During the machining process of a machining center, the reasonable selection of positioning benchmarks determines the quality of the parts, which has a significant impact on whether the dimensional accuracy and mutual positional accuracy requirements of the parts can be guaranteed, as well as the processing sequence arrangement between the surfaces of the parts. When installing workpieces with fixtures, the selection of positioning benchmarks also affects the complexity of the fixture structure. This requires the fixture to be able to withstand both large cutting forces and meet positioning accuracy requirements. Therefore, the selection of positioning benchmarks is a very important process issue.

When determining the positioning reference of the parts, the following principles should be followed:

1. The origin of the workpiece coordinate system, also known as the "programming zero point," does not necessarily have to coincide with the part positioning reference, but there must be a definite geometric relationship between the two. The selection of the origin of the workpiece coordinate system is mainly considered for the convenience of programming and measurement. For parts with high dimensional accuracy requirements, when determining the positioning reference, it should be considered whether the origin of the machining center coordinates can be accurately measured through the positioning reference.

2. When both the machining reference and the machining of each workstation are completed on the machining center, the selection of the positioning reference should consider completing as much machining content as possible. For this reason, it is necessary to consider positioning pipes that are easy to machine on all surfaces. For example, for a box, it is best to use a positioning pipe with one side and two pins to facilitate the machining of other surfaces by the tool.

3. Try to choose the design reference on the parts as the positioning reference. This requires consideration of how to use rough benchmarks to machine the various surfaces of the precision benchmark during rough machining, that is, the positioning benchmarks used on the machining center should be machined in the front ordinary machine tool or other machine tools, so as to ensure the precision relationship between the machining surfaces of each workstation.

4. When the positioning reference of the machining center part is difficult to coincide with the design reference, the assembly drawing should be carefully analyzed to determine the design function of the part's design reference. Through the calculation of the dimension chain, the geometric tolerance range between the positioning reference and the design reference should be strictly defined to ensure machining accuracy.

5. When it is not possible to simultaneously complete the workstation processing including the design reference on the machining center, the positioning reference should be aligned with the design reference as much as possible. At the same time, it is also necessary to consider that after using this benchmark for positioning, all key precision parts can be machined in one clamping operation.