The study objective is to predict the range of surface roughness obtained when milling a 45 steel workpiece on CFPE-250 machine. To achieve this objective, it is necessary to solve the following task: to assess experimentally the roughness of the resulting surface when milling a workpiece made of 45 steel on CFPE-250 machine and to identify the mathematical dependence of the roughness range on the treatment mode for this machine. Based on the theory of experiment planning, the mathematical dependence of the roughness range on the cutting speed, feed and cutting depth is found out. The dependence found allows calculating the roughness range and changing the parameters of the cutting mode. The probability of the measured roughness values belonging to the calculated ranges Ra ± ΔRa, micron is 75%. The novelty of the conducted study is in calculating the range of roughness described by the system of equations. This system of equations will be valid for the used CFPE-250 machine tools, the material to be machined (steel 45) and the tool material (hard alloy), subject to the ranges of cutting modes (cutting depth t=0.25...1 mm, feed S=50...200 mm/min and cutting speed V=15.7...62.8 m/min). Similarly, it is possible to find out a system of equations describing the range of roughness obtained for other machine tools (the more rigid the technological system is, the narrower the roughness range will be).