Drilling Guide
How to Stop Drill Bits Breaking or Overheating
Snapped bits and burnt holes almost always trace back to one of a handful of causes — here's how to spot and fix each one.
A drill bit that keeps snapping, or a hole that comes out discoloured and work-hardened, is frustrating but rarely random — it's almost always one of a small set of causes. Working through them in order usually finds the culprit fast.
Start from the right numbers
Most breakages trace back to wrong RPM or feed rate — check yours against the calculator first.
Feed rate too high for the RPM
If the drill is being pushed into the material faster than the flutes can clear chips, pressure builds up rapidly and the drill snaps — usually right at the moment it should be cutting cleanly. See the speeds and feeds guide to check your feed per revolution matches your drill diameter.
Workpiece not secured firmly
Any flex or movement in the workpiece transfers straight into the drill as side load, which HSS tolerates poorly and carbide tolerates even worse. Clamp the work solidly, as close to the hole location as practical, before starting.
Chips packing the flutes
On deeper holes, chips that can't escape up the flutes get compacted and jam the drill, spiking torque and snapping it. This is exactly what peck drilling — periodically retracting to clear swarf — is designed to prevent. See peck drilling explained for when and how often to peck.
Running too slow or too fast for the material
Too fast overheats the cutting edge and dulls it rapidly, especially in stainless steel and other tough alloys, which then generates even more heat as the dull edge rubs instead of cuts — a vicious cycle. Too slow, particularly with too light a feed, causes the drill to rub rather than bite, which work-hardens the surface and makes the next pass even harder to cut.
Not enough — or the wrong — cutting fluid
Cutting fluid does two jobs: carrying heat away from the cutting edge, and helping chips slide out of the flutes. Dry drilling in materials that need lubrication is a common, easily-fixed cause of overheating.
Quick troubleshooting checklist
- Check RPM and feed rate against your material and drill diameter
- Confirm the workpiece is clamped securely, close to the hole
- Use peck drilling for holes deeper than about 3× the drill diameter
- Apply cutting fluid appropriate to the material
- Inspect the drill point for wear before starting a new job
Frequently asked questions
Why do my drill bits keep breaking?
Usually feed rate too high for the RPM, an insecure workpiece, chips packing the flutes, or drilling too deep without retracting to clear swarf.
Why does my drill overheat when drilling stainless steel?
Stainless work-hardens quickly with too light a feed or a dwelling drill. Running too fast, or letting the drill rub instead of cut, both generate excess heat.
Should I use cutting fluid when drilling metal?
Yes for most metals, especially stainless steel and harder alloys — it reduces heat and friction, extends tool life and helps clear chips.