Crater wear: what it looks like, why it happens, how to fix it
A hollow or dish worn into the rake face a short distance behind the cutting edge, where the chip slides over the insert. Heat and chemical diffusion at high cutting speed. Where the hot chip slides over the rake face, carbon and tungsten diffuse into the chip and the carbide is carried away. Steel at high speed is the classic case.
| What you see | Mechanism | First fix | Most affected |
|---|---|---|---|
| A hollow or dish worn into the rake face a short distance behind the cutting edge, where the chip slides over the insert. | Heat and chemical diffusion at high cutting speed. | Lower Vc and switch to a CVD grade with an aluminium oxide (Al2O3) layer as a thermal barrier. | ISO P (steel) |
Why it happens
- Rake-face temperature: diffusion wear rises sharply with temperature, so it appears first at the top of the speed range.
- Long chip contact on the rake: a flat rake or a chip that does not curl early keeps the hot chip on the insert longer.
- Uncoated or thin PVD-coated carbide in high-speed steel turning has no thermal barrier against diffusion.
How to measure and judge it
ISO 3685 measures crater depth KT. A common tool-life criterion for carbide is KT = 0.06 + 0.3 f (mm), where f is the feed in mm/rev. A deep crater weakens the wedge from behind, so the edge often fails by fracture once the crater reaches it.
Fixes, in the order to try them
- Lower Vc: the crater is a temperature problem first.
- Use a CVD grade with an Al2O3 layer, which slows diffusion between chip and carbide.
- Open the rake or choose a chipbreaker that shortens the chip contact length.
- Improve coolant delivery to the rake face.
Do not confuse it with
Material stuck on the rake face is adhesion, not a crater; under the deposit the insert may be intact. See built-up edge.
Other wear modes
Flank wear · Notch wear · Built-up edge (BUE) · Plastic deformation · Thermal cracking · Edge chipping and fracture · All eight failure modes in one table · Mechanisms in depth
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Frequently asked questions
What causes crater wear?
Crater wear is caused by heat and chemical diffusion at high cutting speed where the chip slides over the rake face. It is most common when turning steel fast with a grade that has no thermal barrier coating.
How do you reduce crater wear?
Lower the cutting speed, use a CVD-coated grade with an aluminium oxide layer, and choose a geometry that shortens the contact between chip and rake face.