Thermal cracking: what it looks like, why it happens, how to fix it
Fine cracks perpendicular to the cutting edge, like the teeth of a comb; small pieces between the cracks flake out and the edge then chips. Thermal fatigue. Each entry heats the edge and each exit or burst of coolant cools it. The surface layer expands and contracts against the cooler core, and cracks open across the edge.
| What you see | Mechanism | First fix | Most affected |
|---|---|---|---|
| Fine cracks perpendicular to the cutting edge, like the teeth of a comb; small pieces between the cracks flake out and the edge then chips. | Thermal fatigue. | Use a tougher grade with thermal-shock resistance and run either dry or with full, consistent coolant, never intermittent. | ISO P (steel), ISO K (cast iron) |
Why it happens
- Interrupted cuts and milling: every revolution or tooth pass is a heating and cooling cycle.
- Intermittent coolant that hits the hot edge only part of the time multiplies the temperature swing.
- High cutting speed raises the peak temperature and therefore the size of each swing.
How to measure and judge it
Count and position matter more than size: cracks running across the edge, several in a row, mean thermal fatigue. Cracks along the edge point to mechanical overload instead.
Fixes, in the order to try them
- In milling, try dry cutting with an air blast; if coolant is needed, flood it consistently.
- Choose a tougher grade with thermal-shock resistance.
- Reduce Vc to lower the peak temperature of each cycle.
- Check the cutter position: a smaller exit angle reduces the shock at each exit.
Do not confuse it with
Small fractures along the edge with no cracks across it are mechanical chipping, not thermal cracks. See edge chipping.
Other wear modes
Flank wear · Crater wear · Notch wear · Built-up edge (BUE) · Plastic deformation · Edge chipping and fracture · All eight failure modes in one table · Mechanisms in depth
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Frequently asked questions
What causes thermal cracks on a milling insert?
Thermal cracks are caused by thermal fatigue: the edge is heated at every entry and cooled at every exit or coolant burst. The cyclic expansion and contraction opens cracks perpendicular to the edge.
Should I use coolant when milling to avoid thermal cracks?
Coolant that reaches the edge only part of the time makes thermal cracking worse. Either mill dry with air, or apply full, consistent coolant.