A mechanical part is composed of several surfaces. To study the relative relationship of parts’ surfaces, a datum must be determined. The datum is the point, line and plane on which the position of other points, lines and surfaces can be determined. According to the different functions of datum, machining datum can be divided into design datum and technological datum.
Machining design datum: the datum used to determine the position of other points, lines and planes on the part drawing, which is called design datum.
Machining process datum: the datum used in the process of machining and assembling parts, which is called process datum. According to different uses, process datum can be divided into assembly datum, measuring datum and locating datum.
(1) Assembly datum: the datum used to determine the position of parts in parts or products during assembly, which is called assembly datum.
(2) Measurement datum: the datum used to check the size and position of machined surface, which is called measuring datum.
(3) Positioning datum: the datum used for workpiece positioning during machining, which is called positioning datum. The surface (or line or point) used as the reference for positioning
In the process, only the rough blank surface can be selected. This positioning surface is called the rough datum. In the subsequent processes, the machined surface can be used as the positioning reference, and the positioning surface is called the fine basis
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