How to choose the positioning reference during the machining of parts on a machining center

In the process of machining workpieces with a CNC machining center, the choice of positioning reference determines the quality of the parts. It significantly impacts whether the dimensional accuracy and mutual positional accuracy of the parts can be guaranteed, as well as the arrangement of the machining sequence between the surfaces of the parts. When using fixtures to install workpieces, the choice of positioning reference also affects the complexity of the fixture structure. This requires the fixture to withstand high cutting forces while meeting the positioning accuracy requirements. Therefore, the choice of positioning reference is a very important process issue. So, how do we choose a positioning reference when machining parts with a CNC machining center? Here's a brief introduction:

The chosen benchmark should ensure accurate positioning of the workpiece, facilitate the loading and unloading of the workpiece, enable rapid completion of workpiece positioning and clamping, provide reliable clamping, and have a simple fixture structure.

2. The benchmark selected for the CNC machining center and the various dimensions of each machining part are computationally simple, minimizing the calculation of dimensional chains and avoiding or reducing calculation steps and errors.

3. Ensure the accuracy of various machining processes. When specifically determining the positioning reference for 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 defined geometric relationship between the two. The selection of the origin of the workpiece coordinate system primarily considers ease of programming and measurement. For parts with high requirements for dimensional accuracy, when determining the positioning reference, it should be considered whether the coordinate origin of the CNC machining center can be accurately measured through the positioning reference.

2) When both machining the reference and completing the processing of various stations on a CNC machining center, the selection of positioning reference should consider completing as much processing content as possible. To this end, it is necessary to consider positioning methods that facilitate the machining of all surfaces. For example, for a box body, it is best to adopt a one-side two-pin positioning method to facilitate the machining of other surfaces by the tool.

3) Try to choose the design reference on the part as the positioning reference. This requires considering how to use the rough reference to machine the surfaces of the fine reference during rough machining. That is, the various positioning references used on the CNC machining center should be machined on the preceding conventional machine tool or other machine tools, which makes it easier to ensure the accuracy relationship between the machined surfaces at each workstation.

4) When the positioning reference of a part in a CNC machining center is difficult to coincide with the design reference, it is necessary to carefully analyze the assembly drawing, determine the design function of the part's design reference, and strictly specify the geometric tolerance range between the positioning reference and the design reference through the calculation of dimensional chains to ensure machining accuracy.

5) When it is impossible to complete the processing of workstations, including the design reference, simultaneously on a CNC machining center, efforts should be made to ensure that the positioning reference coincides with the design reference. Additionally, consideration should be given to using this reference for positioning, so that all key precision parts can be processed in a single clamping operation.