The focus of pure basic level consideration is how to reduce errors and improve accuracy, so the basic principles for choosing the essential surface are:

First: the principle of benchmark coincidence. The design basis as much as possible should be selected as the positioning basis. Especially in the final completion, to ensure accuracy, this principle should be paid more attention to. This can avoid causing position error reference deviation;

Second: the principle of unification of benchmarks. Various surface treatments of a unified positioning reference surface should be selected to ensure the position accuracy between each surface as much as possible; sometimes the principle of common reference is also followed and repeated processing;

Third: the principle of self-reference. Some small machining allowances and a unified finishing process ensure the quality and productivity of the machining, and the precision machining surface itself serves as a basic level.