The actual problem of modeling the influence of geometric errors of vertical milling machines on the accuracy of machined surfaces is considered in the paper. For the first time, a mathematical model of the accuracy of vertical milling machines is presented when processing a complex of keyway surfaces. The model was developed on the basis of a variational method for calculating the accuracy of metal-cutting systems, which is a development of the well-known variational method for calculating the accuracy of machine tools. The results of modeling the influence of geometrical errors of vertical milling machines on the accuracy of the machined surfaces of the keyway are given. Directions for further research of milling accuracy modeling with variable values of machine tool geometries are proposed. The work is useful for researchers engaged in research of the accuracy of technological equipment.