Datum refers to the point, line and surface used to determine the geometric relationship between the geometric elements of the production object.
For a mechanical part, datum is the point, line and surface on which the position of other points, lines and surfaces on the part is determined.
In the design and machining of machine parts, which points, lines and surfaces are selected as the datum according to different requirements is one of the main factors that directly affect the machining technology of parts and the dimension and position accuracy between surfaces. According to different functions and applications, benchmarks can be divided into design benchmarks and process benchmarks.
（1） Design basis
Design basis refers to the basis used to mark design dimensions on the part drawing. In a machine part drawing, there can be one or more design bases. In figure 1-1a, a and B are the design basis for each other; in figure 1-1b, 40mm outer circle is the design basis for 60mm outer circle; in figure 1-1c, plane 1 is the design basis for plane 2 and hole 3, and hole 3 is the design basis for hole 4 and hole 5; in Figure 1-1d, the center line is the design basis for 30mm inner hole, 48mm gear indexing circle and 50h8mm top circle.
（2） Process benchmark
Process benchmark refers to the benchmark used in the process of parts. According to different uses, process datum can be divided into process datum, positioning datum, measurement datum and assembly datum.
1. Process benchmark
In the process diagram, the datum used to determine the size, shape and position of the machined surface in this process is called the process datum. If the machining surface is D-hole, and the center line is required to be perpendicular to plane a and keep a distance from plane C and plane B, dimension L1 and L2, then a, B and C are the process datum of this process.
2. Positioning datum
During machining, the datum used to determine the correct position of the workpiece on the machine tool or in the fixture is called positioning datum.
3. Measurement datum
During or after machining, the datum used to measure the shape, position and dimension error of workpiece is called measurement datum. As shown in Figure 1-3, dimension L1 can be measured by depth caliper, and end face t is the measurement benchmark of end face a.
4. Assembly datum
During assembly, the datum used to determine the relative position of parts or components on the product is called assembly datum. For example, the inner hole of the gear is the assembly datum of the gear. In general, the design basis is given on the part drawing, and the process basis is determined by the technologist according to the specific process.
When analyzing the benchmark problem, we must pay attention to the following two points:
- 1) The datum points, lines and surfaces do not necessarily exist on the workpiece (such as hole center line, axis line, etc.), but are represented by some specific surfaces, which are called base surfaces. For example, the center line of the inner hole is reflected by the surface of the inner hole, the center line of the inner hole is the datum, and the surface of the inner hole is the base plane. Therefore, the problem of choosing datum is the problem of choosing proper datum. In particular, sometimes for the convenience of description, datum and datum can be collectively referred to as datum.
- 2) As a datum, it can be a point or line without area or a very small surface, but the base surface representing this datum always has a certain area. For example, in a lathe, a long shaft is installed with the center, and the datum is the axis line, which has no area; the datum is the cone of the center, which has a small area but has a certain area.
