An automatic welding system uses an anti-gravitational welding controller to minimize or eliminate the undercutting and concavity of the roots of the welds caused by gravitational effects. The pressure within the pipes to be welded is continually changed as the welder orbits the pipes. At the upper section of the weld, positive pressure is applied to the pipes to eliminate undercutting at the upper surface of the weld and negative pressure (vacuum) is applied to the lower section of the weld to eliminate concavity of the root of the weld. In one mode, the internal pressure within the pipes is varied as a function of the position of the orbital welder. In another mode, the arc voltage at the tip of the weld, i.e., the distance between the tip of the weld and the surface of the weld, is maintained constant by varying the pressure within the pipes. The system allows for the completion of high quality welds with less weld material, less passes, and less time. It also allows the welding to be performed by welders who are less experienced and skilled as the present welders.