Design of vacuum flexible fixture suitable for thin-walled parts processing

0 Preface

Vacuum fixtures are widely used in the processing of thin-walled parts, such as the vacuum clamping system designed by SCHMALZ (Schmalz), which is suitable for processing thin-walled metal parts with a flat bottom and a large area. In response to the clamping problem of thin-walled structural parts such as aircraft skins, multi-point flexible fixtures have appeared in recent years, such as the TORRESTOOL multi-point flexible tooling system of Spain’s M.Torres company. This fixture can be used for aircraft fuselages, wings and other aircraft covers. Edge trimming and drilling and milling of the skin. Beijing University of Aeronautics and Astronautics has established a digital stretch test system for aircraft skins based on reconfigurable flexible multi-point molds, which has broken through the automatic generation technology of aircraft skin CAD digital model process supplementary surface. One of the measures taken by AVIC Harbin Aircraft Industry Group Co., Ltd. to deal with the deformation of large thin-walled parts is to design a vacuum fixture for both front and back processing. The design of these vacuum fixtures provides an effective method for the processing deformation control and processing accuracy of thin-walled components.

The surface of a typical aircraft thin-walled component is composed of multiple cavities. The thickness of the web part of these small cavities is less than 1 mm at the thinnest point, and the structure is complex, and the shape and size are changeable. Such structural parts usually cannot use the above-mentioned vacuum clamps, and if a dedicated vacuum clamp is used, there are many types and numbers of clamps, and the versatility of the clamps cannot be guaranteed. In this paper, a set of vacuum flexible fixtures is designed for the processing of thin-walled structure of the whole wall panel, and the influence of the structure of the suction cup and the thickness of the plate on the thin-walled structure is studied.

1　The design of vacuum flexible fixture

The thin-walled components of an aircraft are shown in Figure 1. Because the web is thin, it is easily deformed by the axial cutting force of the tool during processing. Therefore, a set of vacuum flexible clamps is added to each web, which can provide uniform clamping force and effectively support the web. The structure of the vacuum flexible fixture is shown in Figure 2.

Figure 2 　 Vacuum flexible fixture structure

The working principle of the vacuum flexible clamp is: when the oil pressure is not supplied, the clamp is in a released state; after the oil supply is started, the piston rises, and the plunger spring drives the suction cup to rise until it fully contacts the workpiece; then, the taper sleeve moves down under the action of oil pressure , The steel ball generates a large radial force on the sleeve, thereby generating a strong holding force on the plunger; finally, the sealed cavity is vacuumed by a vacuum pump to generate a uniform clamping force in the adsorption area, and the workpiece is processed. After that, make the airtight cavity communicate with the atmosphere to loosen the workpiece. The vacuum flexible fixture is easy to install and self-adaptive. It does not need to manually adjust the height of the suction cup. For web structures of different cavity sizes, only the components above the vacuum suction fixture can be replaced.

2　The establishment of finite element model

In the case of the same cutting amount and cutting method, the deformation law of the web is similar. Here, only a certain web structure is taken for analysis. The simplified geometry model of the clamping is shown in Figure 3. The seal is made of rubber, and the material is soft. The deformation of the web during processing is negligible. Therefore, the three-dimensional model of the seal is ignored in the finite element simulation, and the distributed load q is applied to the corresponding area of the workpiece instead of negative pressure. Select q= 55 kPa. In order to prevent the sucker from cutting the aluminum alloy workpiece, the material of the sucker is aluminum alloy. The material parameters of the workpiece and the sucker are shown in Table 1.

The established 3D model is imported into the finite element analysis software, the 8-node hexahedral element C3D8R is used to mesh the bearing plate and the workpiece, and the mesh of the contact area between the bearing plate and the workpiece is refined. In order to require accurate results for the displacement solution, the linear reduction integral element is selected; the web bears the bending moment in the force analysis, and at least 4 elements should be divided along the thickness direction. The lower surface of the workpiece web and the upper surface of the suction cup define Surface Intact, the contact attribute is set to Coulomb friction, and the friction factor is 0.09. When applying boundary conditions, the lower surface of the suction cup and the protruding parts of the left and right ends of the integral frame apply fixed constraints.

Analysis of the influence of 3　 suction cup structure on thin-walled parts

The use of vacuum flexible clamps to assist the support of the web, that is, a rigid support is added, and a uniformly distributed clamping force is obtained, thereby improving the rigidity of the workpiece during processing. If the force analysis calculation is performed on all points on the web, the calculation amount is relatively large, so only some reference points are selected for simulation analysis. A total of 7 reference points from A to G are selected from the center of the upper surface of the web to the left.

Take D=40 mm, D1=8 mm, D2=50 mm, a=80 mm, plate thickness h=2 mm, the remaining plate thickness is 5 mm, and load the amplitude in the range of diameter Φ1 mm at each reference point With a variable load of 100 N and other conditions unchanged, the normal deformation of the web with and without a vacuum flexible fixture as an auxiliary support is simulated by the finite element software.

Figure 4(a) shows the force and deformation of the web caused by the load applied at different reference points when there is no vacuum fixture as the support. The curves 1-7 in the figure represent the load caused by the separate load at the reference points A to G. The Z-direction displacement at each point of the web; Figure 4(b) shows the force and deformation of the web caused by the load applied at different reference points when a vacuum fixture is used as a support. The curves 1-7 in the figure represent the reference points respectively. The Z-direction displacement of each point of the web caused by a separate load from A to G.

It can be seen from Figure 4 that with and without vacuum fixtures as support, the Z-direction displacement of the node in the center of the web is larger than that of other nodes; in the case of auxiliary support, the web The Z-direction displacement of the nodes in the central area is significantly reduced compared to the unsupported state. When the load is applied at the reference points A, B, C, and D, the Z-direction displacement of the node caused on the web is generally larger, and the closer to the load, the greater the Z-direction displacement of the node. The closer the force-bearing area is to the frame wall, the smaller the overall deformation, and the Z-direction displacement of the maximum node in the force-bearing area is also reduced.

Analysis of the influence of 4　 plate thickness on web deformation

The smaller the thickness of the thin-walled part, the weaker the ability to resist deformation. During the machining process, it is easy to produce "knife letting phenomenon" and chatter due to cutting action. When the vacuum flexible fixture is used as the auxiliary support and the other conditions are the same as those in the previous section, different web thicknesses are selected for analysis. Table 3 lists the maximum Z-direction displacement of the nodes at each reference point as a function of thickness.

It can be seen from Table 3 that when the plate thickness h is 3 mm and 4 mm, the Z-direction displacement of the nodes at each reference point of the web is generally smaller. This is due to the increase in the thickness of the web and the greater rigidity; When the thickness h=2 mm, the node displacement caused by the force in the center area of the web increases significantly, and the Z-direction displacement of the node increases when the web is stressed at the other reference points. When the web thickness is small, it is necessary to increase the vacuum flexible clamp.

5　 Improvement of the suction cup structure

It can be seen from Section 4 that for the same set of suction cups, when the thickness of the web becomes smaller, due to the greater thickness of the seal cavity relative to the web, the stiffness of the web is insufficient, and the node Z-direction displacement increases significantly. In response to the above problems, the following improvement measures are proposed: reduce the boundary distance of the vacuum chamber, divide it into multiple vacuum chambers, and increase the diameter of the suction cup at the same time. The improved simplified clamping model is shown in Figure 5. Other conditions are the same as those in Section 4, the finite element simulation analysis is carried out on the web. Figure 6 shows the deformation of the web caused by the load applied at different reference points when the improved vacuum suction unit is used as the support. Curves 1-7 in Fig. 6 respectively represent the Z-direction displacement of each point of the web caused by a separate load at the reference points A to G.

It can be seen from Figure 4 and Figure 6 that after the improvement of the suction cup structure, the Z-direction displacement of the web node is significantly reduced when the central region of the web is loaded; at the same time the magnitude of the node displacement in the central region of the web is basically the same, which is more effective than before the improvement. improve.

6　 Conclusion

A new type of vacuum flexible clamp is designed, and its main structure and working principle are introduced. Compared with ordinary vacuum clamps, it has better versatility, is suitable for processing various types of cavity webs, and can greatly reduce the number of special clamps.

The finite element analysis software is used to numerically simulate the overall deformation of the web when the web is stressed at different points. The study shows that the vacuum fixture as an auxiliary support for the thin-walled parts can effectively improve the deformation of the thin-walled parts. For the web, auxiliary supports should be added to reduce the deformation of the web, and the boundary distance of the vacuum chamber should not be too large.