The machining factory emphasizes that the machining must meet certain precision, which means that the main parameters of the actual geometry of the steel parts after machining are consistent with the idealized values required by the part drawings. The inconsistent level among them is called production and processing deviation.

Machining precision includes three levels: on the one hand, specification precision, which limits the specification deviation between the two standards of the production and processing surface not to exceed the necessary scope; on the other hand, geometry appearance precision, which limits the macro-economic geometry appearance deviation of the production and processing surface, such as roundness, cylindricity, parallelism and straightness.

On the other hand, it is the precision of each other's parts, which limits the deviation of each other's parts between the standards of the production and processing surface of the machining plant, such as the degree of axis, flatness and parallelism. The precision of production and processing is the conformity of the main parameters of several geometric figures of the actual specification, appearance and position of the surface layer of the parts after production and processing with the main parameters of the idealized geometric figures specified in the engineering drawings.

The main parameters of idealized geometry, for specifications, are mean specifications; for surface geometry, they are circles, cylinders, plans, spheres, parallel lines, etc.; for the mutual parts in the middle of surface, they are parallel planes, verticals, coaxial output, symmetry, etc.

The deviation between the main parameters of the actual geometry and the main parameters of the idealized geometry is called the manufacturing deviation. The precision and deviation of production and processing are all professional terms to comment on the main parameters of surface geometry of production and processing. The precision of production and processing is considered by tolerance grade, the smaller the grade value is, the higher the precision density is; the deviation of production and processing is expressed by standard value, the larger the standard value is, the greater the deviation is.