Tool wear: reading the edge and knowing when to change it

📅 Published 28 September 2026 · A practical guide from mecanizador.com

A tool does not fail all at once: it warns you. The problem is that there are five different ways for it to wear and each one comes from a different cause, so changing the insert without looking at why it wore out is condemning yourself to repeat it.

Flank wear: the normal one

An even wear band on the flank face, just below the cutting edge. This is the good kind of wear: it means the tool is working as it should and is being used up predictably.

The usual change criterion is around 0.3 mm of band for roughing and considerably less, 0.1 to 0.15 mm, for finishing. What matters is that it is measurable and predictable, which is why it lets you plan changes through a batch instead of waiting for a breakage.

If it arrives too fast, the cutting speed is high for that material.

Crater wear

A hollow on the rake face, where the chip flows off. The hot chip erodes the carbide by diffusion. Dangerous because it thins the edge from behind and there comes a point where it collapses all at once.

Causes: high speed, insufficient cooling, or a coating that does not suit the material. It is corrected by dropping the speed, improving coolant flow or picking a grade with a coating more resistant to diffusion.

Built-up edge

Workpiece material welds to the edge, builds up, then tears away taking pieces of coating with it. It leaves a torn surface and unstable dimensions.

It turns up above all in aluminium, soft steels and stainless at low speed. It is the most counter-intuitive kind of wear, because it is cured by raising the cutting speed, not lowering it. A sharper, more positive edge and more lubrication help too.

Chipping

Small nicks along the edge, like bites taken out of it. A mechanical cause: interrupted cutting, vibration, an abrupt entry, or material with hard inclusions.

It is corrected with a more robust geometry, a protective chamfer on the edge, gentle entries and, above all, improving the rigidity of the set-up. If it always chips at the same point along the path, look there: there is probably a sudden entry or an interruption.

Breakage

The edge snaps. It is the end point of any of the above if they are not dealt with, or the result of an impact: too deep a cut, a collision, a part that moved. It is almost never "bad luck"; what is usually behind it is wear that had been warning for a while.

Plastic deformation

The edge sinks or bulges without actually breaking. It is purely thermal: too high a temperature for the substrate grade. It happens with high speeds in hard materials or with badly aimed coolant. It is corrected with a harder grade, less speed or better cooling.

Thermal cracking

Cracks perpendicular to the edge, like the teeth of a comb. They come from cycles of heating and cooling: typical of milling, where each tooth enters and leaves, and made worse by intermittent coolant.

Here is a counter-intuitive piece of advice: in milling with poor coolant delivery, it is sometimes better to cut dry than with a jet that only arrives now and then. The thermal shock does more damage than steady heat.

A quick mental table

• Even band below the edge → normal; if it comes fast, drop the speed.
• Hollow on the rake face → drop the speed, improve cooling.
• Material welded on and a torn surface → raise the speed.
• Nicks → rigidity, gentle entries, more robust geometry.
• Sunken edge → harder grade or less speed.
• Comb cracks → steady coolant or dry.

When to change it

In production, the efficient approach is to measure how many parts it lasts and change on a counter before it fails, rather than waiting for a bad one. Changing with wear under control costs an insert; changing after a breakage costs the insert, the part and sometimes the set-up.

And if the finish gets steadily worse through a batch, look no further: it is wear. It is also among the mistakes that break the part or the tool.