Reference refers to the point, line and plane on which the geometric relations between production object geometric elements are determined. For a mechanical part, the reference is the point, line, or surface on which the position of the other points, lines, or surfaces on the part is determined.
In the design and processing of machine parts, it is one of the main factors that directly affect the machining process, surface dimension and position accuracy of the parts according to different requirements to choose which points, lines and surfaces as the benchmark. According to the different functions and application occasions, the datum can be divided into design datum and process datum.
(I) Design criteria
Design benchmark refers to the benchmark used to mark the design dimensions on the part drawing. There may be one or more design benchmarks on a machine part drawing. In Figure 1-1A, A and B are the design benchmarks for each other. In Figure 1-1B, the 40mm outer circle is the design benchmark of the 60mm outer circle. In Figure 1-1C, plane 1 is the design datum for plane 2 and hole 3, while hole 3 is the design datum for hole 4 and hole 5. In FIG. 1-1D, the centerline is the design datum for the inner hole 30mm, the gear dividing circle 48mm and the top circle 50h8mm.
(2) Process benchmark
Process reference refers to the reference used in the technological process of the parts. According to the different uses, the process datum can be divided into sequence datum, position datum, measuring datum and assembling datum.
1. Process benchmark
In the process drawing, the reference used to determine the size, shape and position of the processed surface of the process is referred to as the process reference. , the processed surface is hole D, and its center line is required to be perpendicular to Surface A, and the dimensions L1 and L2 should be kept apart from surface C and Surface B, so A, B and C are all the procedure benchmarks of this procedure.
2. Positioning reference
During processing, the datum used to determine the correct position of the workpiece on the machine tool or fixture is called positioning datum.
3. Benchmark of measurement
During or after processing, the datum used to measure the shape, position and dimensional error of the workpiece is called the datum of measurement. As shown in FIG. 1-3, the dimension L1 can be measured with A depth caliper, and the end face T is the measurement benchmark of end face A.
4. Assembly benchmark
During assembly, the reference used to determine the relative position of parts or components on the product is referred to as the assembly reference. For example, the inner hole of the gear is the assembly reference of the gear. In general, the design benchmark is given on the part drawing, the process benchmark is determined by the process personnel according to the specific process selection.
In the analysis of the benchmark problem, the following two points must be noted:
1) The point, line and surface as the reference may not exist in the workpiece (such as the center line of hole, axis line, etc.), but are reflected by some specific surfaces, which are called the base surface. 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 reference, and the surface of the inner hole is the base surface. Therefore, the problem of choosing a benchmark is the problem of choosing an appropriate base plane. In particular, benchmarks and base surfaces are sometimes referred to collectively as benchmarks for descriptive purposes.
2) As a benchmark, it can be points and lines with no area or very small faces, but the base surface representing such a benchmark always has a certain area. For example, on a lathe you mount a long shaft with a center point, the reference being the spindle line, which has no area; The base plane is the cone of the center, which is small but has a certain area.