The focus of pure basic level consideration is how to reduce the error, improve the accuracy, so the basic principle of selecting the essential surface is: first: the principle of reference coincidence. The best possible design benchmark should be selected as the positioning benchmark. Especially in the final completion, to ensure accuracy, more attention should be paid to this principle. This can avoid the position error reference deviation; Second: the principle of benchmark unification. Various surface treatments of unified positioning datum should be selected to ensure the position accuracy between surfaces as far as possible; Sometimes also follow the principle of common benchmark, repeated processing; Third: self - reference principle. Some small machining allowances and uniform finishing processes ensure machining quality and productivity, with the precision machined surface itself as a base level.
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