This paper presents a study for the development of tool-life models for machining operations by means of a statistical approach called multi-linear regression analysis. The study was applied to a milling process for machining SAE 121 cast iron in a factory without interrupting the mass production. Different cutting tool materials under dry conditions were used in the cutting tests. Several machining experiments were performed and mathematical models for tool life have been postulated by using least-square regression analysis. The analysis was based on a first-order model in which the tool life is expressed as a function of two independent variables; cutting speed and feed rate. Analysis of variance was applied to check the adequacy of the mathematical models and their respective parameters. In order to demonstrate the usefulness of the developed models, tool-life contours have been generated and presented in different plots.