Application of the six-point positioning principle
The six-point rule is explained in detail in general fixture design books, so I won’t repeat it here. However, there are often two misunderstandings in the design of the body welding fixture: one is that the six-point rule is not applicable to the thin plate welding fixture; the other is that the thin plate welding fixture has a non-adjustable over-positioning, so no analysis is needed. Believes that the positioning principle is wrong. It should be affirmed that the six-point rule is applicable to the body welding fixture, and this principle should be observed when designing. On the other hand, it is also necessary to analyze the characteristics of the body stamping parts, only to correctly understand the production characteristics and at the same time correctly understand the six-point rule In order to correctly apply this principle.
1) The rigidity of sheet stamping parts is poor, and it will be elastically deformed during storage and transportation. In the assembly process, in order to overcome the elastic deformation, external force must be used to make the elastic workpiece and the positioning part of the fixture tightly close together, forming a rigid body together with the positioning part, and then welding into a rigid body with high rigidity and qualified size. Space shell — body assembly. While the rigid body workpiece is positioned in the fixture, the super-positioning support can be adapted by floating or adjustable support. If the super-positioning support is also designed to be floating for the elastic workpiece, it is to install the elastic workpiece on the elastic body. A definite assembly size will never be obtained.
Figure 2 Classification of welding fixtures
2) Body stamping parts have a length or width of 1 to 2 m, and the size tolerance and shape tolerance are relatively large. Due to the gap between the positioning part and the workpiece, the assembly position change of the large part is more obvious in the boundary part. In order to correct the misalignment in the assembly, so that the assembly errors can be evenly distributed, process positioning parts are appropriately added to the important parts of the large welding fixture to prevent assembly errors from concentrating in a certain direction. The lower the accuracy of the stamping parts, this process Positioning is more necessary. But doing so will undoubtedly increase the phenomenon of super positioning.
Figure 3 Three-dimensional flexible welding fixture
3) Since the thin plate is easy to deform, there must be a corresponding supporting block at the point of application of the clamping force. Due to the structural limitation of the workpiece, the point of application of the clamping force often cannot directly fall on the original positioning support point. At this time, the support point must be increased. From the perspective of positioning principle, this kind of support is redundant, but it is indispensable for thin plate parts. Over-positioning will make the contact point unstable and cause interference in the assembly position. It should be avoided as much as possible. However, over-positioning is not allowed under any circumstances, as long as the undesirable consequences of over-positioning do not exceed the scope allowed by the assembly requirements of the workpiece. , Super positioning is allowed. For thin sheet stamping parts, super positioning is sometimes necessary. At present, some automobile manufacturers have clearly stipulated the positioning datum of the body parts assembly and welding process in the form of guidance documents, which is beneficial to the unification of the datum and the improvement of quality. As shown in Figure 3, six positioning datums are established according to the six-point rule, which are represented by solid arrows in the figure, which limit the six degrees of freedom of the piece in space. Due to the large workpiece, in order to prevent the upward deviation from being concentrated on the front side, a process support point is added to the upper part, and the rigidity of the skirt is poor. In order to prevent elastic deformation, two process supports are added, which is shown by the hollow arrow in the figure. Location. In this way, after the workpiece is clamped, it can form a qualified rigid body with all the positioning supports, and finally weld it into a qualified part.
The positioning of thin-plate parts should comply with the six-point rule, but at the same time it has its characteristics. Different qualitative contradictions should be solved by different qualitative methods. This is the integration of theory with practice.