In the subsequent steps, the processed surface is used as the positioning reference. This positioning reference is called the fine reference. The selection of the precision reference directly affects the mutual position accuracy of each surface of the part. Therefore, when selecting the precision reference, it is necessary to ensure the machining accuracy of the workpiece and the convenient and reliable clamping. The principle of choosing the fine benchmark is:

• (1) Principle of reference coincidence: Use the design reference as the fine reference as much as possible to avoid positioning errors caused by non-coincidence of reference.
• (2) The same principle of datum: The same precision datum should be used for as much surface processing as possible to reduce the error caused by changing the positioning datum and make the fixture structure uniform. For example, the center hole is used as a precision reference for machining shaft parts. It is always used as a precision reference in turning, milling, grinding and other processes, so that the coaxiality between the journals of each section can be ensured and the productivity can be improved. As another example, when machining gears, the inner hole is usually processed first, and then the inner hole is used as a fine reference.
• (3) The principle of mutual reference: Use two surfaces on the workpiece that have mutual position accuracy requirements as the positioning reference alternately. For example, to process a short sleeve, in order to ensure the coaxiality of the hole and the outer circle, the outer circle should be used as the positioning reference to grind the hole first, and then use the polished hole as the positioning reference to grind the outer circle.
• (4) is convenient for installation and makes the structure of the jig simple.
• (5) Try to choose a surface with a simple shape and a large size as a fine reference to improve the stability of the installation.