This Small Business Innovation Research (SBIR) Phase I project will demonstrate the feasibility of development of an affordable, flexible, and trackless apparatus and process for full penetration welding of large steel structures. The project will address two critical issues in arc welding; complete joint penetration and joint line tracking. The project will develop an automated welding system including a movable weld backing and trackless mechanism for welding large steel structures. A prototype machine will be designed and experimentally verified based on the requirements of arc welding production. The successful development of the technology will improve significantly automatic equipment for fabricating heavy weldments of large steel structures, productivity and weld quality, and reduce welding cost. The broader impacts (commercial applications) are expected in fabricating heavy weldments within the shipbuilding, high-pressure vessel, pipelines, heavy crane, power generation and aerospace