The focus of the pure fundamental level consideration is how to reduce the error and improve the accuracy, so the basic principles of selecting the essential surface are
First: the principle of datum overlap. Should choose as much as possible the design reference as the positioning reference. Especially in the final completion, to ensure accuracy, more attention should be paid to this principle. This can avoid causing position error benchmark deviation;
Second: the principle of unity of the benchmark. Should choose a unified positioning datum surface of various surface treatment, as far as possible to ensure the position accuracy between the surfaces; sometimes also follow the principle of common datum, repeated processing;
Third: the principle of self-referencing. Some small machining allowance and unified finishing process to ensure the quality of processing and productivity, with precision machining surface itself as a base level.