It’s worth a mention firstly that there are two common terms used in industry, tube bending and pipe bending. Tube is measured by the outside diameter whereas pipe is measured by the inside diameter. Pipe sizing refers to the nominal inside diameter, sometimes called nominal bore, rather than the actual inside diameter.
Tube is bent around a fixed radius die with a mandrel insert in to prevent it from collapsing during bending. The tube or pipe is clamped against the die and this is then rotated whilst the pressure die moves in a straight linear motion. This is the most common form of bending.