Development of automatic weld bead finishing system using plasma arc gouging

Water turbine runner has been used in hydroelectric power generation, and it consists of crown-band-vane. Each part is jointed by arc welding process. Weld line varies in three dimensions that the weld bead is shaped since it may cause a turbulent flow of water, it is necessary to shape and to finish smooth the weld bead after welding. Weld bead finishing has been done manually by grinder, and the accounted percentage for this work of manufacturing step is large. This grinder work forces a worker to a tough work. Considering fabrication of water turbine runners, tough work reduction, stabilization of quality, man-hour reduction and weld bead finishing, automation has been required. We have studied a grinder to the tip of the robot as an automated weld bead finishing. However, there are many problems, such as vibration of the grinder and reaction force of the grinder and variations in the amount of removal due to wear of the grinder. These factors cause problems which becomes more complex, such as increasing the size of facilities, equipment and methods of control. To meet this demand, we focused on the plasma arc gouging of noncontact as a way of finishing the weld bead for the rough grinding step. In order to control the gouging depth, experiments were conducted in the various control parameters. As a result, it was confirmed that the gouging current and gouging speed is a factor that most affects the gouging depth. We established the method of controlling the gouging depth by using the control parameter within ±0.5 mm. After application of this automatic weld bead finishing system to the actual water turbine runner, bead shape finishing errors were ±1.0 mm or less; weld bead shapes were less affected by gradient and inclination were obtained. Also, the cost in man-hour was greatly reduced by the application of the automatic finishing system with plasma arc gouging compared with the manual grinding.