The stable back bead in a first layer of the one-side multilayer welding is important to obtain a good quality of welding. For this purpose, the switch back welding method was applied for the root gap from 1 mm to 5 mm in V groove welding without the backing plate. This method requires the cooperative control of power source, wire feed motor and robot manipulator by the personal computers. The validity of welding conditions for each root gap were confirmed by observing the arc and the weld pool, and the back bead appearance. Next, the feed-forward control of the back bead for the root gap variation was discussed. The root gap on start and end points of welding were detected by using the spot laser sensor. The welding conditions, welding current, wire feed rate, weaving width, forward/backward stroke of the switch back motion and stroke speed, were continuously controlled corresponding to the root gap variation, and the stable back bead was obtained by the fundamental experiment regardless of changing the root gap from 4.4 mm to 1.9 mm.
