Large tower crane lower rotation tailor-welding tool

The utility model discloses a lower rotary tailor-welding tool for a large tower crane, and relates to the technical field of machining. The positioning device comprises a supporting frame and a first positioning plate, the first positioning plate is clamped and fixed to the upper end of the supporting frame, and the first positioning plate is clamped and fixed to the periphery of the supporting frame. Through the supporting frame and the first positioning plate, the problems that the size precision and stability cannot be ensured due to the fact that most of existing lower rotary workpieces are subjected to part positioning welding through manual measurement and lineation by workers, a positioning device of a lower rotary tailor-welding tool is complicated, difficult to disassemble and assemble, time-consuming and labor-consuming, and poor in welding quality due to welding deformation are solved. The X-direction and Y-direction dimensional precision and the end face height of the angle steel cannot be ensure