The assembly clearance requirements of the welding machine seam are high. A gap can be zero. However, in actual construction, it is sometimes impossible to guarantee. Although the edge of the seam is beveled by machining methods, such a long edge (such as 12m) will inevitably have a certain deflection, and the deformation generated during the assembly tack welding is also objective. existing. The out-of-tolerance gap not only makes it difficult to control the reverse side of the weld, but also may cause the arc to weld through the lower edge of the groove, so that the copper pad is partially melted and penetrated into the molten pool. into the base metal of the steel near the welded joint). Therefore, it is very necessary to check the gap of the groove before welding. Once it is found to be out of tolerance, the remedial measures of "filling welding" must be adopted to prevent the occurrence of such cracks.