To study the relative relationship of the surface of a part, a datum must be defined. The datum is the point, line and surface on which the part is used to determine the position of other points, lines and surfaces. Depending on the different functions of the benchmarks, precision machined parts consist of several surfaces. Benchmarks can be divided into two types of design basis and process basis, the following is a detailed explanation.
1. Precision machining design basis
In the part diagram used to determine the other points, lines, surface location of the benchmark, known as the design basis. Precision machined sleeve parts, the design of the outer circle and the bore hole is the axis of the part, the end face A is the design criteria for the end face B, C, the axis of the bore is the base of the outer runout.
2. process basis
Precision machined parts in the processing and assembly process used by the benchmark, known as the process base. Process base according to different uses is divided into assembly benchmarks, measurement standards and positioning standards.
(1) The reference used to determine the position of a part in a part or product when assembling a reference assembly is called the assembly reference.
(2) Measurement Basis The standard used to test the size and position of the machined surface is called the measurement standard. As shown in Figure 32-2 parts, bore axis is to test the radial runout measurement standards; surface A is the length of inspection L size l and measurement standards.
(3) Positioning Benchwork processing workpiece positioning reference, known as the positioning of the benchmark. As the positioning of the surface (or line, point), in the first process can only select the rough blank surface, the positioning of the surface, said rough bench in the subsequent process can be used as the positioning of the surface of the base This positioning surface is called fine reference.