The parts of the thin-walled semi-circular ring structure are intermittently processed, and the parts are easily deformed due to the poor rigidity of the parts, and the processing is difficult. In order to solve the deformation problem of the part, some measures must be taken in the process.

First, the problems encountered in the processing of thin-walled semi-circular parts

Figure 1 shows the parts of a thin-walled semi-circular piece. Wall thickness: diameter = 1:26, the part material is 45 steel, quenching and tempering, high precision requirements. If it is processed in a single piece, it is more difficult. Because it is not a full-round piece, it is intermittently cut on a lathe, and it is difficult to position and clamp, requiring complicated fixtures. The use of two pieces of turning together, that is, processing a full circle, and then separating, can solve the problem of intermittent turning and clamping. According to this scheme, the process route is as follows: rough rounding → heat treatment (tempering) → semi-finishing round → milling (cutting) → grinding (split surface). When the parts were machined according to this route, it was found that the qualified full-round parts were divided into two thin-walled semi-circular parts and then retracted in the two-port part of the part, and the reduction amount was 0.5 to 1 mm, as shown in Fig. 2. Therefore, the parts must be repaired during assembly, but the repaired parts do not meet the requirements of the drawings. The reason is the deformation of the parts.

Figure 1 Thin-walled semi-circular parts

Second, the analysis of the causes of deformation

The reason for this error is mainly due to the internal stress of the part. The internal stress is caused by heat treatment and turning. In the heat treatment, the parts are heated, heated to about 780 ° C, and then tempered by rapid cooling (water cooling) and high temperature 550 ~ 600 ° C, to complete the quenching and tempering of the parts. It can be seen that during the quenching and tempering process, the heat→cold→heat→cold, the parts are thermally expanded and contracted by cold. If the parts are irregular in structure and have different thicknesses, the degree of heat and cold, the degree of thermal expansion and contraction, and the heating and cooling rates are different. The wall is hot and the heat is swollen fast, and the cold shrinkage is also fast; the wall thickness is slow and the cold shrinkage is slow. This speed difference creates an internal stress that interacts in the transition between the thick and thin parts of the part. The internal stresses are balanced within the entire round part. When the part is cut, the internal stress seeks a new balance. In this way, the part will be deformed, and the magnitude of the deformation is related to its stiffness. Larger stiffness results in less distortion. For thin-walled parts, the deformation is relatively large. The part has a thick wall in the middle. After the heat treatment is cooled, the middle portion is cooled slowly. At the thin ends of the two ends, the temperature has been lowered, and the middle portion is still cooled and contracted, and then the thin ends are formed at the ends. Compressive stress, as shown in Figure 3.

Figure 2 The split port is inwardly shrunk

Figure 3 Thin wall cylinder stress state

Figure 4 welding of the two halves of the ring

Finishing and milling also produce internal stress, which is caused by cutting heat. It is relatively small and has little effect on part deformation. As for the clamping force, deformation will also occur, but the accuracy of the roundness is mainly affected, and the influence on the opening of the part is not large. Therefore, when the part is milled, its compressive stress causes a new phenomenon to be created, causing a shrinkage at the opening of the part. The main cause of this shrinkage deformation error is the internal stress generated by the heat treatment.

Third, the solution to the problem

To solve this problem, we must start by eliminating internal stress. One method is to subject the workpiece to aging treatment after heat treatment. Another method is to cut first, then heat treatment, and then weld the two pieces together. In the two methods, the former process route is unchanged, but only an aging process is added. Since the internal stress of the aging treatment cannot be completely eliminated, the deformation can only be reduced, but the deformation cannot be completely eliminated. The latter process route will undergo major changes. However, this method can eliminate deformation errors more effectively. The latter approach is detailed. The process route is changed to: full round roughing (remaining more margin) → milling (separate) → heat treatment → spot welding (combination) → semi-finished car → clamp (separate) → grinding. The key to this change is to cut first and then heat treat. In this heat treatment, the workpiece can be sufficiently deformed. This is because the shape is a semicircle, not a full circle, at which point the internal stress is much reduced due to the deformation of the part. Moreover, in the subsequent processing, only the remaining amount is removed, and the structure itself is not greatly changed (unlike the original full circle cut), so the balance of the internal stress is not broken, and no deformation is caused by the internal stress. . After the heat treatment, the two pieces are spot welded together in a free state (synthesizing a circle) as shown in FIG. After spot welding, there will be weld stress at the weld. Since there is shrinkage after welding, there is tensile stress at the solder joint. There is also a certain compressive stress on the thin-walled parts at both ends of the part, which can offset some of them, so that the welding stress does not affect the parts. After the welded parts are turned, only a small amount of welds are left in the weld. The parts can be separated by hand, and then the split surface is polished to obtain complete qualification (substantially no deformation at the opening). Components. If you want to be more precise, you can add an aging process after spot welding, the effect will be better. Namely: rough car full circle → milling → heat treatment → spot welding → aging → semi-finished car → clamp → grinding.

In summary, for the part, the two-in-one and first separate heat treatment methods are mainly used in the process to solve the processing problem and the deformation problem. Therefore, to solve the processing problems of thin-walled and semi-circular parts, it is necessary to analyze according to different situations, and a suitable process route will be found. Of course, the process route introduced in this article is more complicated. If the part requirements are not high, it may be economical to use the aging process.

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