There are often several surfaces to be machined on a CNC workpiece. These surfaces not only have certain accuracy requirements, but also have certain position requirements between the surfaces. In order to achieve these accuracy requirements, the machining sequence of each surface of CNC machining parts can not be arranged randomly, but must follow certain principles, that is, the selection and conversion of positioning datum determine the processing sequence, and the principle that the former process prepares the positioning reference for the subsequent process.

1. Before machining the important surface, the precision datum should be adjusted once to ensure the machining accuracy of the important surface.

2. After finishing datum machining, roughing, semi finishing and finishing should be carried out on the main surfaces with high precision requirements. The surface with high precision needs to be finished.

3. The surface as a precision datum should be machined at the beginning of the CNC machining process, because it is used to locate other surfaces in subsequent processes. That is, "benchmark first, then others".

4. When machining the precise datum plane, it is necessary to use the coarse datum to locate. In single piece production, small batch production and even batch production, for castings and forgings with complex shape or large size, as well as blanks with large size error, the marking process should be arranged before the machining process, so as to provide alignment reference for precision reference processing.

5. For the process prone to waste products, precision machining and comprehensive processing machining can be appropriately placed in front, and some minor surface processing can be placed behind.

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