Part of the theory of a single pass, maximum thickness recommendation comes from the welders ability to provide enough heat into whatever “weldment” you are welding to achieve that amount of thickness of weld.
Let’s think about that for a second. A machine of 200 amps is usually rated for a ¼” or a little more. That means that it is capable of providing enough heat input to overcome the “heat sink” capacity of the metal to be able to provide a weld that thick. That doesn’t mean that you should do it, though. The best application of this is to put it in magnified terms so it is easier to understand.
Take a piece of heavy duty thick wall pipe. Let’s say it has a wall thickness of 2 inches. You certainly aren’t going to find a welder that is capable of welding that in a single pass. So it will take many multiple passes. Each pass is layered on top of each other, side by side, until the grooved joint is filled with metal.
But a welder that is labeled for this type work can weld up to a ¼” in a single pass by providing enough heat to make the weld puddle stable because a 2 inch thick piece of pipe will wick away the heat pretty quickly and make it difficult to maintain a molten state in the puddle. That’s enough to understand, but when you shrink that down to a ¼” plate, butted together, and try to make a single pass weld, the dynamics change.
