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    in the process of mechanical processing, the selection of the positioning datum is reasonable or not determines the quality of the parts. It has a great influence on whether the size accuracy and the mutual position accuracy of the parts can be guaranteed, as well as the processing sequence arrangement between the parts of the parts. When the fixture is installed the workpiece, the selection of the positioning datum will also affect the quality of the workpiece. The complexity of the fixture structure. Therefore, the selection of positioning datum is a very important technological problem.
    on the time of selecting the coarse base, the main requirement is to ensure that the machining surface has enough allowance to make the position of the machined surface and the non machined surface conform to the drawing requirement, and to pay special attention to obtaining the fine base as soon as possible. The following principles should be considered in the specific selection:

(1) choose the important surface as the rough datum     in order to ensure that the machining allowance of the important surface on the workpiece is small and even, the surface should be chosen as the rough datum. The so-called important surface is generally the machining precision of the workpiece and the surface with high surface quality, such as the guide surface of the bed and the spindle hole of the main axle box of the lathe, all of which are the important surfaces of each. Therefore, when processing the lathe bed and spindle box, the guide rail or spindle hole should be the rough datum.

(2) choose the unprocessed surface as the rough datum   in order to ensure the location requirement between the machined surface and the unmachined surface, the rough surface should be chosen as the rough base. If there are many non machining surfaces on the workpiece, the non machining surface which is higher in the position of the machining surface should be selected as the coarse reference, so as to ensure the requirements and the symmetry of the shape.

(3) the surface of the minimum processing allowance is a coarse reference   if there is no requirement to ensure that the important surface processing allowance is uniform, if each surface is to be processed on the part, the surface of the smallest machining allowance should be selected as the coarse reference to avoid leaving some blank surface in the process of insufficient allowance. Cause the workpiece waste.

(4) choose a more smooth and clean surface with a larger processing area as the rough reference  , so that the workpiece is reliable and easy to clamp.

(5) the rough datum can only use one   in the same dimension, because the rough datum itself is unmachined blank surface, its surface is rough and the precision is low, if repeated use will produce larger error.