Those who are engaged in precision machining of mechanical parts know that from the beginning of machining to the time of machining, the positioning datum must be selected. So what is the positioning reference?
The positioning datum is used to determine the relative position of the workpiece with respect to the surface of the machine and the tool. The data used at the beginning is unprocessed and is called coarse data. The positioning datum used in subsequent processes is the machined surface, which we call the fine datum.
In the machining process design of mechanical parts, how to choose as a datum surface is a very important issue; whether the choice of positioning datum is reasonable will directly affect the processing quality of precision parts and the complexity of the machine tool fixture structure. The role of the boutique benchmark is different, the selection principles of the two are also different.
Rough benchmark selection principles: one is to ensure that each machined surface has sufficient margin, the second is to ensure that the size and location of the unmachined surface in line with the drawing requirements. The selected coarse datum should be easy to position the part, clamping and processing, so that the fixture structure is simple. If you ensure that the workpiece has been machined surface and the position between the unmachined surface is accurate, the unmachined surface should be used as a rough reference. 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 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 on the blank more uniform, the rough machining surface should be selected so that the rough machining surface is based on the selected rough machining datum. A rough datum surface with uniform positional error is a rough datum surface. The rough datum surface should be flat, without defects such as sprue, riser or flying edge, to ensure reliable positioning. General rough datum can only be used once, especially the main positioning datum, so as not to produce a large position error.
Mechanical parts machining accuracy benchmark selection principle: the selected positioning benchmark should be easy to locate, clamping and processing, and must have sufficient positioning accuracy. Uniform benchmark principle. When the workpiece is positioned with a certain group of fine datum to facilitate the processing of most of the remaining surfaces, the same group of datum should be used for positioning in the processing of these surfaces, which can reduce the design and manufacture of tooling and avoid datum. Conversion errors and increase productivity. Reference overlap principle. When the machined surface of a precision mechanical part needs to ensure the positional accuracy of the end, the design reference should be selected as the positioning reference. When using the reference consistent principle for positioning and the positional accuracy of these surfaces cannot be guaranteed, reference overlap must be used. The principle is the self-referencing principle. When some surface finishing process requires a small and uniform margin, the machined surface itself can be used as a positioning reference for positioning, when the position accuracy requirements are ensured by the prior process.