Selection of positioning datum for machining center
As with ordinary machine tools, in machining center processing, parts of the clamping still adhere to the 6-point positioning principle.
In selecting the positioning datum, we should consider the processing situation of each position comprehensively and achieve three purposes:
① the selected datum should be able to ensure the accurate positioning of the workpiece, convenient loading and unloading workpiece, can quickly complete the workpiece positioning and clamping, clamping reliable, and simple fixture structure.
② the selected datum and each processing part of the size of the operation is simple, as far as possible to reduce the size chain calculation, to avoid or reduce the calculation link and calculation error.
③ guarantee the processing accuracy. When determining the location datum of a part, follow the following guidelines:
A) to choose the design datum on the part as the positioning datum as far as possible. In the formulation of the part of the processing program, first of all, to select the best precision benchmark to process machining center. This requires that in roughing, consider how the rough datum of the precision of the various aspects of the processing, that is, the processing center used on the various positioning benchmarks in the front of ordinary machine tools or machining centers in the process of processing, so easy to ensure that the processing surface of each position of the precision relationship between each other When some of the surface depends on multiple clamping or other machine tools to complete, select the same benchmark as the design benchmark positioning, not only can avoid the datum caused by the positioning error, to ensure machining accuracy, and can simplify the programming.
B) When machining centers can not be completed at the same time, including the design of the base of the machining, should be as far as possible to make the positioning datum and design benchmarks coincide. At the same time, we should also consider using the benchmark positioning, one clamping can complete all the key precision parts processing. In order to avoid finishing the parts after a number of unimportant size processing, multiple turnover, resulting in deformation of parts, scratches, in considering as much as possible to complete the processing of content (such as screw hole, free hole, chamfer, not important surface, tool inspection, etc.), the General Processing center on the completion of the process in the final arrangement.
C) When machining the center on both the basis and the completion of the processing of the various stations, the choice of the location of the benchmark should be considered to complete as much processing content. To this end, to consider the convenience of all surfaces are processed positioning methods, such as for the box, preferably the use of a two-pin positioning method, so that the tool on the other surface processing.
D) When the position datum of the part is difficult to coincide with the design datum, the assembly drawing should be analyzed carefully, the design function of the design datum of the part is determined, and the form and position tolerance range between the positioning datum and the design datum should be strictly defined, and the machining precision can be ensured. For the machining center with automatic measuring function, the coordinate system measurement and inspection work step can be arranged in the process, that is, before each part is processed by the program automatic control probe design datum, the CNC system automatically calculates and revises the coordinate system, thus ensures the geometrical relationship between the machining parts and the design datum.
E) The original point of the workpiece coordinate system is "programming 0 points" and the part positioning datum does not necessarily coincide, but there must be a definite geometric relationship between the two. The selection of the origin of workpiece coordinate system is mainly considered for easy programming and measurement. For the parts with higher dimensional accuracy requirements, it is necessary to consider whether the coordinate origin can be accurately measured by positioning datum, and the measurement method should be taken into account.