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Precision Machining Technology-Positioning and datum of parts

by:NJPE     2020-02-13
I. Datum, which refers to the points, lines and surfaces used to determine the geometric relationships between geometric elements of production objects. For a mechanical part, the datum is the points, lines and faces on which the positions of other points, lines and faces on the part are determined. In the design and processing process of machine parts, which points, lines and surfaces are selected as benchmarks according to different requirements, it is one of the main factors that directly affect the manufacturability of parts, the size and position accuracy between surfaces. According to different functions and application occasions, benchmarks can be divided into two categories: Design benchmarks and process benchmarks. (I) Design benchmark design benchmark refers to the benchmark used to mark the design dimensions on the part drawing. On a machine part drawing, there can be one or more design benchmarks. In Figure 1-In 1a, A and B are design benchmarks for each other; In Figure 1- In 1b, the 40mm outer circle is the design standard of the 60mm outer circle; In Figure 1- In 1c, plane 1 is the design datum of Plane 2 and Hole 3, and hole 3 is the design datum of hole 4 and hole 5; In Figure 1- In 1d, the center line is the design reference of 8mm inner hole, gear indexing circle and 50h top circle. (II) Process benchmark, process benchmark refers to the benchmark used in the process of parts. According to the different uses, the process benchmark can be divided into process benchmark, positioning Benchmark, measurement benchmark and assembly benchmark. 1. Process datum. In the process diagram, the datum used to determine the size, shape and position of the machined surface in this process is called process datum. , The processed surface is D hole, which requires its center line to be perpendicular to surface A and keep distance from surface C and surface B to dimensions L1 and L2, then A, B and C are all the process benchmarks of this process. 2. Positioning datum, the datum used to determine the correct position of the workpiece on the machine tool or fixture during processing, is called positioning datum. 3. Measurement benchmark, which is used to measure the shape, position and size error of the workpiece during or after processing, is called measurement benchmark. As shown in Figure 1- As shown in figure 3, dimension L1 can be measured by depth caliper, and end face T is the measurement reference of end face A. 4. Assembly datum, the datum used to determine the relative position of parts or components on the product during assembly, is called Assembly datum. 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 pattern, and the process benchmark is determined by the craft personnel according to the specific process selection. When analyzing the benchmark problem, the following two points must be paid attention to: 1) Points, lines, and surfaces as benchmarks do not necessarily exist on the workpiece (Such as hole center line, axis line, etc) , And by some specific surfaces, these surfaces 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 surface. Therefore, the problem of choosing a benchmark is the problem of choosing an appropriate base surface. In particular, sometimes for the convenience of narration, the reference and the base surface can be collectively referred to as the reference. 2) As a benchmark, it can be a point with no area or a line or a very small surface, but the base surface representing this benchmark always has a certain area. For example, a long shaft is installed on the top of the lathe, and the reference is the axis line, which has no area; The base surface is the top conical surface, which has a small area but does have a certain area.
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