One of those things I would love to learn how to do is to build bike frames. Specifically, I want to braze frames together, either with lugs or fillet-brazed. So far, I have only managed to do very basic brazing. My torch is a cheap "hobby-level" model, and I really haven't taken the time or spent the money to deal with major projects.
The first brazing job I did was on my first Formula One frame. I cut out the tig-welded vertical dropouts and replaced them with track ends, because I wanted a fixed-gear 20"-wheeled bike. I struggled with that conversion, and actually ended up burning a hole in the tube on the drive-side chainstay.
One problem I was having was that I was using nickle-copper brazing rod, which requires a higher heat than brass. I didn't now that, when I bought the brazing rod, and only found out after I was having the problem with overheating the tubes.
Nickle-copper brazing rod is made of the same alloy that U.S. 5-Cent pieces are made of (that's why we commonly refer to the 5-Cent piece as a "nickle"). So, to repair the hole I had left in the chainstay, I took a "nickle" coin and brazed it into place, then ground everything down smooth.
That frame modification held up fine for the whole time I owned that bike, and I never heard any complaints from the guy I sold it to. About a year after I had done that work, I read somewhere that using a 5-Cent coin to patch nickle-copper brazed joints was an old trick that a lot of hot-rod builders had used, back in the day.
Since then, I have found a good source for 1/16" brass brazing rod, and that has simplified things. I have replaced a couple of broken rear dropouts on bikes, with no problem. But, I still need to work on my technique before I trust myself to buld a whole frame.