A mechanical part is composed of several surfaces. To study the relative relationship of the surface of a part, a reference must be established. The reference is the point, line, or face on the part used to determine the position of other points, lines, or surfaces. According to the different functions of the benchmark, the benchmark can be divided into two categories: design benchmark and process benchmark.
1. Design basis
The reference used on the part drawing to determine the position of other points, lines, and surfaces is called the design datum. As shown in [cc2] of the sleeve part shown in Figure 32-2, the design reference of each outer circle and inner hole is the axis line of the part, and the end face is the design reference of the end face B and C. The axis of the inner hole is the outer circle. Radial jumping reference.
2. Process reference
The reference used in the processing and assembly of parts is called the process reference. Based on different uses, the process standards are divided into assembly standards, measurement standards and positioning standards.
(1) Assembly reference A reference used to determine the position of a part in a part or product when it is assembled is called an assembly reference.
(2) Measurement datum A datum used to check the size and position of a machined surface is called a datum. As in the part in Figure 32-2, the inner hole axis is the measurement reference for checking the radial runout of the outer circle; and the surface A is the measurement reference for checking the length and size of the large sum.
(3) Positioning reference The reference used for positioning the workpiece during machining is called the positioning reference. As the surface (or line, point) of the positioning reference, only the rough surface of the blank can be selected in the first operation. This positioning surface is called a coarse reference. In the subsequent processes, the machined surface can be used as a reference for positioning. This positioning surface is called a fine reference.