the minimum essentials for tube bending

A bend die, clamp die, and pressure die are the minimum essentials for tube bending. The bend die helps to prevent the tube from flattening and forms a given radius of bend.
The clamp die holds the tube in position while bending. The pressure die forces the tube into the bend die.
Springback is excessive when a mandrel is not used. This should be considered when selecting a bend die. Springback is the term used to describe the tendency of metal that has been formed to return to its original shape. Springback will cause the tube to unbend from two to ten degrees depending on the radius of bend, and may increase the bend radius of the tube. The smaller the radiuses of bend the smaller the springback.
The tube may kink or buckle, this may be due to hard material which will not compress on the inside radius of the bend. The material, not being able to compress, pushes in toward the centerline of the tube. This condition can be corrected (provided the tube is not too hard) by proper set up of the tooling. A plug mandrel is indicated if the tube buckles and is still within the wall factor and the diameter of the bend.

The purpose of a plug mandrel is to prevent the tube from flattening and to bend without wrinkles or kinks. The mandrel is held in a fixed position while the tube is pulled over it. The tube stretching process is localized on the outer radius of the bend and the material is work hardened to retain its shape and not flatten. The material stretching is done on the forward tip of the mandrel. This force, acting on the mandrel tip, supports the inner radius of the bend, holding it firmly into the bend die groove. CNC bender is used on CNC benders from 0.25″ inch to 6″ inch tube bending machines.

The pressure die should be adjusted for a light pressure against the tube. The purpose of the pressure die is to keep the tube against the bend die through the duration of bending. The pressure die also keeps the mandrel from bending and maintains a straight tube between tangent points of bends (the portion of ubing left on the mandrel after steel bending). The location of the mandrel affects the amount of springback. The mandrel in a forward position (toward tangent) will stretch the material on the outside of the bend more than is necessary. This increases the length of material on the outside beyond that which is required to make a bend. When the bent tube is removed from the bend die, it will conform to the die and there will be little or no springback.