For engineers and operators starting out with bending sheet metal, they might assume that the angle they bend ends up becoming the angle that occurs permanently in the metal. However, that is not the case. After bending, the metal will tend to slightly "spring back", known as springback, and it's known as an undesirable result in press brake operation if not taken advantage of. If you notice this, rest assured that there is nothing wrong with your operator or the metal, but it's simply a reaction the metal has when bending.
What is Springback?
Springback is how the sheet metal tries to revert back into its straight form once it is bent. The formation of sheet metal requires elastic-plastic bending and stretching of the metal, which contributes to springback.
Bending is one of the most frequent actions done to metal in press brake operation. Springback is known as the reaction when this metal is bent, and causes complications when forming the metal into objects such as seams and flanges. Springback is not only an occurence in sheet metal, but also in wires, rods, bars, strips, and other thin-enough forms of metal.
Why Does Sheet Metal Spring Back?
All materials have some degree of elasticity, including metal, which is highly known for its stiffness. Following a plastic deformation that we make to bend the metal, an elastic recovery followed once we remove the load, causing the metal to attempt moving back in place.
This happens for two reasons. First, the molecular density is greater on the inside of the bend than it is on the outer surface that is being stretched. When this happens, the molecular density becomes great on the inner side of the metal than on the outer side. Additionally, when a sheet of metal bends, it experience a great amount of stress. On the metal's stretched side, the tensile forces are greater than its compressive forces, which also contributes to the metal's springback capabilities.
What Is Springforward?
Springforward is the state of the metal bend briefly bending again following the springback. The force of springforward counteracts with springback to get the bent angle that the metal eventually stays with. Springforward doesn't necessarily negate springback, but rather slightly limit how far back the metal springs. When the two forces average out, we get a process known as coining. Coining used to be the traditional means of compensating for springback, but rarely do companies ever count on it anymore thanks to other elaborate and more effective means.
How Is Springback Calculated?
Springback is typically measures starting with the difference between the angles of the part that is bent and the part that does the bending. The number you get from this then has to be divided by the tooling angle.
The Springback factor in formulaic equations is commonly represented by "Ks", and is considered the relation between the initial and final angle. Here is how it was originally written out:
Ks = Initial Angle / Final Angle
Ex: 90° / 87° = A springback factor of 1.0344
If Ks equals 1, there is no springback.
Ex: 90° / 90° = 1
If Ks equals 0, there is total springback.
Ex: 65° / 0° = Total Springback
There is a more advanced formula that you can try. To find the springback before testing, it is crucial to understand that the angle and bend radius vary depending on the springback. You can find the initial angle by doubling the initial radius by 2, dividing the number by the thickness of the metal, and then adding 1. The same is also done with the final angle and final radius.
Ks = ((2 x initial radius) / material thickness) + 1) / ((2 x final radius) / material thickness) + 1)
With this formula we can determine the yield stress and elasticity into our formula. A greater yield stress causes more springback, whereas a greater elasticity leads to less springback.
How Do Companies Take Advantage of Springback?
Springback must be anticipated and limited as much as possible so that operators can control the metal better to get it in its desired form. It is necessary to understand the difference between the bending angle and bent angle when using a brake press operator.
What Is Springback Compensation?
Springback compensation is considered the counter measure to springback. This compensation allows operators or engineers to decrease the time and cost to correct the metal to the intended angle.
This common solution to springback is for operators to bend the metal even more so that when it springs back, it is positioned into the angle that is originally intended. This allows the right stress distribution so that less controlled is required on the user's end. With practice and optimization of this compensation, the right bends can be achieved every time like clockwork.
Springback is something that companies with brake press operators should anticipate, but also know how to deal with in one simple way. Understanding springback is the simplest way to control it and make the products you need with no deformations