Springing when bending metal
Springing
Reverse springback is one of the most important factors affecting the bending result. As already mentioned, the lower side of the sheet is stretched during bending, and the upper side is compressed. Between the compression and tension zones there is a "middle line" where only elastic deformation is present. It is here that the material tries to take its initial position as much as possible and, accordingly, "straighten" the bending. After removing the acting force, the bending angle becomes slightly larger. This difference is called the spring angle
In order to compensate for this factor, bending is done with a sharper angle. The properties and thickness of the metal may differ in different parts of the same sheet, the parts are located on the sheet at different angles. Therefore, it is not uncommon for a bending adjustment to be required even within one batch of metal. The springback angle depends on many factors. Let's consider the main ones.
Material properties . The stiffer the material, the greater the springback angle. So for thin-sheet soft copper, the angle usually does not exceed 0.5 °, for steel it is already about 1 °, and for stainless steel it can exceed 3 °.
Sheet thickness . Thin metal has less springback than thick metal. That is why, for bending thick sheets at an angle of 90 °, manufacturers have in the standard program dies with a large opening and an angle of 80 ° or less.
Bending speed . Of course, everyone strives to bend as fast as possible to maximize productivity. However, this does not have the best effect on accuracy. The lower the bending speed, the longer it takes for the metal to take its final shape. The delay at the bottom point also helps to reduce the springback angle. However, productivity, as a cost factor, is often critical. Therefore, it is necessary to achieve greater repeatability and make a preventive inflection, selected for specific conditions.
Bending radius . The smaller the bending radius (closer to the metal thickness), the less back springback. Smaller bending radius is achieved by smaller V-die opening and punch radius. Accordingly, the impact on the metal is greater and less elastic deformation. This is one of the reasons for using dies with different V-opening according to the type and thickness of the metal. Of course, for bending steel of all thicknesses up to 3 mm, you can use a matrix with an opening of 20 mm, but bending steel of 1 mm will be significantly less accurate than if you use a matrix with V = 6 mm.
Direction of rental . It has already been mentioned that parts located on the sheet at different angles will bend in different ways. This is due to the direction of the internal stresses of the sheet. If the bending line coincides with the direction of the rolling, then the springback angle will be minimal. If across - then the maximum. It is easy to see this on parts that have at least two perpendicular bends. Therefore, such bends must have different angle settings. If the most economical layout of the blanks on the sheet assumes their perpendicular arrangement, then it is recommended to sort the parts by their location on the sheet after cutting. Next, on a Cutter Bending Copper Machine , first tune in and bend the first batch, located at one angle, and then the second, "perpendicular".
The type of bend has a significant effect on the back spring angle, because the impact on the material is significantly different.
Many machines have various devices and internal programs for preventive "bending". To obtain a part “from the first blank”, closed-loop systems are used. After the calculated bending and removal of the force, such systems measure the resulting angle and make a bend to obtain maximum angle accuracy. However, such systems are quite expensive and require additional time at each bend.
