Fundamentals How does the folding of sheet metal work?
Folding of sheet metal is part of the manufacturing technologies bending and forming. It involves the folding of a surface part of sheet metal.
A short definition of the term folding is already given in the title. It remains to be added that during folding, the bending process is carried out simultaneously along the entire length of the bending line and that it is a manufacturing process for the machining of sheet metal, i.e. thin, semi-finished metal products. Folding is used to produce profiles, more precisely, folded profiles.
Roll forming is part of the same area of application. Since the forming process takes place successively along the bending line, however, roll forming does not belong to the folding category. With regard to the material properties, a further prerequisite for folding is sufficient plastic deformability. A complete definition of folding is therefore: "Plastic change of the shape of sheet metal parts by cold forming along straight lines, simultaneously over the entire length".
How is forming classified in the standard scheme of manufacturing processes?
The DIN body of standards divides manufacturing processes into six main groups under DIN 8580. It assigns folding to group 2, under forming, which is standardized according to DIN 8582. The further subdivision assigns forming to subgroup 2.4, bending forming. It contains a subset of the manufacturing processes contained therein. DIN 8586, which covers this subgroup, basically distinguishes two categories of forming by bending: bending forming with straight-line and rotating tool movement. The latter is typical for folding. The first category includes the process groups of free bending and press-brake bending. The following bending processes meet the criteria of folding:
- Free bending with one-sided clamping
- Free bending with press brake (three-point bending)
- Free round bending (successive production of curves by repeated free bending)
- V-die bending
- U-die bending
How does folding work?
Since the term describes a group of forming processes, there are different modes of operation. Common to all processes is that the workpiece is bent around one or more straight edges of a tool by applying pressure. The machine tools used for this purpose are press brakes, folding benches or folding machines.
Functionality of folding with one-sided clamping and folding
In free bending without a die, the workpiece is clamped along the edge to be produced and a punch moves past the clamp at right angles to the sheet surface. During folding, the plate is bent around the folding wing opposite the punch. Folding works very similarly. The difference is that instead of a punch moving in a straight line, a folding wing rotates around the bending edge. The tool thus performs a movement that results in a corresponding bending leg of the sheet metal profile.
What happens during press-brake bending?
In bending processes using a die, the bending edges are generally located on the punch of the machine tool. When more complex parts are folded, for example Sigma-shaped profiles in a single operation, additional bending edges may also be required on the die. Every simple bending operation with a die begins with free bending, which is also referred to as three-point bending in analogy to the corresponding bending test. The workpiece rests on the two upper edges of the die and is pressed into the die by a V-shaped punch without the apex of the workpiece coming into contact with the die.
If the forming process stops before the punch reaches the die bottom, this process is classified as free bending. Otherwise, the pressing phase starts with press-brake bending, i.e. the workpiece takes on the exact shape of punch and die. This reduces the springback after the pressure has been relieved and thus enables more precise production. On the other hand, a specific tool set consisting of punch and die is required for the production of different profile angles.
Special cases of press-brake bending
In U-die bending, two edges are created simultaneously on the workpiece. C-shaped profiles can thus be produced in a single operation. If the die has a movable, e.g. spring-suspended counterholder, the workpiece rests on the underside of the punch during the entire forming process. This eliminates the holding pressure phase, which requires up to three times more force than free bending. In contrast, only about one third of this bending force is required to press the counterholder against the workpiece. In another special case, the forming process done with special machine tools stops when the adjustable counterholder is reached. This ensures that the vertex of the workpiece is in contact with the punch and assumes the bending radius of the punch.
What is the bending edge?
In bending forming, the bending edge refers to the deformation of the workpiece produced in a single work step. During folding, this consists by definition of two legs of bent sheet which are positioned at a given angle to each other. These legs are connected to each other by a rounding with a certain bending radius.
How is the bending angle defined?
The bending angle refers to the distance the leg of the profile moves during the forming process. It is the angle between the originally flat position of the inner edge of the leg and its end position. The value of the opening angle - that is measure between the two inner surfaces of the bending edge - added to the bending angle results in 180°. At the end of the forming process, the bending angle is reduced by the spring-back effect of the workpiece that occurs when the contact pressure is removed. The extent of this elastic deformation depends on the chosen bending method and causes varying dimensional deviations typical to the specific process.
What does the bending radius refer to?
The bending radius is measured at the inner edge of the workpiece after the bending process is completed. Processing flaws can cause wrinkling and thus measurement inaccuracies, since it occurs on the inside of the bending edge, which is compressed during folding. The radius must not be below that of the minimum bending radius, which depends on the material and the geometry of the semi-finished product as well as on the pre-treatment and the bending angle. According to DIN 8586, sheets should be bent transverse to the rolling direction, as the grain structure changes anisotropically as a result of rolling. However, folding is usually carried out in the longitudinal direction. In some cases, different minimum bending radii apply. Bending radii should preferably correspond to the dimensional series listed in DIN 250.
Seaming: Folding as a joining process
During seaming, two sheet metal parts or two edges of the same sheet metal are joined in an interlocking manner. This is done by simultaneously folding or pressing the prepared and fitted edges. According to the DIN 8580 series of standards, this process belongs to subgroup 4.5, joining by forming, and is specified by DIN 8593. Seams can be produced in numerous variations. In the simplest case, the standing seam, two sheet edges are bent at right angles so that the leg of one sheet part is about twice as wide as that of the other. Then the two parts are placed next to each other with the legs touching each other. Finally, the longer leg is bent by 180° so that it encloses the second one.
Which bending angles can be produced by folding?
In folding, the most important requirement placed on the material is cold formability. Under DIN 17100, which was effective until 1994, the suitability for bending hot-rolled steels was indicated in the material number by its own code letter Q. Following its replacement by DIN EN 10025, only the letter C is provided for all cold-formable steels. The type of material is a limiting factor for the bending radius. For steels, further information on cold formability and minimum bending radii for folding is provided by DIN EN 10025; for sheets and strips made of aluminum and its alloys, for example, specifications are given by DIN 5520.
What is the maximum folding length?
The maximum profile length that can be achieved by folding depends on the machine tool and is constantly increased by technical progress. The Variopress 1250-80 Tandem folding machine from EHT Werkzeugmaschinen in Teningen can, for example, bend sheets with a maximum length of 16 meters or two sheets up to eight meters long simultaneously in a single operation.
This article was first published by belchnet.