The 6-point positioning principle for the selection of machining center positioning datum

Release time:

2021-05-18

When processing on a machining center, the clamping of parts must comply with the 6-point positioning principle.

When selecting the positioning datum, it is necessary to fully consider the processing conditions of each station to achieve three objectives:

①The selected datum should be able to ensure the accurate positioning of the workpiece, convenient to load and unload the workpiece, can quickly complete the positioning and clamping of the workpiece, the clamping is reliable, and the fixture structure is simple.

②The selected datum and each size of each processing part are easy to calculate, so as to minimize the calculation of the dimensional chain, and avoid or reduce the calculation links and calculation errors.

③Ensure various machining accuracy. When determining the positioning datum of the parts, the following principles should be followed:

A) Try to choose the design datum on the part as the positioning datum. When formulating a part's processing plan, first select the best precision benchmark for machining center processing. This requires that during rough machining, consider what rough datum to use to machine each surface of the fine datum, that is, each positioning datum used on the machining center should be processed in the previous ordinary machine tool or machining center process, so that it is easy to ensure that each work The precision relationship between the machining surfaces, and when some surfaces need to be completed by multiple clamping or other machine tools, choosing the same datum positioning as the design datum can not only avoid the positioning error caused by the datum misalignment , To ensure the processing accuracy, and can simplify the programming.

B) When the station processing including the design datum cannot be completed on the machining center at the same time, the positioning datum and the design datum should be overlapped as much as possible. At the same time, it must be considered that after positioning with this reference, all key precision parts can be processed in one clamping. In order to prevent the finished parts from undergoing multiple non-important size processing and multiple turnovers, resulting in deformation of the parts, bumps and scratches, consider completing as many processing contents as possible at one time (such as screw holes, free holes, chamfers, etc.) In addition to non-important surfaces, tool inspections, etc.), the processes completed on the machining center are generally arranged at the end.

C) When both the processing datum and the processing of each station are completed on the machining center, the selection of the positioning datum should consider the completion of as much processing content as possible. For this reason, it is necessary to consider a positioning method that facilitates all surfaces to be processed. For example, for a box, it is best to use a positioning method of two pins on one side to facilitate the processing of other surfaces by the tool.

d) When the positioning datum of the part is difficult to coincide with the design datum, the assembly drawing should be carefully analyzed to determine the design function of the design datum of the part. Through the calculation of the dimensional chain, the geometric tolerance range between the positioning datum and the design datum should be strictly specified to ensure Precision. For machining centers with automatic measurement functions, coordinate system measurement and inspection steps can be arranged in the process, that is, the program automatically controls the probe to detect the design basis before each part is processed, and the CNC system automatically calculates and corrects the coordinate system to ensure that each part is processed. The geometric relationship between the processing part and the design basis.

E) The origin of the workpiece coordinate system, that is, the "programming zero point" and the part positioning datum do not necessarily have to coincide, but there must be a definite geometric relationship between the two. The main consideration for the selection of the origin of the workpiece coordinate system is to facilitate programming and measurement. For parts with high dimensional accuracy requirements, when determining the positioning reference, it should be considered whether the coordinate origin can be accurately measured by the positioning reference, and the measurement method should be taken into consideration.