What Is Press Brake Bending?

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Bending is one of the important procedures in the sheet metal processing industry. It is the process of forming sheet metal into the desired profile by applying force to the workpiece.

The bending process is generally performed on the press brake - a mechanical processing tool that is mainly used for sheet metal bending and forming.

The bending process is mainly used for manufacturing parts and workpieces in various industrious fields. Bend parts can deal with small parts as well as large workpieces.

Due to the different thickness, hardness of the sheet metals, and different shapes of anticipated profiles. Therefore, we need press brakes with different tonnage and pressure to bend sheet metals.

Bending stretches and compresses the sheet metal. The external force will only change the shape of the metal plate.

The length of the outer part of the sheet metal will be elongated, while the inner part will be compressed and the length will be shortened. But the length of the neutral axis is unchangeable.

The ductility of sheet metal allows its shape to change, but other parameters remain unchanged, such as volume and thickness.

Although in some cases, bending may alter the external characteristics of the sheet metal. Additionally, bending can change the moment of inertia of workpieces. Tonnage is determined by the driving source of press brakes.

According to the different force application methods, press brakes can be primarily divided into mechanical press drive, hydraulic press drive, pneumatic press drive, and servo press drive.

What’s more, it is also necessary to match the punches (upper dies) and dies (lower molds) with different heights, shapes, and V-opening sizes.

Generally, the materials of bending dies are gray cast iron or low carbon steel. But the materials of punches and dies vary from hardwood to carbide according to the workpiece.

The sheet metal is properly placed on the bottom die. The punch is lowered on the die through the power of the ram.

The bending process will repeat the bending stroke several times to form desired profiles.

The metal plate will rebound slightly after bending.

In order to ensure that the predetermined bending radius and bending angle remain unchanged, usually during operation, the bending radius should be set to a value greater than the predetermined bending radius, and the final bending angle will also become smaller.

Types of Press Brake Bending

There are different types of sheet metal bending methods. These methods are based on the relationship of the end tool position to the material thickness.

The bending methods are also different in the method of plastic deformation of the plate.

Although the bending techniques are different, the toolings and configurations are basically the same.

Bending methods are also determined by the material, size, and thickness of sheet metals.

Bending dimension, bending radius, bending angle, bending curvature and bending position in the workpiece are also essential for the bending methods.

V bending is one of the most common types of sheet metal bending methods. It needs to be equipped with a V-shaped punch and die.

In the process of bending, the metal plate should be placed on the V-shaped die, and the punch presses the metal plate into the V-shaped die under the action of pressure.

The bending angle of the metal plate is determined by the pressure point of the punch.

The angles and shapes of the dies include acute angle, obtuse angle, right angle, etc.

V-bending can be subdivided into air bending, bottoming, and coining.

Air Bending

Air bending is also called partial bending because the workpiece is not in full contact with the die.

In air bending, the sheet metal is only connected with the shoulder of the die and the tip of the punch.

The punch is pressed onto the plate and passes through the top of the die into the V-shaped die opening, but does not contact the surface of the V-shaped opening.

Therefore, the distance between the punch and the sidewall of the die must be greater than the thickness of the metal plate.

Air bending is the bending method with the least contact with sheet metal.

The equipment only needs to contact the sheet metal at three points, namely punch, punch tip, and die shoulder.

Therefore, the correlation between bending angle and tooling angle is not so large. 

The depth of the punch pressing into the V-shaped opening is the important factor affecting the bending angle. 

The depth of the stroke, that is, the greater the pressing depth of the punch, the sharper the bending angle.

The bottom die and punch of air bending do not need to have the same radius, because the bending radius is determined by the elasticity of the sheet metal.

Air bending is the most widely used bending method because it has many advantages.

Because the punch tip doesn't need to be pushed past the surface of the metal, it requires less bending force or tonnage.

Additionally, it does not need too many tools, and the operation is simple and flexible.

However, air bending also has some disadvantages. Air bending will have a certain springback after bending.

So during bending, the actual bending angle should be sharper than the preset bending angle, so as to get the final bending angle.

Moreover, in air bending, because the metal plate and die are not in full contact, the bending accuracy is difficult to ensure. Its stroke depth cannot be kept very accurate either.

Bottoming

Bottoming is also known as bottom pressing, bottom bending, or bottom striking. Like air bending, bottom bending also requires punch and die.

In the bottom bending, the punch presses the metal plate to the bottom of the die, so the angle of the die determines the final bending angle of the metal plate.

The release of the punch will cause the sheet metal to spring back and contact the die.

Excessive bending helps reduce springback. Using more force will also reduce the springback effect and provide good accuracy.

The difference between air bending and bottom bending is that they have a different radius.

The die’s radius determines the inner radius of the bending sheet metal.

The width of the "V" shaped opening is usually 6 to 18 times the thickness of the sheet plate.

In the bottom bending, because the angle of the die is fixed, the bending accuracy is higher and the springback is smaller.

But a larger tonnage of force is required. Moreover, each bending angle, plate thickness, and materials need a separate die.

Coining

Coining is also a widely used bending method. The word “coining” comes from coin making.

In the United States, in order to print Lincoln's profile on a coin, a large tonnage machine is used to compress the coin to obtain the same image as that on the mold.

In coining, its punch and metal plate are at the bottom of the die.

The force produced by the punch is 5 to 8 times that of air bending. In this way, the sheet metal will hardly spring back.

The bending accuracy of coining is extremely high and the bending radius is small. Therefore, its cost for manufacturing is also very high.

However, in this bending process, the press brake and tooling are easy to be damaged by friction.

Moreover, more toolings need to be equipped. Basically, each plate thickness needs different punches and dies. Also consider the angle, radius, and die opening.

What Needs To Be Considered When Bending a Metal Using a Press Brake?

Types of the Bending Materials

Before sheet metal bending, we must first make sure that what materials are good for bending.

Some metal materials have good ductility. These kinds of metals are more suitable for bending.

While some metals are less malleable or are brittle. They are easy to be damaged or broken during bending.

Metal materials like mild steel, annealed alloy steel, 5052 aluminum, copper, etc are malleable and easy to bend.

While some materials, such as brass, 6061 aluminum, and bronze are miserable to bend and are more likely to crack.

Bending Springback

In the bending process, the sheet metal inner surface will be compressed and the outer surface will be stretched.

Because the metal plate has a good bendability, the compressed surface will produce a certain springback after the load is relieved.

As a result, over-bend is necessary when bending.

The bending radius affects the springback of the plate. The larger the bending radius, the greater the springback.

Using a sharp punch can reduce springback. Because the sharp punch has a small inner radius.

Bend Allowance

Bend allowance is the length of the neutral axis between the bend lines.

Once the bending allowance is calculated, it is added to the total flat length to obtain the material length required to produce the desired workpiece.

In order to reduce bending errors, bend allowance needs to be considered.

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