① Try to choose the design basis as the positioning basis;
② When the positioning datum and the design datum cannot be unified, the positioning error shall be strictly controlled to ensure the machining accuracy;
③ When the workpiece needs to be clamped for more than two times, the selected benchmark can complete the processing of all key precision parts in one clamping and positioning
④ The selected benchmark shall guarantee to complete as many processing contents as possible;
⑤ In batch machining, the positioning datum of parts should coincide with the tool setting datum of workpiece coordinate system as much as possible;
⑥ When it is necessary to clamp several times, the benchmark shall be uniform.
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