May 15, 2001
Knowing how metal bends and what factors come into play during bending—especially wipe bending—can make a positive difference in your stamping operation.
Editor's Note: This article is Part I of a two-part series discussing bending. Read Part II.
Of all of the operations that typically are done in a stamping die, it seems that achieving and maintaining a 90-degree bend are two of the most difficult.
To accomplish these tasks successfully, you must have a basic understanding of what happens to metal when it is bent, as well as a basic grasp of all the variables that influence the bending operation.
This article focuses primarily on the fundamentals of wipe bending. All of the following are factors that control or influence the success of a bending operation:
Ignoring or failing to control any one of these factors most certainly affects your ability to achieve a controlled bend.
Figure 1 shows a simple wipe bending operation. Springback, also known as elastic recovery, is the result of the metal wanting to return to its original shape after undergoing compression and stretch.
As Figure 2 shows, the inside bend radius is squeezed together, or forced into compression, and the outside bend radius is forced into tension, or stretch. After the forming tool is removed from the metal and the load is released, the workpiece relaxes, forcing the bent portion of the metal to return slightly to its original shape.
To visualize this better, imagine the metal is similar to Silly Putty when it is deformed. A good rule of thumb to remember is that approximately one-third of the metal's thickness from the inside bend radius undergoes neither stretch nor compression. This zone often is referred to as the neutral zone.
The key to obtaining the correct bend angle is to overbend the metal and allow it to spring back to the desired angle. All metals exhibit a certain amount of springback, and depending on the part tolerances, overbending usually is required.
The mechanical properties of the metal grossly affect the amount of springback. Higher-strength ferrous and nonferrous metals exhibit more springback than lower-strength metals do. This means that they require more overbending to achieve the desired result.
1. Thickness. Metal thickness also affects the amount of springback. The thicker the material, the less the springback. This is mainly because of the metal's stiffness.
2. Tolerance. The thickness tolerance of the metal also contributes to inconsistency in a bending operation. When metal is thicker or thinner, it is squeezed less or more in the bending operation, respectively. This contributes to inconsistency in the amount of springback.
3. Size. The size of the inside bend radius also affects the amount of springback. The larger the bend radius, the more the springback, because when metal is bent over a large radius, more surface area is compressed and stretched.
4. Speed. The speed at which the bending takes place also affects springback. Generally, faster forming speeds reduce the amount of springback. This is mostly attributable to the inertia created when the metal is deformed. Because the metal is forced to rotate very quickly, it continues to rotate even after the forming punch has stopped. This results in more overbending.
5. Grain direction. The grain direction of the metal also contributes to the amount of springback that occurs in a bending operation. The grain direction is established during the metal rolling process. Bending with the grain gives a different result than bending against it. You must pay careful attention to grain or rolling direction when bending high-strength metals, especially when trying to achieve a small inside bend radius.
When bending high-strength materials, such as spring or high-carbon steel, you should orient the part so that it can be bent against the grain. Bending with the grain may result in cracking or even breaking in the deformation area.
Figure 3 shows a poor nesting configuration for a simple box that is made of 0.125-inch-thick 980X spring steel. The inside bend radius is 0.125 inch, and the box is bent in four areas.
If the part is nested as shown, the two outside bends (with the grain) most likely break off. However, if the part is nested as shown in Figure 4, bending takes place 45 degrees to the grain direction and produces a more even amount of springback, greatly reducing the likelihood of breakage.
6. Friction. The coefficient of friction also contributes to the amount of springback. During bending, the metal is forced between the lower die section and the forming punch. If the clearance between these two sections is less than the metal thickness (as it usually is), intense friction is created.
The amount of friction determines how much the bend will be stretched. If the inside bend radius is large enough and the metal can be stretched over it, the amount of compression is reduced or eliminated. If compression is eliminated, both sides of the radius are in tension, thus reducing the amount of springback (see Figure 5).
Keep in mind that, although an increase in friction may reduce the amount of springback, intense friction may result in severe wear to the die sections. For this reason, you should use lubricants. Be certain to control the amount, type, and mixture of the lubricant consistently—it's critical.
Despite all these factors, by far the most critical is the design of the tool itself.
Because it is nearly impossible to control all of the aforementioned variables exactly (everything has a tolerance), the die must be designed to be easily adjusted for slight variations in metal thickness and mechanical properties, as well as changes in the key variables.
A good tool has features designed and built into it that allow tooling personnel to make changes to the bending operation simply, quickly, and safely and to increase or decrease the amount of overbend.
Unless there is a great deal of bend angle tolerance, or all of the key factors can be duplicated and controlled precisely, it is unrealistic to expect a nonadjustable bending operation to produce quality parts consistently.