Is it necessary to use clamps or braces to hold the tube while doing the flare or swage?

No, there’s no need of clamps or holders, as long as you keep the tube stable enough to apply the tools. A firm grip normally suffices, however if you have a very powerful drill, it may not be enough. In those cases, for short-lenght tubes, we do recommend to make the swage before cutting the tube out of the pancake, for instance. It will provide a better stability and will require less effort from the user to keep a firm grip on the tube. If the other side requires another flare or swage, you probably will need some sort of holder to clamp it tight. For bigger lengths there’s no need of clamps of any type.