A convertible single or double spot welding machine operating by the electrical resistance method for producing grids or bar grates by single spot welding or double spot welding, has a transverse row of electrodes and at least one welding transformer on one side of the welding plane. Electrically conductive connections, which pass through the welding plane, connect the welding transformer to the electrodes located on the other side of the welding plane and close the welding current circuit for single spot welding. In order to convert from single spot welding to double spot welding, the connections which pass through the welding plane are removed, selected electrodes on the side of the welding plane remote from the welding transformer are connected in pairs by current bridges, and the supply lines from the welding transformer are respectively connected to the two electrodes opposite the electrodes connected by the current bridges.