Metal bending basics on the stamping press

What Is Metal Bending?

One of the most common forming methods performed in metal stamping dies, bending involves deforming metal along a straight axis. (This makes it different from flanging, which uses a curved axis.) It can be used to obtain any desired bend angle, although 90-degree bends are most common.

Items such as tabs and channels are created using the bending process. When bending is used to create U-shaped parts, it’s called U forming or channel forming.

Springback and Overbending

One of the biggest challenges in metal bending is springback. Also known as elastic recovery, it is the tendency for material to return to its original flat shape when subjected to deformation.

Metals such as copper and mild steel are softer and have lower springback values than their higher-strength counterparts, such as high-strength steel or spring steel. Regardless of the springback value of the metal, to achieve the proper final bend angle, you have to bend the metal past the desired bend angle and allow it to return to the correct angle. Tool- and diemakers and engineers commonly refer to this process as overbending.

Variables Affecting Springback

Despite the efforts of steel manufacturers, it’s nearly impossible to keep the metal’s mechanical properties consistent throughout an entire coil, and those differing properties affect the amount of springback that occurs.

The higher the yield and tensile strength of the material, the more likely that springback values will increase, requiring greater overbending to compensate. Thickness also makes a difference: Thicker metals exhibit lower springback values than thinner metals of the same type, mainly because a larger volume of material has been deformed and work-hardened in the radial area. In addition, thicker material is inherently stiffer than thinner material, so it retains its original shape more effectively.

The size of the inside bend radius also has a large impact on the amount of springback that will occur during the bending process. Larger radii will result in greater springback values, while smaller radii reduce the amount of springback. If the radius is too small, however, it may result in the metal splitting on the outside radius where it is in the greatest amount of tension.

All coil material is rolled and has a grain direction. Bending or forming with respect to the grain direction will affect the amount of overbending required, as well as the likelihood of splitting in the radial area. When splitting is a concern, bending transverse (across the grain) to the rolling direction is more desirable than bending parallel to the rolling direction. Be sure to pay close attention to both the size of the inside bend radius and the bending direction with respect to the rolling direction, especially if the material is high-strength or has poor stretchability.

Metal deformation speed also affects the amount of springback that occurs. Remember that metals are sensitive to strain rate, meaning that different forming velocities result in different amounts of stretch and stretch distribution.

The type, amount, and severity of strain being used to create the bend are other variables. When metal is strained or work-hardened, springback decreases. Tensile strain and compressive stresses are generated naturally during bending as the metal is stretched and compressed, respectively. Strain also can be created, such as by coining, which is squeezing the metal between a punch and a die to reduce its thickness and cause it to work-harden.

One of the keys to getting the desired bend angle is to design the die so that it can be adjusted quickly, safely, and effectively to compensate for incoming material variability.

Coining the Radius

I remember being on the shop floor as a young tool and die apprentice, working on a die and trying diligently to get a simple 90-degree bend on a part. One of the older toolmakers told me that if I wanted to make a successful 90-degree bend, I would have to coin the radius. I asked him what coining was, and he said it was a way of setting the radius and bend angle by squeezing the metal in the radial area between the punch (or anvil, as he called it) and the die. I followed his instructions, and lo and behold, I ended up making the desired 90-degree bend.

At that point in my career, I understood that it worked, but not why it worked. Today, having about 40 more years of experience, I will explain why coining the radius works and how to achieve it.

No Strain, No Gain

To understand why coining works, you must understand what strain is and how it affects the final part shape. For a stamping to retain its shape with minimal springback, it must be adequately strained. This means you must form the metal in a way that imparts work hardening (also known as strain hardening) to adequately meet the metal’s yield point (the point of permanent plastic deformation) and eliminate springback. Permanent deformation is basically the product of work hardening.

There are two basic types of permanent strain: tensile strain and compressive strain. Tensile strain happens when the metal is stretched, and compressive strain happens when the metal is compressed or squeezed together. This process is commonly known as coining.

One of the more effective ways to create a 90-degree bend by coining is to design the die in such a way that the coining takes place in a small, localized area within the bend radius (see Figure 1). Coining the entire profile of the radius will help to reduce springback and achieve the necessary overbending. Keep in mind that using this method will increase the force or tonnage required to achieve the final desired bend angle.

The key is to impart compressive strain where it is the most effective, near the tangent closest to the metal being bent. By making the height of the forming block adjustable, you can change the final bend angle by shimming the forming section up or down. Doing so controls the amount and severity of the coining in the radial area.

Rotary Bending

Rotary, or rocker, benders are very effective mechanical methods for creating a given bend angle. Unlike coining dies, rotary benders create a great deal of tensile strain (on the outside radius) and compressive strain (on the inside radius) by overbending metal significantly beyond 90 degrees to an acute angle (see Figure 2). All of the strain is the product of naturally occurring straining that happens during the metal bending process.

These benders have many advantages. They can overbend the metal to as much as 30 degrees beyond 90. They are easily adjustable and can be used for bending up or down. Because they're essentially wrapping the material around the punch, the amount of force needed to create the bend is significantly lower than with the conventional wipe bending process. This is especially true when the wipe bending process uses coining as well. Rotary benders also can be designed with high-pressure nylon or plastic for bending prepainted materials.

One disadvantage of a rotary bender is that it uses mechanical motion to create the bend, so it requires periodic maintenance to ensure precise, low-friction rotation.

Die Design for Flexibility

There are at least half a dozen other ways to design a die to achieve the desired bend angle on a part, and all of them use both tensile and compressive strain. Sometimes the strain is natural, but sometimes the strain is created by additional coining.

Regardless of the method you choose, remember to design the bending process to allow for quick, safe adjustment within the boundaries of the press. This is especially true if the desired bend angle is very critical.