A novel welding device comprises a frame body. A first sliding groove with a downward opening is formed in the frame body. A first sliding frame is mounted in the first sliding groove in a manner of being capable of sliding up and down. Second sliding grooves with downward openings are formed in the first sliding frame in a manner of being symmetric left and right, wherein the second sliding grooves extend front and back. Guide grooves extending front and back are formed in the upper ends of the two side walls of each second sliding groove. A second sliding frame is mounted in each second sliding groove in a manner of being capable of sliding front and back. A grinding wheel is fixedly mounted at the lower end of each second sliding frame. Guide blocks mounted in the corresponding guide grooves in a sliding manner are symmetrically arranged on the left side and the right side of each second sliding frame. A containing groove extending front and back is formed in each second sliding frame. An upper gear strip is fixedly arranged in the upper end wall of each containing groove. A lower gear strip is fixedly arranged in the lower end wall of each containing groove. A mounting groove is further formed in the first sliding frame. A second gear is operably mounted in the mounting groove through a second rotary shaft. A first gear cooperating with the second gear is operably mounted in the mounting groove through a first rotary shaft.