concept
A mechanical part is composed of several surfaces. To study the relative relationship of the parts' surfaces, 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. According to different functions of datum, datum can be divided into design datum and process datum.
classification
Design basis: the basis used to determine the position of other points, lines and surfaces on the part drawing, which is called design basis.
Process benchmark: the benchmark used in the processing and assembly of parts, which is called process benchmark. According to different purposes, process datum can be divided into assembly datum, measurement datum and positioning datum.
(1) assembly datum: the datum used to determine the position of parts in components or products during assembly is called 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 machining is called positioning datum. For the surface (or line, point) as the positioning reference, only the rough surface can be selected in the first process. This positioning surface is called the rough reference. In each subsequent process, the machined surface can be used as the positioning reference, and this positioning surface is called the fine reference.
