Basic knowledge Bending of sheet metal and other semi-finished products

Bending is one of the most important tasks in the metal processing industry. In order to achieve the best possible result, physical characteristics have to be taken into account. There are different types of machines that can be used for bending. Read more about these interesting process in this article.

Definition: What is bending?

To bend materials, industrial companies mainly alter the shape of metallic workpieces such as sheet metal or tubes. The addition or removal of other materials does not occur during the actual bending process. The workpieces to be bent may be light such as sheet metal, hollow like tubes and to a certain extent solid like a massive body. In addition to punctual bending, it is also possible to bend a linear section, for example in sheet metal bending.

During bending, a force, the so-called bending moment, must act on the workpiece. In many cases, certain machines are used because they control the force more accurately and evenly than humans. This applies in particular to large or solid workpieces. There are various bending processes that belong to the main group of forming as defined by DIN 8580. Experts also refer to this process as forming by bending.

Difference between elastic and plastic bending

When bending, the industry strives for permanent deformation, which in technical jargon is called plastic, i.e. permanent deformation. However, not every force that is applied leads to this result. Up to a certain degree, the materials deform only elastically. The deformation is only temporary, and the workpiece returns to its original state as soon as the force is released. The degree to which a workpiece no longer bends elastically but plastically is determined not only by the type of material but also by its strength.

The maximum force that can be applied with no permanent deformation occurring, is called yield strength. Bending shops must therefore exceed the yield strength in order to achieve permanent deformation. At the molecular level, metal sheets and other metal materials consist of crystal-like metal atoms, the so-called crystal structure. If this structure changes after exceeding the yield point in such a way that the regularly arranged metal particles assume a completely new position, they can no longer leave this position without further ado. Then the deformation remains.

In elastic deformation, on the other hand, the individual molecules of the crystal structure move closer together or move away from each other. The individual atoms store the applied force in the form of mechanical energy, which they release again as soon as the external force is released. All particles return to their original position, i.e. bending failed.

Which materials can be bent?

Bending often refers to metals, but not exclusively. There are many different materials that are interesting for the industry, for example:

• Plastics,

• Wood,

• Iron,

• Aluminum,

• Copper,

• Brass,

• Alloys, i.e. mixtures of a metal with other metals or non-metals.

Two of the most important alloys used in bending are steel and stainless steel. Steel consists primarily of iron, which also contains carbon. High degrees of purity are particularly important for stainless steel, in addition further materials such as titanium, chrome or nickel are added. When bending and folding, bending shops often process steel or stainless-steel materials, for example in the form of sheet metal. Bending of sheet metal is one of the most common activities in bending.

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The type of metal can be crucial

If you want to bend sheet metal or workpieces, you should know which metal you are dealing with. Every metal and every alloy has its own individual yield and tensile strengths, which have an impact on successful bending. Data of this type is summarized in complex tables, each of which specifies the properties in N/mm². If you want to know how much force is needed to bend a metal of a certain strength without having to do your own test series, you should have a good look at these tables. The data is very useful for ensuring precise bending.

The load capacities can also be visualized in so-called stress-strain curves. Scientists, for example, collect data of this kind by means of tensile tests conducted on standard pieces. However, they only ever reflect statistical values. The complex curves show:

• at which deformation the yield point is reached,

• when constrictions (unwanted cavities) occur,

• when the metal breaks due to excessive force,

• and much more.

Semi-finished products as workpieces in bending

Bending shops often bend sheets in various thicknesses and sizes. This also includes perforated plates and other sheet metal variants. Pipes are also among the semi-finished products that can be optimized by bending, as pipes are often laid on non-linear routes. The challenge with pipes is that the pipe cross-section changes from round to oval as soon as it is bent at one point. This effect can only be reduced, but not completely eliminated, by certain measures. In addition to sheets and tubes, profiles, rods and wires can also be bent or folded.

Types of bending machines

Machines are often used for sheet metal bending. In the course of industrialization, a large number of variations emerged, which were initially operated manually, but have been increasingly automated. Nowadays, bending shops and locksmiths frequently use CNC bending machines for mechanical bending. These machines allow a high degree of precision and fast results. A CNC machine is equipped with a modern computer system and is automated in most cases.

Tools for manual bending

Some sheets, tubes and other semi-finished products can be bent without machines. Since a force is always required in bending, craftsmen use mobile bending devices as aids to increase human strength. Manually operated pipe benders, bending devices or bending pliers are also used. Especially in the field of bending pliers, a large number of shapes and areas of application have emerged, for example pliers for manual sheet metal bending, pipe pliers or simple flat pliers. The bending tools for use in machines are also described in the procedures.

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Common shapes in sheet metal bending

Bending sheet metal can result in a variety of shapes. These are often simple angles. In the range between 0° and 360° everything is possible. The right (90°) and the acute angles (0° to 90°) are particularly common in bending. Sheets bent in this way often serve as container or box components, for example for computer housings or fuse boxes. For the different bending shapes, specific designations have been established, e.g. a 90° angle at the edge of a sheet is called a standing seam. With the suitable technology and the right tools, round or wavy shapes can also be bent into sheet metal.

In addition to folding, bending can also involve flanging. The bending department bends the edge of a sheet in such a way that it obtains a much higher stiffness, whereby several sheets can be joined together. Flanging is also a technique for the production of non-detachable pipe connections in the automotive industry and in refrigeration technology. However, bent metal tubes are also a valuable component in other industries, for example within pipe systems. They are used in aviation, construction and the food industry.

Bending: The most important procedures at a glance

In most cases, bending in the manufacturing industry is carried out by means of die bending (folding) or swivel bending. Roll bending also plays an important role in this respect. Machines are used in all of these techniques.

1. Die bending

In the case of die bending or folding, the machine used for this purpose has three essential elements that simplify bending and make it more precise:

• Stop: The sheet must be inserted in such a way that it remains in the desired position during bending and the die is located directly below the bending point.

• Die: Refers to the master mold. Its individual template determines the shape of the sheet after bending. Dies can also be found, for example, in printing plants or in casting technology, and even nature uses them to reproduce DNA in cells. Instead of a die, the term bending die is also commonly used.

• Punch: The punch, which in contrast to the die is movable, is located above it. The die and punch often have an interlocking V-shape, but U-templates are also common in bending.

How bending with punch and die works

Once the sheet is in the correct position, the machine lowers the punch. The sheet is deformed by the force applied from both sides. Often the workers have to manually adjust the workpiece to obtain the desired shape. It is not necessary for the punch to lower as far during bending as to press the workpiece completely into the die. If there is still a gap between the punch and die such that the angle of the sheet remains more obtuse than permitted by the die shape, experts speak of free bending.

Angles in sheet metal bending can therefore be varied manually, which requires a lot of experience. If, on the other hand, the punch presses the workpiece completely into the die so that it reaches the so-called form-fit, this is referred to as embossing. To ensure that each workpiece can be bent into the desired shape during embossing, there must be an exact shape of punch and die for each sheet metal bending. Usually, there are different punch geometries available in a bending shop.

2. Swivel bending

The so-called swivel bending machine is used for swivel bending. In contrast to die bending, the workpiece is not pressed into a die by a punch. Its most important elements for efficient bending are:

a fixed beam (lower beam),

a lowerable beam (upper beam),

a clamping beam (bending beam), and

a stopper or a stop system.

How bending with a swivel bending machine works

The workers first insert the sheet into the area of the machine intended for this purpose until it touches the stopper. The machine then lowers the lowerable beam. Then the operator verifies that the sheet metal is still in the desired position for bending. A correction may already be necessary at this point.

Just like the stopper, the upper beam serves for the stabilization of the workpiece. When fully lowered, the sheet is firmly clamped between the fixed and lowerable beam. The bending beam finally creates the desired angle in the sheet metal by making a pivoting movement during bending. In the final step, the bending beam and the upper beam move back to their starting position so that the bent sheet can be removed from the machine. The described process consists of a combination of manual and mechanical working steps. In addition, fully manual and fully automated procedures are possible.

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Due to its numerous advantages, bending shops often prefer swivel bending to die bending. For instance, unlike die bending, swivel bending does not cause scratches on the surface of the metal. These scratches occur when the sheet metal rubs against the edges of the die. The machine wear, the total costs and the possible bending variety are also more favorable in direct comparison.

3. Roll bending

For roll bending, machines are equipped with rotating rollers. The rollers can be smooth or profiled and can even transfer patterns to the sheet metal during bending, including, for example, the corrugations of corrugated sheet metal. Roll bending is a completely different technique than milling. The following two areas illustrate this very well.

(a) The purpose of the forming

• Roll bending is used to bend workpieces, for example to form round, straight or profiled edges.

• Milling serves to reduce the cross-section of workpieces, for example to produce thin sheets from solid steel plates.

b) Materials

• Roll bending: thin sheets and solid metals of low thickness in various formats.

• Milling: only solid metals of different sizes.

Four techniques in detail: Bending with rollers

• Operators clamp the workpiece between up to four rollers. The movement of the rollers ensures uniform rounding. This process can be used, for example, to produce round containers.

• Roll straightening provides other results. A large number of straightening rollers, which are always arranged offset from each other, ensure straight sheets without tension or unevenness. The rollers coarsely corrugate the sheet during bending and then flatten it again until the workpiece moves out of the machine evenly and in a relaxed state. This process enables the production of high-quality workpieces.

• Corrugated sheets can be produced by means of roll bending. By incorporating corrugates, sheet metal bending produces sheets that can carry more load and are stiffer than other sheets. The rollers are equipped with a profile that is shaped like a star with several points or like a gear wheel. During bending, the sheet passes through the rollers, where the profile presses the corrugates into the metal in a similar way to die bending.

• Another important process in the field of roll bending is roll forming, in which even complex cross-sections of sheet metal can be produced quickly and inexpensively in large plants.

This article was first published byMM Maschinenmarkt.

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