External thread cutting is a complex 3-D process in which the cutting conditions vary over the thread cutter profile. It is accepted as a mature technology. However, thread machining is heavily experience-based technology and there are few academic works published. Determining the cutting forces during machining is crucial to explain formation of the surface layer, residual stresses, selection of the most appropriate machine tool, and optimizing the process. Mechanistic force model is known to be the most developed method in machining processes. In orthogonal cutting, the material is removed by the cutting edge that is perpendicular to the direction of relative tool-work motion (direction of the cutting velocity). Although the most cutting operations are three-dimensional, orthogonal cutting is used to explain the general mechanics of the material removal. In this study, tangential and radial cutting forces during thread cutting which has API-V0.040 thread profile were predicted from the orthogonal cutting data of the AISI 4140 steel. Huge differences up to 60 % were observed between predicted and measured cutting forces. Therefore, mechanistic force model was improved for thread cutting operations and cutting forces were predicted with average errors between 3 and 18 % depending on machining parameters.