... to the welding seam bottom of the box beam is to prevent large current burning through bottom ... back welding is composed of moving cart, column, cantilever beam, welding arm, welding seam tracking device, wire feeder, ... makes dragging plate and welding beam moving levelly on the beam.The cantilever box beam welding arm has two ... 4.5 m/minl Cart gauge 1400 mm l Main machine power 1.4 kWl Gas source ...