1、精基準的選擇原則 1, the selection principle of fine benchmarks 選擇精基準一般應遵循以下幾項原則： Generally speaking, the following principles should be followed in selecting fine benchmark. （1）基準重合原則：應盡可能選擇被加工表面的設計基準作為精基準，這樣可以避免由于基準不重合引起的定位誤差。 (1) the principle of reference coincidence: the design datum of the machined surface should be chosen as the fine reference as much as possible so that the positioning error caused by the non coincidence of the datum is avoided. （2）統一基準原則：應盡可能選擇用同一組精基準加工工件上盡可能多的表面，以保證各加工表面之間的相對位置精度。 (2) the principle of unified reference: it is necessary to choose as many surfaces as possible with the same group of fine datum to ensure the relative position accuracy between the machined surfaces. （3）互為基準原則：當工件上兩個(gè)加工表面之間的位置精度要求比較高時(shí)，可以采用兩個(gè)加工表面互為基準反復加工的方法。 (3) the principle of mutual reference: when the position precision between two machined surfaces on the workpiece is higher, the two machining surfaces can be processed repeatedly. （4）自為基準原則：一些表面的精加工工序，要求加工余量小而均勻，常以加工表面自身作為精基準。 (4) self - based principle: some surface finishing processes require small and uniform machining allowance, often using the machined surface itself as a fine benchmark. 上述四項選擇精基準的原則，有時(shí)不可能同時(shí)滿(mǎn)足，應根據實(shí)際條件決定取舍。 The above four principles of selecting fine benchmarks are sometimes impossible to meet at the same time, and decisions should be made according to actual conditions. 2、粗基準的選擇原則 2, the principle of selecting rough benchmarks （1）工件加工的第一道工序要用粗基準，粗基準選擇得正確與否，不但與第一道工序的加工有關(guān)，而且還將對工件加工的全過(guò)程產(chǎn)生重大影響。 (1) the first process of workpiece processing should be used with coarse datum, and the selection of rough datum is correct or not. It is not only related to the processing of the first process, but also has a great influence on the whole process of the workpiece. （2）合理分配加工余量的原則：從保證重要表面加工余量均勻考慮，應選擇重要表面作粗基準。 (2) the principle of reasonable allocation of machining allowance: from the uniform consideration of ensuring important surface machining allowance, the important surface should be selected as the rough datum. （3）便于裝夾的原則：為使工件定位穩定，夾緊可靠，要求所選用的粗基準盡可能平整、光潔，不允許有鍛造飛邊、鑄造澆冒口切痕或其它缺陷，并有足夠的支承面積。 (3) the principle of easy to clamp: in order to make the workpiece stable, clamped and reliable, the coarse reference is required to be as smooth and clean as possible. It is not allowed to have forged flown, cast in situ riser cut marks or other defects, and has sufficient supporting area. （4）粗基準一般不得重復使用的原則。 (4) rough datum should not be repeated in principle. 上述4項選擇粗基準的原則，有時(shí)不能同時(shí)兼顧，只能根據主次決擇。 The above 4 principles of selecting rough benchmarks can not be considered at the same time.