Projection welding equipment and working principle
1. Projection welding equipment
The projection welding machine is a kind of spot welding machine, which is generally composed of a fixed spot welding machine mounted with a projection welding head. Figure 1 shows two common projection welders.
(a) ordinary projection welder
(b) Shaped projection welder
Figure 1 projection welding machine
2. The working principle of projection welding
During the projection welding, the plate member is placed on the lower electrode, and the positioning pin passes through the projection welding hole formed in the plate member, and then the pin is inserted into the threaded hole of the projection nut, and the upper electrode is lowered to complete the projection welding process.
Figure 2 Working principle of projection welding
As shown in Fig. 2, the projection welding generally consists of four processes. The first is the preparation process, in which the preparation of the upper part and the electrode pressing is completed; the second is the pressing process, which is preset on the nut at this stage. The bump is deformed under the action of the electrode pressure, the height is lowered and the oxide film on the surface of the plate is broken, and the nut is well combined with the convex welded plate member; the third is the energization process, which includes the bump crushing process. And the nugget formation process; the fourth is the holding process, in which the current concentrates through the position of the bump, and the bump is completely flattened.
Projection point of projection welding
The application of projection welding on sheet metal parts has certain limitations in the process. Not all profiles and all positions of all the plates can be convexly welded. There are also some points of attention. Touching these aspects will deteriorate the quality of the projection welding. Or the cost of projection welding, the cycle becomes higher, and the length becomes longer.
1. Point one
1) The position of the convex welding of the part is perpendicular to the axis of the projection nut and bolt. The minimum dimension from the edge of the part is less than the depth of the welder's throat C (the depth of the welder of Jianghuai Company is 420-770mm) to avoid interference with the welder.
2) When using ordinary DC electrodes to meet special design requirements, special electrodes can be considered (the production of special electrodes will increase the cost of the product, and the quality of the solder is not guaranteed, try not to use).
3) Long strip stamping parts, in order to facilitate the convenience of workers, the length dimension must meet the requirements of L ≤ 1500mm.
4) Square stamping parts, in order to meet the convenience of workers' operations, the size of the side must meet the requirements of L ≤ 1000mm.
5) In order to reduce the labor intensity of workers, the weight of stamping parts should meet M≤7.5kg.
6) Do not use M5 and below projection welding nuts to avoid problems such as low projection welding strength, difficult procurement, long cycle and high cost.
7) For the same part, try to use the specifications (such as M6) and the same size of the projection nut.
8) The projection nut of the same part should be welded to the front and back sides as much as possible to increase the unnecessary workload.
2. Point 2
The avoidance space between the projection welding tip and the workpiece is at least 3 mm, of which 1.5 mm is the stamping single piece tolerance, 0.5 mm is the projection welding tolerance, and 1 mm is the safety space to avoid welding interference. In addition, it is recommended to re-arc weld after the projection welding of M10 and above.
3. Point three
In order to ensure the stability of the welding quality, the correspondence between the projection welding nut (bolt) and the thickness of the part is shown in Table 1.
Table 1 Correspondence between the projection nut (bolt) and the thickness of the part 
4. Point four
In the case where the same plate member has both projection welding and stud welding, since the plate member is generally welded by the first stud after welding, the center of the stud welding screw is from the edge of the projection welding bolt or the projection nut. The length should be greater than the outer radius of the stud gun nozzle by 15mm.
Figure 3 Stud welding avoidance convex welding diagram
Projection welding quality inspection
The projection welding inspection generally includes the welding strength inspection and the installation performance inspection. The installation performance inspection is generally a visual inspection. If the visual inspection cannot be confirmed, the bolts (nuts) matched with the projection welding nuts (bolts) are used for on-site matching. The installation performance is checked; the welding strength is checked by the torque wrench to check whether the welding strength reaches the process requirement value (see Table 2). If it has fallen off if the specified torque value is not reached, the welding parameters should be re-adjusted until the strength test is passed. .
Table 2 Contrast welding specification and test torque comparison 
Conclusion
The projection welding technology has a long history in the manufacture of body-in-white, and its equipment and technology are quite mature. However, with the application of a large number of galvanized sheets and high-strength boards in the body manufacturing in recent years, the attention of the projection welding technology has been improved, especially The optimization of the process and the attention of the quality of the projection welding are constantly strengthening. The body is an important assembly of the whole vehicle, and the quality of the body directly affects the service life of the vehicle. As one of the four welding processes in automobile manufacturing, namely spot welding, projection welding, arc welding and stud welding, the projection welding technology completes more than 90% of the assembly work of the vehicle body, so the quality of the projection welding quality for the vehicle Has an important impact. With the continuous improvement of the performance of automobiles, especially the wide application of high-strength steel sheets, galvanized steel sheets and hot-formed steel sheets, people pay more attention to the quality of projection welding than ever before.
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