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Precision machining process - part positioning and reference

2018-07-13 Administrator home station clicks:

A benchmark,

The reference refers to the point, line and surface on which the geometric relations between the geometric elements of the production object are determined. For a mechanical part, a reference is to determine the points, lines, and surfaces on which other points, lines, and surfaces are located.

In the design and processing of machine parts, which points, lines and surfaces are selected as the reference according to different requirements is one of the main factors that directly affect the processing technology of the parts and the dimension and position accuracy between the surfaces. According to the different functions and applications, the datum can be divided into design datum and process datum.

Design criteria

Design datum refers to the datum used to mark the design dimensions on the part drawing. On a machine part drawing, there can be one or more design benchmarks. In FIG. 1-1a, A and B are design reference for each other. In figure 1-1b, 40mm outer circle is the design reference for 60mm outer circle; In figure 1-1c, plane 1 is the design reference for plane 2 and hole 3, and hole 3 is the design reference for hole 4 and hole 5. In FIG. 1-1d, the centerline is the design reference for 30mm inner bore, 48mm gear indexing and 50h8mm top circle.

Process standards

Process datum refers to the datum used in the process of parts. According to the different purposes, the process reference can be divided into sequence reference, positioning reference, measurement reference and assembly reference.

1. Process benchmark


                        Figure 1-1 example of a design benchmark

In the process diagram, the reference used to determine the size, shape and position of the processed surface of the process is referred to as the process reference. As shown in FIG. 1-2, the workpiece has D holes on the machining surface, and its center line is required to be perpendicular to surface A, and distance from surface C and surface B to dimensions L1 and L2. Therefore, A, B and C are all process standards of this process.

2. Positioning benchmark

The reference used to determine the correct position of the workpiece on the machine or in the jig during processing is called the positioning reference.



 Figure 1-2 sample process benchmarks     FIG. 1-3 schematic diagram of a workpiece procedure


In FIG. 1-3, the workpiece is clamped on the three-jaw self-centering chuck and mounted on the outer circle, then the positioning reference of size is the hole center line.

3. Measurement base

The datum used to measure the shape, position and size error of workpiece during or after processing is called the datum of measurement. As shown in figure 1-3, the size L1 can be measured by depth caliper, and the end surface T is the measurement benchmark of the end surface A.

4. Assembly reference

The reference used to determine the relative position of a part or component on a product during assembly is called the assembly reference. The inner bore of the gear, for example, is the assembly reference of the gear. Generally, the design reference is given on the part drawing, and the process reference is determined by the technologist according to the specific process selection.

The following two points must be noted in the analysis of the benchmark problem:

1) the points, lines and surfaces that serve as the reference may not exist on the workpiece (such as hole center line, axis line, etc.), but are reflected 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 base line, and the surface of the inner hole is the base surface. Therefore, the problem of selecting a baseline is the problem of selecting the appropriate base. In particular, sometimes benchmarks and bases can be collectively referred to as benchmarks for narrative convenience.

2) as a benchmark, it can be a point and line without area or a small surface, but the base surface representing this benchmark always has a certain area. For example, on a lathe a long shaft is mounted on the top, and the base is the center line, which has no area. The base is the top cone, it's very small but it does have a certain area.