The purely basic level of consideration is how to reduce errors and improve accuracy, so the basic principle of choosing the essential surface is:

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

Second: the principle of unified standards. Should choose a variety of surface treatment of the uniform positioning datum, try to ensure the positional accuracy between the surfaces; sometimes follow the principle of the common benchmark, repeated processing;

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