Accurate selection of positioning datum is the key to drawing up mechanical processing technical regulations and fixture planning. The selection of benchmarks is an important issue in the planning of technical regulations. Whether the selection of benchmarks is reasonable affects the processing quality, productivity and processing costs. Whether the selection of positioning benchmarks is reasonable or not will directly affect the quality benchmarks of the planned part processing technical regulations. Incorrect selection of quality benchmarks will increase the process, or make the technical path unreasonable, or make fixture planning difficult, or even fail to achieve the processing of the workpiece. Precision requirements. In the process of planning the technical regulations of the precision piston rod , the fine datum should be selected first according to the parts and workpiece drawings, and then the fine datum of the material. Consistent thinking must be carried out in connection with all technical processes, and the first process sequence must create conditions for subsequent processes.
(1) Selection of rough benchmark
The rough selection should be able to ensure the azimuth accuracy between the machined surface and the non-machined surface, and reasonably allocate the margin of each machined surface to provide a precise reference for the subsequent processes. Therefore, in order to facilitate the positioning, clamping and processing, the outer circle surface of the optional shaft can be set as the reference, or the outer circle and outer surface holes can be used as the positioning reference. When positioning with an external surface, since the reference surface processing and work clamping are relatively convenient, the clamping is usually carried out by a chuck. In order to ensure that the rough machining allowance of the important surface is small and uniform, the surface should be selected as the rough reference, and the orientation accuracy between the processed surface of the workpiece and the other unprocessed surfaces should be ensured. The rough reference is the outer circle of the cast blank. Three-jaw self-centering chuck is used for the middle hole processing to clamp the outer circle of the blank, and the middle hole is drilled by turning the end face. Usually you can’t use the outer circle of the blank to drill the two middle holes twice. Instead, you should use the outer circle of the blank as the rough reference. First process an end face, drill the middle hole, turn the outer circle of one end, and then use the already turned outer circle as the benchmark. Use a three-jaw self-centering chuck to clamp the other end surface and drill the middle hole to ensure that the two middle holes are coaxial.
(2) Selection of fine benchmarks
Based on the piston rod technology needs and installation requirements, should be selected at the mercy of the piston rod end surface and the two intermediate holes as fine reference plane. Many appearances on the parts can be processed on both sides as a reference. It can prevent benchmark conversion errors and also comply with benchmark consensus guidelines. The middle axis at both ends is the planning reference. The middle axis is selected as the reference, which can ensure the final processing azimuth accuracy of the appearance, and complete the overlap of the planning reference and the technical reference.
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