Rough reference is the reference for positioning on the rough surface in the initial processing steps. When selecting a rough reference, ensure that each surface has sufficient machining allowance so that the processed surface has a proper mutual position with the non-processed surface. The selection principles are:

• (1) Use the surface of the workpiece that does not need to be processed as a rough reference to ensure that the position error between the processed surface and the unprocessed surface is small.
• (2) If the machining allowance of an important surface of the workpiece is uniform, the surface should be selected as the rough reference.
• (3) Try to use a flat, large enough rough surface as a rough reference.
• (4) The rough reference cannot be used repeatedly, because the surface accuracy of the rough reference is low, and it is not guaranteed that the workpiece will maintain the same position during the two installations.