Mechanical processing plants emphasize that machining should meet certain precision, which refers to the level at which the main parameters of the actual geometry of the steel parts after machining are consistent with the idealized values required by the parts drawings. The discordant levels among them are called production bias.

The precision of production and processing in machinery processing plants consists of three levels. On the one hand, the precision of specifications limits the standard deviation between the surface of production and processing to be within the necessary scope. On the other hand, the geometric shape precision limits the appearance deviation of the macroeconomic geometric shape of the surface layer, such as roundness, cylindricity, parallelism and straightness.

On the other hand, it is the precision of the mutual parts, which limits the deviation of the mutual parts between the standard of the production and processing surface of the mechanical processing plant, such as coaxiality, flatness and parallelism. The precision of production and processing refers to the level of the actual specifications, appearance and main parameters of several geometric shapes of the surface of parts after production and processing and the main parameters of the idealized geometric shapes stipulated in engineering drawings.

The main parameters of the idealized geometrical figure, in terms of specifications, are the mean specifications; In terms of surface geometry, it refers to circles, cylinders, plane graphs, spheres and parallel lines. For the mutual parts in the middle of the surface, it refers to the parallel plane, vertical, coaxial output, symmetry, etc.

The deviation between the actual main parameters of the geometrical figure and the main parameters of the idealized one is called the production and processing deviation. The precision of production and processing and the deviation of production and processing are the technical terms to comment on the main parameters of surface geometry of production and processing. The precision of production and processing is considered by the tolerance grade. The smaller the grade value is, the higher the precision is. The deviation of production and processing is expressed by the scale value. The larger the scale value is, the greater the deviation is.