Flow forming wheels - GIF

[schuylergrace]

If you watch, first the drum is spun up, then the profile on the left presses against the rotating drum and then the profile on the right does the same. The spinning action lets the sidways force deform the cylinder evenly.

Oh! I didn't see that second forming tool, despite all the times I watched the video! It all makes sense, now. I thought there must have been some materials voodoo going on to do the finish forming.