The Bending Process

INTENT The intents of this part of the tutorial are to discuss some basic mechanical concepts used in describing bending of sheet metal and to introduce a limiting condition on the amount of deformation that can be imposed in bending. A superficial summary is
- how can deformation be described?
- what is the minimum bend radius that can be produced?
Introduction
Bending is one of the most common metalworking operations. Parts are made by bending of sheet stock and bending also is a component of more complex sheet metal forming operatins. Bending is the plastic deformation of metals about a linear axis called the bending axis with little or no change in the surface area. When multiple bends are made simultaneously using a die, the process is sometimes called forming. What distinguishes bending is that the bend axes are linear and independent. Independence means that bending about one axis has no effect on the bending about the other axis. For example, a blank with four separate tags along the edges of a rectangular section can be bent into a box by bending each tab separately. In contrast, forming a box or rectangular pan from a rectangular sheet by using a punch and die is called a drawing operation. The corners of the pan are formed simultaneously and the deformation around a corner is determined by both edges and how they interact at the corner. If the axes of deformation are not linear or are not independent, the process becomes drawing and/or stretching, not bending.
The bending process is used not only to form parts such as angle sections, flanges, seams, and corrugations, but also to impart stiffness to the part by increasing its moment of inertia. Often changes in cross section shape can lead to increasing section stiffness without the addition of material.
The major concerns in bending are springback, the minimum length of workpiece needed to form a complex shape, the minimum bend radius possible the forces required. In this tutorial, we first present an overview of the deformation imposed in bending with an example of the shifting of neutral axis and its effects on bent part dimensions. We then define the strain imposed in bending since this quantity is needed to describe the details of the process. With a definition of strain and a material failure condition we can determine the minimum bend radius. Finally, the problem of springback in bending is discussed and strategies to compensate for springback are presented.
Deformation in the Bending Process
In shearing a part of the blank is held and force applied to the other part of the blank. We can imagine that if the clearance is increased sufficiently, the result will be plastic deformation of the metals rather than material shearing and fracture. The figure at the right illustrates the forces applied during bending in V-dies, wiping dies, and U-dies. We can also imagine that when the tooling is retracted from the bent workpiece some elastic recovery or springback will occur. The final part shape will not be the shape of the blank as it is held in the die. The obvious solution to this springback problem is to overbend the workpiece. The amount of overbend is the real question and a process model which can predict this is useful.
The tooling used in bending operations results in the metal being deformed in localized areas only. The localized stresses occur only in the bend radius. The remaining flat part of the blank is not stressed during bending. The stresses acting in bending are illustrated in the following figures. The metal on the outside of the bend radius is stretched or elongated. The metal on the inside is compressed. If blank fracturing occurs during bending, it will occur at the outside bend surface. A process model which decribes strain in the bent region and a failure criterion can be used to predict the minimum bend radius (maximum strain) which can be produced. Any wrinkling will occur on the inside surface of the bend. This wrinkling also has to be taken into account when designing parts and processes but is usually less of a concern than is outside bend fracture.
During bending, one area of the blank is usually held stationary by a pressure plate called a pad. A local region of the blank is bent and the free blank areas move up or down to create the change in contour. This free metal movement is often called swinging and is a characteristic common only to bending operations. In design of a bending die, the swinging action must be predicted so that no obstacles are placed in the way. Because of this metal movement, the larger area of the blank is usually held stationary and the smaller blank area is alloowed to move.
(fr Jun Du and Barney E. Klamecki)

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