In the future each process must use the surface has been processed as the positioning datum, this positioning datum is called precision datum. The selection of precision datum directly affects the position accuracy of each surface of the parts, so in the selection of precision datum, it is necessary to ensure the machining accuracy of the workpiece and convenient and reliable clamping. The principles for selecting a fine benchmark are:

(1) Benchmark coincidence principle: as far as possible use the design benchmark as a fine benchmark, so as to avoid the positioning error caused by the benchmark does not coincide.

(2) The principle of the same datum: should make as much as possible surface processing with the same precision datum, in order to reduce the error caused by the change of positioning datum, and make the fixture structure unified. For example, machining shaft parts with the center hole as the precision reference, in the turning, milling, grinding and other processes always take it as the precision reference, so as to ensure the coaxiality between each section of the journal. It can also increase productivity. For example, when gear machining, the inner hole is usually processed well, and then the inner hole is used as a fine benchmark.

(3) The principle of mutual reference: use two surfaces on the workpiece that have mutual position accuracy requirements alternately as the positioning reference. For example, when processing a short sleeve, in order to ensure the coaxiality of the hole and the outer circle, the outer circle should be first used as the positioning reference for grinding the hole, and then the grinding hole as the positioning reference for grinding the outer circle.

(4) Easy to install, and the structure of the fixture is simple.

(5) Try to choose the surface with simple shape and large size as the fine reference, in order to improve the stability and accuracy of installation.