Bending Sheet Metal
Bending Long and Large-Size Sheet Metal
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Bending is a process that permanently deforms the shape of metal. A force is exerted on the material which leads to the bending of this material, known as the bending moment. The metal is stretched on the outside and compressed on the inside. The extent to which a product can be bent, depends on the size of the product and the bend radius. The maximum bend radius is 90°
Bending sheet metal requires enormous force. The maximum pressing capacity of the press brake is distributed across its total length. The thicker the sheet metal, the more force is required. This means that not just any working length of sheet metal can be bent, even on a press brake with a high pressing capacity. The required pressing capacity per meter is plotted against the thickness of the material.
Dies and Punches
The tools used at the press brake are the die and the punch. When the punch is pressed to the bottom of the die, the process is called bottom bending; if it is not, the process is known as air bending. The deeper the punch is pressed into the die, the more acute the angle. The tool and die set can form a variety of different angles and radius.
Crowning is an important aspect of bending on a press brake. During the bending process, the sheet metal is pressed deeper into the die at the sides than in the middle. This means that the bending is not even across the sheet. Crowning can counterbalance this effect by pressing the die in the middle of the sheet upwards so that the bending is even across the sheet. There are various ways of crowning and the technique depends on the type of press brake: Some press brakes press cylinders upwards, others work with edges.
The Back Gauge
The back gauge is an important tool for bending sheet metal. Its main function is to interface with the computer numerical control (CNC) and can be set along the X, Y and Z axes in order to position a piece of metal for forming. A free back gauge can be used to bend complex shapes in large size sheet metal with great accuracy.
Air bending is a bending technique that does not press the sheet metal to the very bottom of the die. The width of the die determines the bend radius. This technique is also known as three-point bending. This is the most commonly used bending technique, but it has one disadvantage. It is not as precise as the rest of the techniques and the material may spring back. However the die may be set deeper to compensate for spring back.
Bottom bending is a bending technique that presses the sheet metal right to the bottom of the die. The shape of the die determines the shape of the sheet. The sheet is pressed completely into the die and the force required for the last stage in particular may be considerable. As consequence, twice as much pressing capacity is required for this bending technique in comparison to air bending. An advantage of this technique is that there is little or no spring back.
Step bending is a bending technique that creates a circular form by performing a large number of bends. The bend radius of each individual bend is small. The exact form of the curve is determined by a number of aspects: the number of bends, the thickness of the sheet and the tolerances. In addition, the price and the appearance of the product naturally plays a role as well. For a perfect circular form, it is better to apply a rolling technique, but this technique allows more limited sheet sizes and thickness.
The width of the die determines to a large extent the distance between the different bends. The minimum distance is also determined by the machine and tooling. The sheet may be pushed aside by the machine or tooling. The type of die selected depends on the thickness of the material and the required bend radius. The following formula can be used to calculate the minimum distance between the bends: v = 8*t, where v is the width of the die and t the wall thickness.
The W-bend is usually used in first when making U-bends to prevent the legs of the sheet from touching the machine or tooling. The W-bend makes it possible to bend products that otherwise could not have been bent.
Creating a Z-section may be problematic because there are two components of the press brake that may interfere with the bending process. These two components are the die and the lower beam. When the legs are short, the sheet may touch the lower beam, which will interfere with the bending process.
Box Forming Die
The box-forming die is used to create a closed contour. Several different profiles can be made using this die, but the blades are the deciding factor. Special extended blades are available for creating longer legs.
Limitations on Bending Sheet Metal
Bending techniques have a number of limitations. One of these is the maximum pressing capacity of the press bake. The larger the radius, the more pressing capacity per meter is required for bending sheet metal. Full capacity may then be reached quickly.
Limitations on Tools
The punches and dies come in many different varieties. However, not all punches and dies can be combined. Sheet metal that requires a small damage zone, for example, must be bent using narrow punches. The force is required for bending with a narrow punch is much higher than that for bending with wider punches.
Limitations as a Result of Cracking
Sheets with a small bend radius could be stretched so much that the metal cracks. The extent to which the metal can be stretched before cracks occur depends on the type of material. Blasted steel, for example, is more susceptible to cracking than unblasted steel.
Limitations as a Result of Holes in the Sheet Metal
Sheet metal with holes in it is more vulnerable during bending. When the hole is too close to the bending edge, the material will bulge out. The metal close to the hole does not bend well enough and therefore protrudes.
Limitations due to the Geometry
The geometry of the product may impose two limitations when bending: the angle and the minimal leg length. the angle concerns the tolerances. In case of thick sheets, it is almost impossible to check the tolerances for all sizes.
Length and thickness of bends for S235 reference material.