The focus of pure basic level consideration is how to reduce errors and improve accuracy, so the basic criteria for selecting substantive appearances are:

Top: Benchmark coincidence criterion. The planning benchmark should be selected as much as possible as the positioning benchmark. Especially in the final conclusion, to ensure accuracy, attention should be paid to this criterion. This can avoid causing azimuth errors and benchmark errors;

Secondly, the principle of benchmark consistency. We should select consistent positioning reference surfaces for various surface treatments, and strive to ensure the accuracy of orientation between each surface; Sometimes they also follow the principles of a common benchmark and repeatedly process;

Third: Self benchmarking principle. Some small mechanical machining allowances and consistent precision machining processes ensure machining quality and productivity, and the surface of precision machining itself serves as a basic level.