When working with press brakes, working tonnage is of keen importance. It is the force required to exert pressure on the metal sheet for bending operations to get the desired shapes and angles.
With different types of metals, thicknesses, bending lengths, and other factors that affect tonnage, the value can vary. So, after a thorough evaluation and detailed calculation with a formula, you can get the right value for specific requirements.
Calculating the correct tonnage is vital. It ensures that the press brake operates safely and efficiently, producing accurate bending results. But when you neglect certain factors, you get substantial errors that may even damage the tooling and the machine.
So, how do you calculate it? What is the formula, and what factors impact the tonnage calculation? In this article, we will cover all this information in detail. This knowledge will help machine operators and engineers to achieve the right tonnages for various small and large bending tasks.
Main Factors Affecting Tonnage
Material thickness
Increased thickness implies increased resistance to bending. This means that more bending force will be required for thicker metals.
For example, there are two metal sheets of the same material with varying thicknesses. One with a 1/2-inch thickness and the other with a 1/8-inch thickness. Here, the 1/2-inch sheet with higher thickness will require a higher bending force than the 1/8-inch sheet with lower thickness.
Bend length
Then we have bend length. When the press brake exerts pressure on the sheet metal, the force is divided across the entire area. So, there is an increase in length, and you will need more tonnage.
Take an example of two metal sheets with lengths of 1 meter and 2 meters, respectively. The 2-meter sheet will need higher tonnage, while the 1-meter sheet will require low tonnage.
Lower die V-opening width
A lower die opening accommodates material when punched; it must be specified, considering the material thickness. If it isn’t, the bending accuracy is compromised. The press brake will need more force to exert pressure. With a broader die opening, the material will be accommodated easily. So, it is recommended that the size of the v-opening be selected up to 9-10 times the material thickness.
Material type and tensile strength (σb)
The tensile strength of different material types varies. This means tonnage for different materials will also be different. It typically refers to the pulling or stretching stress a material can withstand before deforming. Make sure you have the correct value for the material you are working with.
Bending method
The choice of bending method not only impacts the tonnage requirement but also impacts the final bending results. So, there are three most commonly used methods:
- Air bending: lower tonnage requirement
- Bottom bending: higher tonnage, consistent bending results.
- Coining: highest tonnage, accurate bends
Inner bending radius
The inner bending radius affects how sharply the sheet is forced to bend. With a tighter radius, more tonnage is required. But it can lead to a cracked surface of the material. In contrast, a larger radius needs a lower force.
Other factors
Die condition: If the die is worn out or damaged, it can lead to inaccurate bending angles.
Eccentric loading: When the bending concentrates the load on one side of the ram/bed, it can impact the tonnage calculation.
Machine ageing: With an ageing press brake machine, various parameters can differ, causing it to deliver or read force differently than expected.
Detailed Explanation of Tonnage Calculation Formulas
Common metric formula
This formula is used to calculate the tonnage:
P= 650 x S2 x L/V
where,
P stands for bending force (kN)
S represents the thickness of the material
L stands for bending length
V stands for V-die opening
Brief derivation of the formulas
When assuming the tensile strength of the material is 60,000 PSI, which is usually mild steel. So, adjust the calculation based on the material factor for different materials.
So, the formula can be adjusted as follows:
P = 650 x S2 x L (Tensile strength/60,000)/V
where,
Material factor = (Tensile strength/60,000)
It refers to the tensile stress compared to that of mild steel. For example, table below represents the material factor of two different metals:
| Material | Tensile strength | Material factor |
| Stainless steel | ~84,000 PSI | 1.4 (84,000 ÷ 60,000) |
| Aluminum | 3400 PSI | 0.54 (3400 ÷ 60,000) |
Method factor: Each bending method possesses a distinct factor
| Bending Method | Factor |
| Air bending | 1.0 |
| Bottom bending | 5.0+ |
| Coining | 10.0+ |
Multiple-bend tooling factor: This is the additional tonnage when using multiple tools for the bending operations.
So, with the material factor, method factor, and multiple-bend tooling factor incorporated in your calculations, you can set your machine parameter accordingly.
Comparison with imperial formulas
The standard imperial forming tonnage formula is as follows:
P= [1.42 x S^2 x Rm x L]/V
Where:
1.42 is a constant, specifically for air bending, derived from mechanical derivations and unit conversions.
P stands for bending force (in tons)
S stands for the thickness of the material (in inches)
S^2 stands for thickness squared
Rm represents the tensile strength of the material (in PSI ÷ 1000)
L stands for bend length (in inches)
V stands for V-die opening (in inches)
Explanation of impacting variables:
- S^2, which represents the material thickness, acts as a critical factor. Even with a slight miscalculation, the results can vary substantially.
- Rm, representing the tensile strength, directly affects the force needed for bending. If one material possesses twice the tensile strength (for example, some stainless steels compared with mild steel), it will generally need around twice the press force- provided the other variables remain the same.
- L, the bend length, is directly proportional to tonnage. Doubling the length means doubling the tonnage.
- V, representing the V-die opening width, is inverse to the tonnage. So, with a wider opening, a lower tonnage force will be required, and vice versa.
Calculation example
Using the above formula, here is an example of tonnage calculation:
Example of Bending mild steel (using air bending)
P = [1.42 x S^2 x Rm x L]/V
Thickness (S) = 0.125 inches = S^2 = 0.015625 inches
Bend length (L)= 4 inches
V-die opening width (V) = 0.500 inches
Rm = 60,000 ÷ 1000= 60
Now, putting the values in the imperial formula:
P = [1.42 x 0.015625 x 60 x 4]/ 0.500
P = 10.65 US tons
Safety margin recommendations
Although the formula for tonnage calculation is useful, when you apply it in practice, you may encounter certain variations. So, some adjustments must be made accordingly.
- When working with a different press brake model, always check the manufacturer’s guidelines.
- Some material deformation may occur during bend operations. So, a safety margin must be incorporated to ensure accurate calculations.
- Press brake maintenance and calibration also serve an important task. It will enhance equipment longevity and result in accurate readings.
Common errors and precautions
Common errors include:
- Ignoring material tensile-strength differences: It matters most. So, slightly inaccurate tensile strength can lead to incorrect calculation. So, use the correct tensile-strength values and grade data. When the batch is unknown or variable, apply a conservative value and add a safety margin.
- Choosing an incorrect V-die opening width: It can lead to uneven load distribution. So, use V-die opening width values according to the material thickness. It should be approximately 8-10 times the material thickness.
- Ignoring method factors: Each of the three methods has a distinct method factor. When you don’t apply it in your calculations, it overloads a press brake. So, apply the correct value.
Conclusion
So, now you have all the information you need to calculate the correct tonnage for the press. However, tonnage calculation is a daunting task that requires a keen understanding. Well, the importance of the correct tonnage calculation can not be denied, as a slight miscalculation can lead to substantial errors. These include inaccurate bending angles, incomplete operation, and damage to the machine and tooling. So, it’s crucial to understand all aspects of the calculation and the critical factors that influence it. With this information and the high-quality press brake, you can meet your goals with confidence!
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MIHARMLE, a leading press brake manufacturer in China, offers a range of powerful press brakes for reliable, efficient operation. These press brakes combine the latest technology to meet diverse production needs and complexities. With premium quality, you also get a worldwide delivery option and strong technical after-sales support. So, whether you are dealing with small bending tasks or large heavy-duty tasks, the powerful machine with robust performance can do it all for you. Contact us now to get more details and elevate your metal fabrication business!