When a tube is bent, two things happen to metal. The outside wall is reduced in thickness due to the stretching of the material, and the inside wall becomes thicker due to the compressing of the material (Figure 1).
The material is actually formed approximately about the centerline of the tube. The material that forms the outside of the bend has further to travel and therefore is stretched; the inside of the bend has less distance to travel and is compressed.
The function of the Bend Die
When the ratio of the tube diameter to wall thickness is small enough, the tube can be bent on a relatively small radius (Centerline Radius or CLR = 4 x Tube O.D.). Excessive flattening or wrinkling of the bend during tube forming should not occur.
The outside and inside of the bend tend to pull towards the centerline of the tube (flattening). Two factors that help prevent this from happening are a grooved bend die, which supports the tube along the centerline, and the natural strength of the tube, round or square (Figure 2).
Little or no support is needed within the tube when the tube diameter is small, and the wall is thick. As the size of the tube diameter is increased during tube bending, the tube becomes weaker.
If the wall thickness of the tube is decreased, it also becomes weaker. The forces acting on the tube also become greater as the radius of the bend becomes smaller.
Basic Primary Tooling
Bend dies, clamp dies, and pressure dies are the minimum essentials for bending tube and tube forming. (Figure 3).
The bend die helps to prevent the tube from flattening and forms a given radius of the bend. 2 Figure 1 The clamp die holds the tube in position while bending. The pressure die forces the tube into the bend die.