The theory and practice in engineering industry is not the same. Durability of cutting tools is defined in standard ISO 3685. In standard ISO 3685 is defined T-vc dependence for different cutting materials and standard included process evaluation of tool durability for cutting materials made of high speed steel, sintered carbide and cutting ceramic. Standard ISO 3685 contains instructions how to create T-vc dependence for cutting tools made of cutting ceramic. In this standard are only instructions how to create T-vc dependence according to Taylor. The article compares T-vc dependencies for various cutting ceramics (Al2O3, Al2O3+ZrO2, Al2O3+TiCN) with T-vc dependence defined in standard ISO 3685, because this standard describes T-vc dependence for all entered cutting materials together.