I have welded quite a bit of .060 copper, never used preheat. Argon (kind of heavy flow worked better for me, around 25cfm), lanthanated tungsten ground to a fine point, and crank the amperage up a bit. If you want it to come out pretty, set up a jig and purge the backside. I have never welded thicker copper, but the preheat sounds like a good idea, especially if you're having trouble holding a puddle.