The processes of machining have vital influence on the operating properties of machine parts because they ensure the formation of the required geometrical structure of the working surfaces and the condition of the top layer. In this paper the influence of emulsion mist parameters on Ra, Rz and RSm surface roughness parameters is described when finishing the turning of C45 carbon steel. It was found that cutting parameters have considerably greater influence on roughness parameters in comparison with emulsion mist conditions. When increasing cutting speed from 100 to 300 m/min Ra and Rz values decrease nearly 2 times, but RSm value does not depend on speed. When increasing feed rate from 0.1 to 0.2 mm/rev Ra, Rz and RSm values increase more than twice. Changes of emulsion mass flow and compressed air flow affect roughness parameters reach 9–15% of maximum. It was shown that Parameter Space Investigation method can be used efficiently for quick analysis of tested parameters and optimization of their values.