Owing to expense effects, the operation of NC machine tools requires safely calculable technological data. The tool life criteria must form the basis of these, the connection of which with the technological data for given tool-workpiece material coupling can be determined simply, at low cost and quickly, even by the employer of the machine. The study deals with the calculation of tool life of semi-rough turning. The tool life criterion is the depth of crater. Its changes are calculated for length and cross-turning. The analytical method valid for cross-turning offers a direct possibility for the continuous plotting of the Taylor curve, for determination of its constants. The procedure after the solution of still unclarified questions offers a possibility for a decrease in the order of cost and time requirements of tool life tests