To study the relative relationship of the surface of the parts, it is necessary to determine a benchmark, which is used to determine the points, lines and surfaces of other points, lines and surfaces. According to the different functions of the datum, the precision machining parts are made of several surfaces. The benchmark can be divided into two types of design benchmark and process benchmark, the following is a detailed explanation.
1 precision machining design reference
A reference for determining other points, lines, and surface positions on the part drawing is called a design reference. The sleeve parts precision machining, design basis for the outer circle and inner hole is part of the axis line, face a is end B and C design reference, the axis of the hole is cylindrical radial runout of the benchmark.
2 process standard
The benchmark used in the processing and assembly of precision machining parts, called process reference. According to the different uses of the process, it is divided into the assembly datum, the measuring datum and the locating datum.
(1) an assembly reference assembly is used to determine the position of a part in a component or product, called an assembly reference.
(2) a benchmark is used to test the size and position of the machined surface, referred to as the measuring datum. In figure 32-2, the inner hole axis is a measure of the radial run out of the outer circle. The surface A is the test length L and the L and the measuring datum.
(3) a reference for locating the workpiece in a reference position, called the positioning reference. As the positioning reference surface (or line, point), in the first process can only choose the raw surface of the blank, the positioning surface of the said crude benchmark. All the steps in the future can be the machined surface as the locating datum, the positioning surface called precise basis.