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Hi Joe, Undercut comes from faulty weldor maniplation of the electrode mostly from incorrect arc length ie; too long of arc length. This will cause the molten puddle to blow out from the base metal thus underfill the crater thus leave a undercut weld when cooled. Also with a long arc length you do increase the heat of the electrode and base metal as a shorter arc length welds slightly cooler with more molten puddle control. To fast of travel will also cause undercut as it doesn't allow for the crater to be completely full before moving the electrode. It's really important to watch the molten puddle while welding and reconize the difference between the slag pool and the molten pool of the weld puddle. The slag pool forms mostly on the lagging edge then slightly forms on the leading edge of the molten puddle and has a dull apperance. Do not let the slag pool distract your attention away from the liquid weld puddle. The pure molten pool of weld will be slighty shinny with the center being mirror like and this is the pool that you need to pay close attention too. This liquid pool needs to fill the weld zone crater completely on both sides thus leaving behind no undercut before any travel movement. Correct electrode angle is 0� to 15� of vertical in the direction of travel. Too much more than 15� then the molten puddle will be pushed to the back of the heat zone thus leaving behind a slight undercut or more likely to heavy of a bead build up when cooled. I weld pretty hot, with 1/8" electrodes I push 150amps easy but I have seen one other weldor that put me too shame that ran about 170amps and neither weld had any undercut but I do agree it takes alot more concentration to control the molten puddle at the higher amperage. T_Bone
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