# Precision machining process-part positioning and datum

1. Benchmark
Benchmark refers to the point, line, and area on which the geometric relationship between the geometric elements of the production object is determined. For a mechanical part, the datum is the point, line, and surface on which the positions of other points, lines, and surfaces on the part are determined.
In the design and machining of machine parts, which points, lines, and surfaces are selected as benchmarks according to different requirements are one of the main factors that directly affect the advance cnc machining technology of the parts and the size and position accuracy between the 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. On a machine part drawing, there can be one or multiple design bases. In Figure 1-1a, A and B are the design basis for each other; in Figure 1-1b, the outer circle of 40mm is the design basis of the outer circle of 60mm; in Figure 1-1c, plane 1 is the design of plane 2 and hole 3. The benchmark, hole 3 is the design benchmark for holes 4 and 5; in Figure 1-1d, the center line is the design benchmark for the inner hole 30mm, the gear index circle 48mm, and the top circle 50h8mm.
(2) Process benchmark
Process benchmark refers to the benchmark used in the process of parts. According to different purposes, the process reference can be sub-process reference, positioning reference, measurement reference and assembly reference.

1. Process benchmark
In the process diagram, the reference used to determine the size, shape, and position of the processed surface of the process is called process reference. , The machined surface is hole D, and its centerline is required to be perpendicular to surface A, and to maintain distances L1 and L2 from surface C and surface B. Then A, B, and C are the process benchmarks for this process.
2. Positioning reference
During machining, the datum used to determine the correct position of the workpiece on the machine tool or fixture is called the positioning datum.
3. Measurement standard
During or after advance cnc machining, the benchmark used to measure the shape, position and size of the workpiece is called the measurement benchmark. As shown in Figure 1-3, the dimension L1 can be measured with a depth caliper, and the end face T is the measurement datum of the end face A.
4. Assembly benchmark
During assembly, the datum used to determine the relative position of the part or component on the product is called the assembly datum. For example, the inner hole of the gear is the assembly reference of the gear. Under normal circumstances, the design basis is given on the part drawing, and the process reference is determined by the craftsperson according to the specific process.

• When analyzing the benchmark problem, we must pay attention to the following two points:
1) The point, line, and surface used as the reference may not necessarily exist on the workpiece (such as the center line of the hole, the axis line, etc.), but are embodied by some specific surfaces, which are called base surfaces. For example, the centerline of the inner hole is embodied by the inner hole surface, the inner hole centerline is the reference, and the inner hole surface is the base surface. Therefore, the question of choosing a benchmark is the question of choosing an appropriate base. In particular, sometimes for the convenience of description, the reference and the base surface may be collectively referred to as the reference.
2) As a datum, it can be points and lines with no area or a very small surface, but the base surface representing this datum always has a certain area. For example, a long shaft is installed on a lathe with a center point. The reference is the axis line, which has no area; the base surface is the cone surface of the center point. It has a small area but does have a certain area.

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