A benchmark

The reference refers to the point, line and surface on which the geometric relations between the geometric elements of the production object are determined. For a mechanical part, a reference is to determine the points, lines, and surfaces on which other points, lines, and surfaces are located.

In the design and processing of machine parts, which points, lines and surfaces are selected as the reference according to different requirements is one of the main factors that directly affect the processing technology of the parts and the dimension and position accuracy between the surfaces. According to the different functions and applications, the datum can be divided into design datum and process datum.

Design criteria

Design datum refers to the datum used to mark the design dimensions on the part drawing. On a machine part drawing, there can be one or more design benchmarks. In FIG. 1-1a, A and B are design reference for each other. In figure 1-1b, 40mm outer circle is the design reference for 60mm outer circle; In figure 1-1c, plane 1 is the design reference for plane 2 and hole 3, and hole 3 is the design reference for hole 4 and hole 5. In FIG. 1-1d, the centerline is the design reference for 30mm inner bore, 48mm gear indexing and 50h8mm top circle.

Process standards

Process datum refers to the datum used in the process of parts. According to the different purposes, the process reference can be divided into sequence reference, positioning reference, measurement reference and assembly reference.