breakages track how deep you are cutting per pass; easing the depth stops them. Small-diameter bits are especially unforgiving of a deep single pass.
The fix
Reduce depth of cut per pass and take more passes - as a starting rule, keep depth at or below the cutter diameter for general work and much shallower for small bits in hard material. Let the calculator's depth adjustment guide you.
Confirm it's this
Cutting forces have crept up over time, the finish has degraded, or the edge looks rounded under a loupe. A dull bit rubs, heats and eventually fails.
The fix
Replace the bit. A dull cutter needs far more force for the same cut, and that force is what breaks it.
Confirm it's this
You are using, say, a two-flute in a gummy material that clogs, or a general-purpose bit in something abrasive that dulls it fast.
The fix
Match geometry and flute count to the material - fewer flutes and better evacuation for gummy stock, carbide for abrasive stock. See the material how-to guides for the right cutter per material.
The consumables a fix like this usually calls for. Prices are live; we may earn a commission on a purchase, which never affects what we recommend.
A worn collet is one of the most common hidden causes of breakage - it lets the bit run out or creep down under load. Clean it every change and replace it every 3-6 months.
A dull or wrong-geometry cutter needs far more force for the same cut, and that force is what snaps it. Match the bit to the material and keep spares.
Diagnostic patterns are distilled from manufacturer documentation and established tooling references; where the community converges on a real-world tell, we state it. Exact feeds and speeds always come from the calculator, never invented here.
