A CNC machined parts often have several surfaces need to be processed, these surfaces not only have a certain accuracy requirements, and there are certain positional requirements between the surfaces. In order to achieve these accuracy requirements, CNC machining parts of the surface of the machining sequence can not be arranged arbitrarily, but must follow certain principles, which is the selection of positioning datum and conversion determines the machining sequence, as well as the former process for the subsequent process to prepare for the principle of positioning datum.

1, In the important surface machining, the fine datum should be adjusted once, in order to help ensure the machining accuracy of the important surface.

2, After the fine benchmark processing, then the main surface should be roughing, semi-finishing and finishing of the main surface with high accuracy requirements. The surface with particularly high precision requirements also needs to be finished.

3, As the surface of the precision benchmark should be processed at the beginning of the CNC machining process, because the subsequent processes in the processing of other surfaces to use it to locate. That is, "the first benchmark after the other".

4, In the processing of fine datum surface, the need for coarse datum positioning. In a single piece, small batch production, or even batch production, for the complex shape or size of the castings and forgings, as well as larger dimensional errors in the blank, in the machining process should first be arranged before the scribing process in order to provide a corrective benchmark for the processing of precision benchmarks.

5, For easy to produce waste process, finished and finishing processing can be placed in front of the appropriate, some minor small surface processing can be placed after.