The focus of the pure basic level consideration is how to reduce errors and improve accuracy, so the basic principle of selecting the essential surface is:
First: the principle of benchmark coincidence. The design datum as far as possible should be selected as the positioning datum. Especially in the final completion, to ensure accuracy, more attention should be paid to this principle. In this way, the reference deviation of position error can be avoided.
Second: the principle of unified benchmark. A unified positioning datum surface treatment should be selected to ensure the accuracy of the position between the surfaces. Sometimes also follow the principle of common benchmark, repeated processing;
Third: self-benchmarking principle. Some small machining allowance and unified finishing process ensure machining quality and productivity, with precision machining surface itself as a base level.