yogy

参考基准须标全。


Datum Features Must Be Referenced to Be Used.
如图所示，孔的位置公差给出了基准A和B的要求， 而无相对于基准C的要求。

孔只须满足垂直于基准A，距离基准轴线B为20mm的两个要求。则孔的位置不能唯一确定，孔的两种可能位置如右图所示。为了给定孔的位置，还须给出相对于基准C的要求。



There continues to be the misconception that if a feature is labeled as a datum feature, its use is somehow implied. Take the drawing shown. The position tolerance on the hole references datum features A and B but not C. Therefore the hole must be perpendicular to the plane established by datum feature A and 20 mm from the axis established by datum feature B. Datum feature C has nothing to do with the position of the hole. Yes, the hole is shown on a centerline that is perpendicular to datum feature C. So, we know what the goal is. But, since datum feature C was not referenced in the position tolerance, there is no limit to how far the hole can be off the 90° angle. A couple of possible, acceptable parts are shown.



In order to avoid this, all you have to do is add datum feature C to the position tolerance.