The focus of purely basic level consideration is how to reduce errors and improve accuracy, so the basic principles for selecting essential surfaces are:

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

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

Third: the principle of self-based benchmarks. Some small machining allowances and a uniform finishing process ensure machining quality and productivity, with the precision machined surface itself as a base level.