Abstract: The determination of blank dimension datum ignores its specific machining process and uses the datum of parts drawing, that is the dimension of blank is generated in the dimension of the part corresponding modified total allowance, which seems to become a classical design method. But the method don't determine its datum unclearly according with the discipline of the blank size affecting the process, which will lead to a negative impact that more blank dimension error will accumulated to the process. Based on the rule of dimension path, the characteristics of blank dimension and its reference are explained. Aiming at the minimum effect of blank size error on machining process accuracy, the principle of determining the blank dimension datum is given to determine the method of confirming plane blank datum. Firstly, the three datums where the workpiece combined in the plane are abstracted into a vector, and the datum vector is often used. In view of the problem that the rough datum vector is often not in the geometric elements of the blank. Then the principle of proximity put forward to sum up the method of determining the dimension datum of plane blanks. Through the case analysis and calculation of the box and other parts, it is proved that the new method is an effective method for determining the blank dimension reference and reducing the machining allowance and improving the dimensional accuracy of parts.

Key words: datum vector, diagram of dimension path, dimension datum of plane blanks, principle of the proximity