The press brakes process the sheet metal by applying force to the punches and dies.
Press brake tonnage stands for the maximum bending force or bending capacity of the press brake during bending materials.
When selecting a press brake, the most important thing is to determine the size of the press brake according to the bending force and total bending length.
The bending force determines the working capacity of the press brake.
In addition, the actual working tonnage cannot exceed the working capacity of the press brake and the maximum bearing capacity of the die.
The actual tonnage is affected by many factors, such as bending radius, bending method, materials thickness, material tensile strength, die ratio, friction between material and die, the rolling direction of the steel, work hardening, and so on.
Generally speaking, the greater the thickness and hardness of the sheet metal to be bent, the higher the working tonnage, the thinner the sheet metal, and the lower the tonnage.
Factors that Influence Press Brake Tonnage
Using different metal bending methods will lead to different tonnage.
For example, in air bending, the tonnage can be increased or decreased according to the change of die opening width. The bending radius affects the opening width of the die.
In this case, we need to add a method factor to the formula.
When using the bottom bending and coining method, the tonnage required is larger than that of air bending.
If the tonnage of bottom bending is calculated, the tonnage per inch of air bending needs to be multiplied by at least 5 times; If coining is used, a larger tonnage than the bottom bending may be required.
We have learned that in air bending, the larger the die opening is, the smaller the tonnage is.
The smaller the opening is, the larger the tonnage is.
Because the width of the die opening determines the internal bending radius.
The smaller the die radius, the larger the tonnage required.
In air bending, the die ratio is 8:1, that is, the opening distance of the die is 8 times the material thickness.
At that time, the material thickness is equal to the internal bending radius.
Friction and Speed
In air bending, the punch needs to pass over the opening of the bottom die to bend the sheet metal.
If the surface of the metal plate is not lubricated, the friction between the die and the sheet metal will increase.
The tonnage for bending the sheet metal will be greater. What’s more, this will reduce the springback of the material.
On the contrary, if the sheet metal surface is smooth and lubricated, the friction between the die and the sheet metal will be reduced.
The tonnage for bending the sheet metal will be lower.
However, it will increase the springback of the sheet metal.
The tonnage is also affected by the bending speed. As the bending speed increases, the tonnage required will be lesser.
Increasing the speed can also reduce the friction between the die and the sheet plate.
However, this will increase the springback of the sheet metal.
Material Thickness, Tensile strength, and Bending Length
Tonnage is the force applied by the press brake to the metal plate.
Therefore, the thickness and tensile strength of sheet metal determine the range of bending force.
Each material has different tensile strength.
Tensile strength refers to the maximum stress that a tensile material can bear under a steady load.
If this stress is applied and maintained, the material will be broken.
Typical tensile strengths of some materials
The greater the thickness of the material, the greater the tonnage required, and vice versa.
The bending length of the press brake workbench is the maximum length that the metal plate can be bent.
The bending length of the press brake should be slightly longer than the material to be bent.
If the length of the workbench is incorrect, the die or other components may be damaged.
Press brake punch is also a factor to be considered.
These punches also have load limits. Right angle Vee punch can bear a large tonnage load.
Due to the small angle and fewer materials used in the manufacture of acute angle dies, such as gooseneck die, they are not inclined to withstand heavy loads.
When using different dies, their maximum bending force must not be exceeded.
How To Determine Press Brake Tonnage？
Press brake tonnage can be determined by a press brake tonnage chart.
Press brake tonnage chart
- The unit of the press brake tonnage in the table is KN
- The above data is based on bending 1 meter metal sheet with tensile strength 450N/mm²
The tonnage in this table is based on the material tensile strength of σb＝450N/mm2.
The value in the table is the bending force when the length of the metal plate is 1m.
- P=Bending force
- S=Thickness of metal plate
- V=V opening of the bottom die
- B=Min bending flange R=Inside radius
For example, if S=5mm, V=40(the width of the V die opening is 8-10 times the thickness of the sheet), we can see the value in the chart is 400.
From the chart, we can find that bending a 5mm thick and 1m long sheet metal, the tonnage of press brake is 400KN.
Press brake tonnage also can be calculated by the tonnage calculation formula:
- P: Bending Force(Kn)
- S: Thickness of the Plate (mm)
- L: Width of the Plate (mm)
- V: V-Opening of the Bottom Die(mm)
The calculation result of this formula is approximately equal to the value in the tonnage chart.
When the bending materials are different, use the coefficients in the following table to multiply the calculated results.
You can use the tonnage calculator below to obtain the results directly.
No matter what method you choose to determine the tonnage, do not exceed the tonnage range of machines and dies.
Using the wrong tonnage may damage the die or working components.
In the worst case, it may cause deformation of the machine and even affect the safety of bending operators.
Before each bending, it is very important to determine the tonnage and load limit of the press brake.
The press brake under excessive load will lead to inaccurate bending, damage to the machine and even endanger the safety of the operator.
For example, as a result of bending and deformation of the ram and worktable, the workpiece may spring up.
Therefore, before bending, you need to check or calculate tonnage and load limits.
You can refer to the manufacturer's information or use the tonnage calculator here to calculate.