To study the relative relationship of the parts of the surface, a datum must be determined. The datum is used to determine the other parts. Points, lines, and positions are based on points, lines, and surfaces. According to the datum of different functions, the parts are precisely machined by several surfaces. Standards can be divided into two categories, design data and technical data, as explained in detail below.

1. Fundamentals of precision machining design

Parts drawing is used to determine the reference point, line, surface, called design reference. The precision machining of bushing parts, the outer circle and the inner hole are designed to be part of the reference shaft, the end face is designed to be B, C reference, the shaft of the reference hole is radially beating.

2. Process metrics

Benchmarks are used during machining and assembly of precision machined parts, a process called benchmarking. The benchmarking process is divided into different ones based on the use of assembly benchmarks, measurement benchmarks, and position benchmarks.

Assembly datum When assembling (1), the positional datum used to determine the components of a component or product is called the assembly datum.

(2) Measurement datum The datum for the size and position of the completed surface is called the measurement datum. As shown in Figure 32-2, the radial runout measurement datum of the inner hole shaft cylinder, check the length dimension L, and the measurement datum of the surface.

(3) Positioning datum The positioning datum of the workpiece is called the positioning datum. As the positioning datum of the surface (or line, point), only the blank positioning surface coarse datum is selected in the first step of the original surface. In each process, the machined surface is used as the positioning datum, and the fine datum of the positioning surface is used.