concept

Mechanical parts are composed of several surfaces. To study the relative relationship between the surfaces of the parts, a datum must be determined. The datum is the point, line, and surface on which the position of other points, lines, and surfaces is determined on the part. According to the different functions of benchmarks, benchmarks can be divided into two categories: design benchmarks and process benchmarks.

Classification

Design datum: The datum used to determine the position of other points, lines and surfaces on the part drawing is called the design datum.

Process datum: The datum used by the part in the process of machining and assembly is called the process datum. Process benchmarks are divided into assembly benchmarks, measurement benchmarks and positioning benchmarks according to different uses.

(1) Assembly datum: The datum used to determine the position of a part in a component or product during assembly is called an assembly datum.

(2) Measurement datum: The datum used to check the size and position of the machined surface is called the measurement datum.

(3) Positioning datum: The datum used for workpiece positioning during processing is called the positioning datum. As the surface (or line, point) of the positioning reference, only the unprocessed blank surface can be selected in the first process. This positioning surface is called the rough reference. The machined surface can be used as the positioning reference in each subsequent process. , this positioning surface is called fine datum.