"slicing and dicing the weld reveals the truth ... "
vertical mig welding technique. Thats because they always weld downhill, never uphill.
For thick metal, Thats just wrong with a capital R.
Mig welding downhill is like falling off a log. It's easy because gravity is your friend and just pulls the welding puddle along like running a bead of caulk. That is probably how we get sayings like: "You can teach a monkey to Mig weld!"
Mig welding downhill on anything thicker than 1/4" will not penetrate into the corner. Unless you are welding with spray transfer (but I am guessing that if you are using spray transfer, you already know what I am talking about here.)
Here is a cross section of what happens when you Mig weld downhill on 3/8" hot rolled steel.
But what separates the Men from the boys or better yet, the welders from the monkeys, is Mig welding uphill. Now gravity becomes your enemy....like the Ice Queen in the Narnia movie...a cold hearted witch!
She doesn't care about how good you can weld downhill or your crappy uphill welding technique or how your coworkers are going to make fun of your weld that looks like hammered dog crap.
Poor welding technique will make your mig weld pile up in the middle because gravity is pulling downward on the puddle. So how do you fight back? With a good Vertical Mig welding technique that's how.
The upside down V is one of the most effective technique for vertical mig welding of lap and tee fillet welds. Sure there are other ways of making a good looking vertical uphill weld, but for ensuring penetration as well as making a good looking uniform weld, the upside down V is a good way to go.
Here is a sketch of the technique and also a down and dirty youtube video of this Vertical uphill welding technique.
how to mig weld video
mig welding overhead
Mig Welding Lesson on the importance of short stickout and ground