Mechanical parts are composed of several surfaces. To study the relative relationship between the parts’ surfaces, a datum must be determined. The datum is the point, line, or surface on which the position of other points, lines, or surfaces is determined. According to different functions of benchmarks, benchmarks can be divided into design benchmarks and process benchmarks.
Design basis: The basis used to determine the position of other points, lines, and areas on a part drawing is called a design basis.
Process benchmark: The benchmark used by parts during processing and assembly is called the process benchmark. 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 benchmark: The benchmark used to check the size and position of the processed surface is called the measurement benchmark.
(3) Positioning reference: The reference used for positioning the workpiece during processing is called the positioning reference. As the positioning reference surface (or line or point), only the unprocessed rough surface can be selected in the first process. This kind of positioning surface is called a rough reference. The processed surface can be used as the positioning reference in each subsequent process , This positioning surface is called fine datum.