Press Brake Tonnage Calculation
Generally, the press brake tonnage calculation can be according to the following table, and set in the processing parameters.
- The value in the table is the bending pressure when the sheet length is 1m.
For example: S=4mm, L=1000mm, V=32mm, check the table to get P=330KN.
- This table is calculated based on the material strength of σb＝450N/mm2.
When bending other different materials, the bending pressure is the product of the data in the table and the following coefficients:
– Bronze (soft): 0.5
– Stainless steel: 1.5
– Aluminum (soft): 0.5
– Chrome-molybdenum steel: 2.0
- Approximate calculation method of bending pressure:
Press Brake 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)
Actual tonnage is affected by many factors, This formula calculates the press brake force requirement for air bending, it’s better that our machine actual force allow for up to 20% additional capacity.
What affect the machine tonnage?
Material tensile strength
We need to carefully consider the tensile strength of the material and the maximum processing thickness and length. Material is one of the important factors.
Compared with carbon steel, the bending force of material stainless steel is usually increased by about 50%, while the soft aluminum bending force is reduced by about 50%. In the case of the same workpiece, the required tonnage per meter is completely different.
Material thickness and length
Assuming that the material thickness is 1/4 inch, length is 10 feet, after calculating by the press brake tonnage formula, air bending force requires 165 tons, and some forming workpiece require at least bending force 600 tons.
If most of the workpieces are 5 feet or shorter, it can be almost halved, which greatly reduces the cost of purchase.
So the length of the workpiece is very important for determining the specifications of the new machine. In the same way, the thickness of workpiece also plays the significant role in the hydraulic metal press brake tonnage calculation.
Now all different kinds press brake machines are designed to bend to a certain proportion of force capacity.
From the press brake tonnage chart, the force capacity is designed according to the ratio of bottom die and sheet or plate thickness: 8-to-1. Usually we can get a good bending effect based on this ratio.
However, this ratio is not fixed.
– For thin material, we can use the ratio of 6-to-1.
– But when we are forming thick material, we can choose the bottom die V groove and material thickness ratio of 10-to-1 or 12-to-1, which can reduce press brake force capacity, and also can get the perfect bending effect.
Friction and bending speed
The air bending is the movement of the punch displacement, and penetration into the bottom V die.
If the surface of the material is clean and not lubricated, the friction will be greater when bending. If there is oil on the surface of the material or lubricant is used, the friction will be reduced, which will reduce the bending force, but the reduced friction will also increase the amount of material springback.
The time of bending movement also will influence the press brake tonnage. When the bending speed increases, the tonnage decreases.
The friction coefficient changes with the bending speed. Increasing the bending speed can reduce the friction coefficient and bending force, but it will also increase the rebound of the material.
When air bending, the bending radius is 0.156 times the opening distance of the bottom die. During the air bending process, the opending distance of the bottom die should be 8 times the material thickness.
For example, when we choose 12mm opening distance for bend carbon steel 1.5mm, the bending radius of the workpiece is about 1.9mm. If the bending radius almost as small as the material thickness, it is necessary to form the bottom bending, However, the bottom bending required pressure is about 4 times larger then the air bending.
When the bending radius is larger than the material thickness, the air bending method can be used directly, we can select the suitable lowest bending machine tonnage. When the bending radius is large, it often does not affect the workpiece quality and future use.