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The rational choice of cooling mixtures (coolant) is a complex technological task. Currently, world industries produce a wide range of such substances. At the same time, manufacturers' recommendations on their use are of a general nature, which makes it difficult to choose them for specific cases. One of the criteria for choosing a coolant is the roughness of the formed surface, especially during constrained cutting, in particular during drilling. Therefore, studies of the influence of cooling mixture on the roughness of the hole surface during drilling are relevant. The task to which the paper is devoted is to find out the dependence of the roughness of the hole surface obtained when drilling workpieces of low-alloy steel on the properties of the coolant used. To achieve this aim, experimental studies were conducted. As a result of laboratory tests of coolants, their lubricating properties and density were determined. The evaluation of the obtained roughness dependencies during drilling on the properties of coolant allowed us to define a formula for determining the predicted surface quality. It is found out that the determinant property of the coolant is its lubricating action, and the parameter for its evaluation is friction factor. The calculation formula makes it possible to predict the roughness of the hole surface when drilling low-alloy steels with an error of 11.8%. The method of coolants testing and determination of their effectiveness by the roughness parameter Raavg is presented.