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Sheet Metal Bending

Bending is one of the most important processes and can also be called edging, folding, braking, flanging or die bending. A press brake has an upper and a lower tool.

How sheet metal bending works

During the bending process the top tool, also called a punch, is pressed into the bottom tool (die) in a controlled way. The metal sheet is placed between the two tools and is bent into the required shape by lowering the punch.

The advantage of professional press brakes is that both angles and radii can be bent. This is done by simply changing the bending punch. The punch has the precise shape required for the metal sheet. The sheet can be bent by differing amounts depending on the lowering depth. The material to be bent is positioned against a stop (back gauge) to make the bending process as efficient as possible.

The setting of all our machines are made by CNC (computerised numerical control). This also enables simulation of the planned bending operation. In this way the insertion depth (stroke depth) can be planned optimally without having to carry out trial bending.

Bending radii and bending radius

We have four press brakes with dimensions 3,500 mm, 6,000 mm 8,000 mm and 16,000 mm. On these press brakes we can use radius dies to bend high-precision inside radii. Radius stamps have to be used if extremely resistant steels, for example HARDOX, are to be bent. These materials are very hard, but tend to tear if the inside radii are too small.

With our radius dies and stamps we ensure that materials are bent optimally according to their specific properties, without losing their stability.
Before a radius can be bent in metal sheets or plates, the minimum bending radius must be known. Minimum bending radii depend on the types of material and sheet thicknesses. Predefined radii are required for many applications. Regardless of whether they are required for visual or design reasons.
With our radius tools (punches and dies) we can produce inside radii with 0.5 mm accuracy. We achieve larger radii through multiple bending or segment bending.

Bending Dillidur or Dillimax wear plates

Despite their high toughness, impact resistance und robustness, Dillinger wear plates are have exceptionally good workability. According to the manufacturer, these plates are suitable for cold forming and machining. Our machines can be set to sheets and plates with higher strengths, so that any spring back of the material during the bending process can be balanced out.

Contact us if you need products made of wear-resistant material. We will be pleased to advise you!

What bending methods exist?

Air bending

schematic representationThe sheet must be inserted in the machine so that the material only sits on the two upper edges of the die. The preprogrammed insertion depth (stroke depth) of the punch is CNC controlled; the upper tool (punch) presses the sheet into the die and in doing so bends the required angle. However, the lowering operation ends before the sheet touches the die. The main advantage of air bending is the flexibility of bending different angles without having to change the tool (die). This is also a reason why air bending is used so frequently.

Coining

The basic principle of coining is the same as that of air bending. The only difference lies in the large pressing force, which acts on the workpiece. Angles bent in this way are more precise, as the workpiece is pressed completely into the die. The disadvantage of this method is its low flexibility.
Each die enables a defined angle to be bent. In the case of workpieces with different angle specifications, the press brake must be retooled. This has a disadvantageous effect on the unit price.

Three-point bending

As with air bending, the sheet to be bent is placed on the die so that the material only touches the bottom tool at the two top edges of the die.
The lowering process is also similar to that of air bending; only the punch is lowered until the sheet touches the die in the middle (at the 3rd point). If the bottom tool is equipped with an adjustable pressure pad or clamp (adjustable height die), it is possible to bend different angles without changing the tool.
These tools are very expensive to buy and are only used by a few manufacturers.

Step bending

We have a large number of radius dies and punches for bending sheets with specific inside radii. However, there are orders that require radii which are larger than the tools we keep in stock. In such case we bend all the required radii by “step bending”.

In this computer-controlled bending process, the metal sheet is bent step-by-step with different size bends until the required inside radius is produced.
The CNC control sets the insertion depth of the punch and the back gauge with millimetre precision. However, the dimensional accuracy of the radius depends on several factors. In principle the radius of curvature or bending radius is defined by the number of bends required and the material thickness.

Why is bending film needed?

bending filmsurface protection

Using bending film (also called bending tape) avoids the risk of marks on sensitive sheets, bent parts made of steel, stainless steel with ground surface, aluminium or mirror polished V2A / V4A steel.

The bending film is placed between the workpiece and die and protects the metal sheet from direct tool contact. It is made of polyester or polyamide, is mechanically extremely resistant and can be used for several bending operations.

Bending film is easy to handle. It can be changed quickly and easily. As the film prevents direct contact between the tool and the material to be bent, there is no soiling or contamination of the bent parts. It is therefore not necessary to rework or finish the metal sheets. This naturally increases the quality of the product.

What is the minimum bending radius?

Bending or folding is a chipless metal sheet forming method, in which metal sheets or plates are formed by the effect of mechanical force.

At Rime the forming is carried out using four modern CNC press brakes with 4, 6, 8 and 16 m dimensions. These presses are CNC controlled and make facilitate the bending operation immensely. Our ability to simulate bending processes helps us to avoid trial bending and therefore to save time and material. Yet machines can’t do all our work for us. In principle, the bending is done by inserting the punch in the die.

The metal sheet is inserted between the two tools, and when the punch is lowered the sheet is pressed into the die and in this way the required angle is bent. Yet this bending process conceals several details, which always have to be taken into account for optimum bending. All metals do not behave the same when they are exposed to large forces.

Minimum bending radius

The minimum bending radius is the smallest possible radius, with which a metal sheet panel can be bent without damaging the material. This value depends on the material and sheet thickness. There is a rule of thumb for steel sheets, which says that the minimum bending radius must at least be equal to the value of the sheet thickness. For aluminium the minimum bending radius is equal to the value of twice the sheet thickness. If the bending radius is less than this value, the sheet can tear at the edges, or material can spall from the surface.

Spring back

If a workpiece is to be bent with a specific angle, it must be bent beyond this angle, because the workpiece springs back slightly after bending. The spring back can be compensated for very well with modern CNC press brakes.
It only becomes critical if a metal sheet is to be bent with precisely its minimum bending radius, as the spring back means that the bending radius must be smaller than the minimum bending radius. For this reason the actual minimum bending radius must always be larger than the theoretical possible radius.

What are large parts needed for?

Metal sheet formats 8 to 16 metres in size Larger, faster, further… – Striving for progress, moving one’s own limits and overcoming technical limits are anchored deep in human nature. And so each year new records are broken. In 2010, the highest building in the world to date was completed in the United Arab Emirates. From the ground to the tip of the roof mast the “Burj Dubai” is a lofty 830 metres high. There are vehicles that drive faster than sound in the salt desert of Arizona and bridges which nor only span wide rivers but also whole straits.

The bridge with the longest span is the Storebælt bridge in Denmark. The distance between two pylons is greater than 1,600 metres. Such structures would not have been possible without high-strength and stable construction materials and modern working methods.

In the machining of metal sheets the trend is towards ever-larger workpieces. There are several advantages in being able to bend sheets with a length of over 8 m. A few years ago it was not possible to machine large parts. For example, if a 16 m long bent part was required; such workpieces had to be welded together from several smaller parts.

Despite modern welding procedures, welds are still a weakpoint. The effect of heat during welding not only melts the material. It also changes its crystalline structure slightly, which then weakens load-bearing capacity and increases the rust susceptibility of the large part made. Making welds is also time-consuming and material-intensive, for example, because welded bridge girders have to be welded using a gas shield and then checked using X-ray testing.

Increasingly large-sized parts are being produced in order to reduce this expense. Machines that are able of machining such parts grow in line with the ever larger sheet metal parts. In Europe there has been a strong demand for large parts in recent years. International transport of long metal sheets or plates is expensive, so that import from Asian countries is only worthwhile for small parts.

Bending thick plates and long sheets

We now follow these new requirements and since 1 December 2011 we have been able to bend very long sheet metal parts. Yet this is only one facet of our new possibilities. Often it is not only a matter of bending large parts with a specific length – the ability to bend large sheet thicknesses or rather metal plates is also very much in demand. To achieve this we have a modern press brake with a total pressing force of 25,000 kN. This means that the machine is practically able to bend 30 mm thick steel plate along its whole length.

This machine even manages to bend plates that are 70 mm thick with a bending length of 2,000 mm. Our existing “XL” offer is becoming ”XXL” and we can offer you sheet metal and plate parts, whose length are only just still transportable with a truck. With such workpieces we enable the construction and development of larger and more stable structures and follow our human goals to erect ever larger and better structures.

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