VMC machining center positioning datum

When machining on a machining center, the clamping of the parts must comply with the 6-point positioning principle.
When selecting the positioning datum, the processing situation of each station should be fully considered to achieve three purposes:

①The selected datum should be able to ensure accurate positioning of the workpiece, convenient loading and unloading of the workpiece, rapid positioning and clamping of the workpiece, reliable clamping, and simple fixture structure

②The selected datum and each dimension of each processing part are simple to calculate, minimize the calculation of the dimension chain, avoid or reduce the calculation link and calculation error.

③Ensure the machining accuracy. When determining the positioning datum of the part, the following principles should be followed:
a) Try to select the design datum on the part as the positioning datum. When formulating a machining plan for a part, first select the best precision benchmark for machining center machining. This requires that during rough machining, consider what kind of rough datum is used to process all sides 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 workpiece is processed. Moreover, when some surfaces are completed by multiple clamping or other machine tools, selecting the same datum positioning as the design datum can not only avoid the positioning error caused by the misalignment of the datum , to ensure machining accuracy and simplify programming.

b) When the machining center including the design datum cannot be processed at the same time, the positioning datum and the design datum should be overlapped as much as possible. At the same time, it should also be considered that after using the reference positioning, the processing of all key precision parts can be completed in one clamping. In order to prevent the finished parts from going through multiple unimportant dimensional processing and multiple turnovers, resulting in deformation, bumping and scratching of the parts, it is considered to complete as many processing contents as possible at one time (such as screw holes, free holes, chamfers, At the same time as non-important surfaces, tool inspection, etc.), the processes completed on the machining center are generally arranged at the end.c) When both the datum and the processing of each station are processed on the machining center, the selection of the positioning datum should consider the completion of as many processing contents as possible. To this end, it is necessary to consider the positioning method that is convenient for all surfaces to be processed. For example, for the box, it is best to use the positioning method of two pins on one side, so that the tool can process other surfaces.

d) When the positioning datum of the part is difficult to coincide with the design datum, the assembly drawings should be carefully analyzed to determine the design function of the design datum of the part. Precision. For machining centers with automatic measurement function, coordinate system measurement and inspection steps can be arranged in the process, that is, the program automatically controls the probe to detect the design datum 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 machining part and the design basis.

e) The origin of the workpiece coordinate system, that is, the “programming zero point” and the part positioning datum need not necessarily coincide, but there must be a definite geometric relationship between the two. The selection of the origin of the workpiece coordinate system mainly considers the convenience of programming and measurement. For parts with high dimensional accuracy requirements, when determining the positioning datum, it should be considered whether the coordinate origin can be accurately measured through the positioning datum, and the measurement method should be taken into account

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