The development of computerization allows the development of mathematical models of physical processes with different methods of blade machining of machine parts. Modern programmable CNC machines have the ability to produce parts in an automated cycle, ensuring the requirements of the drawing. Control of the quality of processing of the main flat surfaces, which are technological and Assembly bases, should be considered in more detail. Their accuracy in accordance with the standards is determined by the shape deviations from the adjacent planes, size and relative position. The standards have instructions on how to create a virtual adjacent plane using adjustable supports and further scan the ordinates of its individual points to calculate the deviation of the form. To quantify the deviations of shape, size and location is not enough to have a field f, the vectors of which are defined from the geometric plane, given the nominal size, it is necessary to establish a base for their reference. The question of establishing bases for reference is not only of fundamental importance, since different bases can be obtained different values of deviations. The paper proposes a mathematical model for determining the named parameters of accuracy from the middle planes, which are determined by the calculated vectors at specific points of the treated surface when processing batches of blanks on milling machines face milling.