How to choose the rough and precise datum for precision mechanical parts machining? People engaged in mechanical parts processing know that the positioning datum must be selected from the beginning to the end of processing. Then what is the positioning datum? It is the surface used to determine the relative position of the workpiece with respect to the machine tool and the tool. The datum used at the beginning is unprocessed and is called the rough datum. The positioning datum used in subsequent machining is the machined surface, which we call the fine datum.
How to choose which plane as a reference is a very important issue in the process design of precision mechanical parts. Whether the positioning datum is reasonable will directly affect the processing quality of precision parts and the complexity of the machine tool fixture structure. However, due to the different functions of precision benchmark and precision benchmark, their selection principles are also different.
Rough datum selection principles: first, to ensure that each machining surface has sufficient margin, second, to ensure that the size and location of the non-machined surface in line with the requirements of the drawings; the choice of rough datum should be easy to position the part, clamping and processing, and make the fixture structure as simple as possible. If the workpiece is first to ensure the position accuracy requirements between the machined and non-machined surfaces, the non-machined surface should be used as the rough datum. In order to ensure that the roughing allowance of the important surface is small and uniform, the surface should be selected as the roughing datum. To ensure that the roughing allowance of the important surface of the base is small and uniform, the surface should be selected as the roughing datum. In order to make the machining allowance of multiple precision machining surfaces above the blank more uniform, the surface of the blank that will have the same position error as the selected rough datum can be selected as the rough datum. Rough datum surface should be flat, no gating, riser, flying edge and other defects to ensure reliable positioning, the general rough datum can only be used once, especially the main positioning datum, in order to avoid large positional errors.
Fine benchmark selection principles: the selection of positioning benchmark should be easy to position, clamping and processing, and has sufficient positioning accuracy; according to the principle of unified benchmark, when the workpiece with a set of more easily processed most of the remaining surface of the fine benchmark positioning, in the precision parts processing these surfaces in each process, should use the same set of benchmarks for positioning, in order to reduce the tooling design and manufacturing, to avoid benchmark conversion errors, improve Productivity. Benchmark overlap principle. When the processing surface of precision mechanical parts ultimately need to ensure the position accuracy, the design reference should be selected as the positioning reference for positioning. In the finishing process according to the principle of unity of the benchmark positioning but can not guarantee the positioning accuracy of the surface, must use the principle of benchmark overlap and the principle of self-referencing. When some surface finishing process requires a small and uniform margin, the processing surface itself can be used as a positioning reference. At this time, the position accuracy requirement is guaranteed by the prior process.