In the machining process of the CNC machining center, whether the selection of the positioning reference is reasonable determines the quality of the parts, and has a great influence on whether the dimensional accuracy and mutual position accuracy of the parts can be guaranteed and the machining sequence arrangement between the parts. When the workpiece is installed with a fixture, the choice of positioning datum will also affect the complexity of the fixture structure. This requires the fixture not only to withstand greater cutting force, but also to meet the requirements of positioning accuracy. Therefore, the choice of positioning reference is a very important technical issue. So how to choose a positioning datum when machining parts with a CNC machining center? Here is a brief introduction:
1. The selected datum should ensure the accurate positioning of the workpiece, facilitate the loading and unloading of the workpiece, quickly complete the positioning and clamping of the workpiece, the clamping is reliable, and the fixture structure is simple.
2. The CNC machining center has simple operations between the selected datum and each size of each processed part, which can minimize the calculation of the dimensional chain and avoid or reduce the calculation links and calculation errors.
3. Ensure machining accuracy. When determining the part positioning datum, the following principles should be followed:
1) The origin of the workpiece coordinate system, that is, the “program zero point”, does not have to coincide with the part positioning datum, but there must be a certain geometric relationship between them. The choice of the origin of the workpiece coordinate system is mainly for the convenience of programming and measurement. For parts with high dimensional accuracy requirements, when determining the positioning datum, it should be considered whether the positioning datum can accurately measure the coordinate origin of the CNC machining center.
2) When machining benchmarks and finishing at each station of the CNC machining center, the selection of positioning benchmarks should consider as much machining content as possible for finishing. For this reason, it is necessary to consider a convenient positioning method for each surface to be processed. For example, for the box, it is best to use the positioning method of two pins on one side, so that the tool can process other surfaces.
3) Try to choose the design datum on the part as the positioning datum. This requires that when roughing, consider what kind of roughing datum to use to machine each surface of the finishing datum, that is, each positioning datum used by the CNC machining center must be processed on the previous ordinary machine tool or other machine tool. In this way, it is easy to ensure the accuracy relationship between the machining surfaces of each station.
4) When the positioning datum of the CNC machining center parts is difficult to coincide with the design datum, the assembly drawing should be carefully analyzed to determine the design function of the part design datum, and through the calculation of the dimensional chain, strictly specify the shape and position between the positioning datum and the design datum Tolerance range to ensure machining accuracy.
5) When the station machining including the design datum cannot be completed at the same time in the CNC machining center, try to make the positioning datum coincide with the design datum. At the same time, it should be considered that after positioning with this datum, all key precision parts can be processed by one clamping.