Using a MIG welder, Cornett welds with the wire touching the slug. The heat must be turned up high enough on the welder that it is capable of fusing both the slug along with the ends of the tubing on both sides. That's why the width of the gap is important. It must be wide enough so that the wire can reach the slug, but close enough so that both ends of the tubing get hot as well. Cornett also adds that he always begins his weld at the bottom of the tube and works his way up. This way he is always working at the edge of the weld puddle. If you start at the top and work your way down, gravity can pull the weld puddle into your work area and keep you from getting good weld penetration.