How to select positioning datum for machining parts with machining center

When selecting the positioning datum, the machining center, like ordinary machine tools, should consider the processing conditions of each station in an all-round way, so as to achieve three purposes: first, the selected datum must be able to ensure the accurate positioning of the workpiece and facilitate the loading and unloading of the workpiece. It can complete the positioning and clamping of the workpiece at the fastest speed, and the clamping is reliable, and the fixture structure should be as simple as possible; Second, the selected datum requirements and the calculation of each dimension of each processing part should be as simple as possible, so as to reduce the calculation of dimension chain and avoid errors or errors in the calculation link as far as possible; The third is to ensure the accuracy of all processing.
When determining the specific positioning datum of parts on the machining center, we must pay attention to the following important principles:

Take the design datum as the positioning datum
When machining parts with machining centers, try to choose the design datum on the parts as the positioning datum when selecting the positioning datum. When it is necessary to formulate the processing scheme of parts, the best precision datum should be selected to complete the processing. Therefore, in the rough machining stage, we must consider how to process all surfaces of the precision datum. In other words, each positioning datum used on the machining center should be processed in the previous ordinary machine tool or machining center process, so as to effectively ensure the accuracy relationship between the machining surfaces of each station. In particular, if some surfaces need to be clamped for many times or processed on other machine tools, selecting the same datum positioning as the design datum can not only avoid the positioning error caused by the non coincidence of the datum, ensure the machining accuracy, but also simplify the programming.

Make the positioning datum coincide with the design datum
However, in some cases, it is impossible to complete the station processing including the design datum on the machining center at the same time, so we should try to make the positioning datum coincide with the design datum. At the same time, we should also consider whether the machining of all key precision parts can be completed in one clamping after positioning with this benchmark. In order to avoid the deformation, bumps and scratches of parts caused by repeated turnover and non important dimensional processing of parts after finishing, the process completed on the machining center is usually arranged at the end.

Positioning mode in which all surfaces are machined
When the machining center needs both the machining datum and the machining of each station, the selection of its positioning datum must consider the completion of as many machining contents as possible. Therefore, it is necessary to use a positioning method that is convenient for all surfaces to be processed. For example, for box processing, it is best to adopt the positioning method of one side and two pins, so that the tool can process other surfaces.
Strictly stipulate the geometric tolerance of positioning and design basis
If the positioning datum and design datum of the part can't coincide, the assembly drawing should be carefully analyzed to determine the design function of the design datum of the part, and the geometric common deviation range between the positioning datum and design datum should be strictly stipulated through the calculation of dimension chain to ensure the required machining accuracy. If you use a machining center with automatic measurement function, you can arrange a program to automatically control the probe to detect the design basis before each part is processed, and let the system automatically correct the coordinate system, so as to ensure the geometric relationship between each processing part and the design basis.

There is a definite geometric relationship between the origin of the coordinate system and the positioning datum
The origin of the coordinate system of the workpiece on the machining center, that is, the "programming zero point", does not necessarily coincide with the positioning datum of the part, but there must be a definite geometric relationship between them. When selecting the origin of the coordinate system, the main consideration is to facilitate programming and measurement. When selecting the positioning datum, the main consideration is whether the coordinate origin can be accurately measured through the positioning datum, especially for parts with high dimensional accuracy requirements, it is more necessary to ensure the accuracy of measurement.