A cutting tool gradually loses useful geometry as it rubs and reacts with the workpiece and hot chip.

Common wear modes include:

• flank wear on the surface rubbing the newly machined workpiece;
• crater wear on the rake face under the flowing chip;
• edge chipping or fracture when mechanical or thermal loading is too severe.

Tool life is often defined as the cutting time until a specified wear criterion or loss of acceptable performance is reached.

A simple empirical relationship between cutting speed $V$ and tool life $T$ is Taylor's tool-life equation:

$$VT^n=C,$$

where $n$ and $C$ depend on the tool-workpiece system and the chosen life criterion.

Suppose $n=0.25$ and a tool lasts $T_1=60$ min at $V_1=100\ \mathrm{m/min}$. If speed is increased to $V_2=120\ \mathrm{m/min}$,

$$V_1T_1^n=V_2T_2^n,$$

so

$$T_2=T_1\left(\frac{V_1}{V_2}\right)^{1/n} =60\left(\frac{100}{120}\right)^4 \approx29\ \mathrm{min}.$$

A 20% speed increase has roughly halved tool life in this example.

This trade-off is why the fastest possible cut is rarely the most economical one. Production rate must be balanced against insert cost, tool-change downtime, dimensional drift and the risk of sudden failure.