"Making a Sheet Metal Flanging Tool"
I have a very good friend whose hobby is sheet metal working. Currently he's re-building a Morgan roadster. This guy is talented and learned sheet metal work through books, Internet and experimentation. So when he needs some custom parts fabricated (or MIG, Gas, Arc, and TIG welded), he comes to the house and we work the problem together.

This time round he needed a "flanging tool" for putting a flange into sheet metal once the hole has been punched. The flange makes fabricated part stronger, lighter, and definitely better looking. The idea came from a sheet metal fabricator's book that was published by famed metal smith Ron Fournier.

This is a scan of the rough picture that he brought along for the project. It's crude, but was good enough for a start.

Sheet metal is sandwiched between the mandrel anvil and the die, then the top of the anvil is hit with an impact hammer. Basically, you hit the hard steel bolt on top of the anvil, and the mandrel puts a flange in the sheet metal.

To make the curved flanges we first needed to grind symmetrical convex and concave cutting profiles on a 3/8" HSS tool bit blank. This takes a little time, but if you have a good bench grinder and a little patience it will not take long. We just had to make sure not to overheat the bit. We used lots of cold water and short grinding sessions to complete the profiles. Below is the bit with the ground concave and convex profiles on the ends.

Next we needed to cut the anvil portion of the mandrel out of aluminum stock. We decided to use an old billet blank from a previous project (we only needed two billets for our previous hydraulic press bushing project, but we had made four) which could be used as the die portion of the mandrel (why waste material).

The HF 4x6 bandsaw came in very handy for cutting the anvil section of the mandrel.
Although I curse this saw sometimes, I highly recommend getting one. If its
dialed-in correctly you save a lot of time and effort.