（1） Selection of positioning datum for CNC machining center
(1) Try to select the design datum on the part as the positioning datum.
(2) One clamping can complete the processing of all key precision parts.
(3) When the machining center not only processes the datum but also completes the machining of each station, the selection of its positioning datum needs to consider completing as many machining contents as possible.
(4) When the positioning datum and design datum of the part are difficult to coincide, the assembly drawing shall be carefully analyzed to determine the design function of the design datum of the part. Through the calculation of the dimension chain, the tolerance range between the positioning datum and the design datum shall be strictly specified to ensure the machining accuracy.
（2） Selection and use of fixture in CNC machining center
(1) According to the characteristics of machine tools and machining needs of machining centers, the commonly used fixture types are special fixture, combined fixture, adjustable fixture, group fixture and workpiece unified benchmark positioning fixture system.
(2) The high flexibility of machining center requires that the fixture is more compact and simple than the general machine tool, the clamping action is more rapid and accurate, the auxiliary time is reduced as far as possible, the operation is more convenient, labor-saving and safe, and the sufficient rigidity and flexibility are ensured. Therefore, pneumatic and hydraulic clamping devices are often used.
(3) In order to keep the workpiece fully exposed to all the surfaces to be processed in this positioning and clamping, the clamp shall be as open as possible, and the space position of the clamping element shall be low or low, so there must be space for the tool movement path.
(4) Considering the minimum distance between the spindle and the worktable and the clamping length of the tool, the installation position of the fixture on the worktable of the machine tool shall ensure that the processing content of the workpiece can be completed within the range of the spindle travel.
(5) It is not allowed to interfere with fixture or workpiece during automatic tool change and worktable exchange.
(6) Sometimes, the positioning block on the fixture is used when installing the workpiece. In the process of processing, in order to meet the requirements of the processing of front and rear left and right stations and prevent interference, the workpiece can be removed after being clamped. Therefore, it is necessary to consider how to maintain the positioning accuracy of the workpiece after removing the positioning elements.
(7) Try not to change the clamping point in the middle of processing.
（3） CNC machining to determine the best position of parts on the machine tool table
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