Non-standard machining benchmarks are divided into planning benchmarks and process benchmarks according to their different functions.

(1) Planning basis. Planning basis is the basis used in the planning of parts drawings. The dimensions and interposition of the elements are identified according to the planning basis. The planning basis of the inner hole, outer circle and indexing circle of the gear is the axis of the gear, and the two end faces can be considered as mutual reference.

(2) Process standard. The process standard is the standard used in the process of making parts and devices. According to its use of different process benchmarks are divided into positioning benchmarks, measurement benchmarks and device benchmarks. They are respectively used for the positioning of the workpiece during processing, the measurement of the workpiece and the device of the part.

The positioning reference refers to the surface on which the workpiece is used for non-standard machining to confirm the relative position of the workpiece to the machine tool and the tool. For example, when the outer circle and the left end face of the gear wheel blank are shown, if the workpiece is installed on the mandature with the inner hole that has been processed, the axis of the hole is the positioning reference for the outer circle and the left end face. It is necessary to point out that the point or line on the workpiece as a positioning reference is always represented by the detailed surface, which is called the positioning reference surface. The axis of the gear hole does not exist in detail, but is represented by the surface of the inner hole, so to be precise, the inner hole in the figure is the positioning datum for processing the outer circle and the left end face.