几何形状位置公差中英对照18-20.doc

基准（续） 基准次序的重要性 18 Datums (Cont.) Importance of Datum Sequence 18 基准（续） 基准关联尺寸vs. 形体尺寸 尺寸 仅那些通过几何公差关系与参考基准体系有关联的尺寸需在参考基准体系中测量。如果某个尺寸与带有几何公差的参考基准体系没有关联，那么该尺寸在基准体系中不参与定位零件。 注：孔的位置是从参考基准体系测量，而36.0-36.2尺寸不必从参考基准体系测量 19  Datums (Cont.) Datum Related vs. Feature of Size Dimensions Only dimensions that are related to a datum reference frame through geometric tolerances should be measured in a datum reference frame. If a dimension is not associated to a datum reference frame with a geometric tolerance, then there is no specification on how to locate the part in the datum reference frame. NOTE: The hole locations are to be measured from the datum reference frame and the 36.0-36.2 dimension should (a FOS) not be measured from the datum reference frame. 19 基准（续） 目标基准识别符号 目标基准应用时使用两个符号：目标基准识别符号和表示被测元素类型的符号。目标基准识别符号如下图A所示。符号以一水平直线分成两部分，下半部分表示参考基准字符和组成基准的目标数目；上半部分是被测元素尺寸大小。来自基准符号的引线表示目标基准是在零件视图显示的可见面还是隐藏面上。如果引线是实线，那么目标基准是在可见面上的（如下图B面的B1，B2）。如果引线是虚线，那么就表示目标基准是在零件的隐藏面上的（如下图B面的A1，A2和A3）。在图纸上使用基准目标符号时，基准识别符号同时使用。 A 基准目标识别符号 20Datums (Cont.) Datum Target Identification Symbol A datum target application uses two symbols: a datum target identification symbol and a symbol that denotes which type of gage element is to be used. The datum target identification symbol is shown in panel A of the figure below. The symbol is divided into two parts with a horizontal line. The bottom half denotes the datum reference letter and the target number associated with that datum; the top half contains gage element size information when applicable. The leader line from the symbol specifies whether the datum target exists on the surface shown or on the hidden surface side of the part. If the leader line is solid, the datu