1. What Is Press Brake Tonnage?
During the bending process, the force between the upper and lower die is applied to the material, which causes the material to undergo plastic deformation. The tonnage of work refers to the bending pressure or press brake tonnage during the bending process.
The factors affecting the determination of work tonnage include bending radius, bending method, die ratio to bending length, thickness, and strength of bending material.
Related reading: Press Brake Buying Guide
2. Press Brake Tonnage Calculator
You can use the following press brake tonnage calculator to determine the necessary bending force for your sheet metal bending.
The calculator provides both metric and imperial units.
The recommended V-opening width for the bottom die
For instance, if the sheet metal to be bent is mild steel, with a thickness of 4mm and a bending length of 3.2m, the theoretical width of the bottom die opening should be 8 times the thickness, which is 32mm. Input these figures into the calculator above (remember the units are in mm), and we get a value of 106.12 Ton.
This means you’ll need a minimum bending force of 106 tons to meet your bending needs. Of course, we generally multiply the final result by a safety factor of 1.1, and the resulting value is the press brake tonnage you can choose.
New Tonnage Calculation Method
If the width-to-thickness ratio (V/S) is not equal to 9, and the radius-to-width ratio is not equal to 0.16, the above calculator will not be valid.
Please review the updated method for calculating the bending force on a press brake machine.
Use the following bending force calculator instead.
- Download Excel: Press Brake Tonnage Calculator Excel Version
3. Press Brake Tonnage Chart
The press brake tonnage chart below can assist you in determining the necessary bending force with ease.
For instructions on how to read a press brake tonnag chart, please refer to this post.
- Air Bending Force Chart: The Most Authoritative Data From Amada
- Download Press Brake Tonnage Chart (in PDF)
(1) Air Bending Force Chart
Data of V, R, B
Below press brake tonnage chart is from Amada:
(2) Hemming & Seaming Tonnage Chart For Mild Steel & Stainless Steel
Hemming is a type of bending that requires a higher amount of tonnage compared to standard air bending.
The following tables illustrate the tonnage needed for hemming and seaming operations.
(1) Hemming & Seaming Tonnage Chart For Mild Steel
Note: Required tonnage is given per 1-meter length
(2) Hemming & Seaming Tonnage Chart For Stainless Steel
Note: Required tonnage is given per 1-meter length
4. Press Brake Tonnage Calculation Formula
There are currently two main formulas for calculating the tonnage of the press brake that are popular.
The first formula is commonly used in China and the second one in other countries.
However, regardless of which formula is used, the calculated required press brake pressure is basically the same. Let me introduce these two formulas separately below.
#1 Tonnage Calculation Formula
- P – Bending pressure, kN
- S – Sheet metal thickness, mm
- L – Sheet metal length, mm
- V – Width of lower die opening, mm
#2 Tonnage Calculation Formula
- P – Bending force (KN)
- S – Plate thickness (mm)
- L – Plate width (m)
- V – Bottom die slot width (mm)
- σb – Tensile strength (Mpa)
If you would like to gain a deeper understanding of how the tonnage of a press brake is calculated, you can refer to an article on the subject.
Related reading: How To Calculate Press Brake Tonnage?
The above article goes into detail on how to determine the required press brake tonnage using three different methods.
Actually, there is a third formula for calculating the tonnage of a press brake, which we have developed into a calculator. You will read about this knowledge later in the content.
5. Press Brake Bending Radius
During sheet metal bending, a bending radius is required at the bending point, which should not be too large or too small, but should be selected appropriately. If the bending radius is too small, it is easy to cause cracking at the bend point, while if the bending radius is too large, the bending may rebound.
(1) Bending radius chart
The optimal bending radius (inner bending radius) for various materials of different thicknesses is shown in the table below.
Minimum bending radius value (mm)
|Material Science||Annealing state||Cold work hardening state|
|Corresponding position between bending curve direction and fiber direction|
|Semi hard brass||0.1t||0.35t||0.5t||1.2t|
The data in the table above are optimal and for reference only. In fact, the rounding of the bending blade of the manufacturer is usually 0.3, with a few bending blades having a rounding of 0.5.
For ordinary low carbon steel plates, rustproof aluminum plates, brass plates, copper plates, etc., an inner rounding of 0.2 is generally sufficient. However, for some high carbon steel, hard aluminum, and super hard aluminum, this type of bending rounding can lead to bending fractures or cracking of the outer rounding.
(2) Bending radius calculation formula
Sheet metal bending parts require a bending radius r at the bend. Typically, the blueprints for sheet metal parts have clear markings for the bending radius. The final size after bending is determined by the punch radius r0 and the amount of springback △r, i.e.,
r = r0 + △r.
In actual production, the punch radius r0 used is predominantly between 0.3 and 0.5mm, which can be considered a constant and has a minor impact on the bending radius, thus it can often be disregarded. This means that the bending radius r is closely related to the springback △r.
However, the magnitude of the springback is related to the bending pressure, which in turn is determined by the die slot width B and the sheet thickness t. An increase in the die slot width B reduces bending pressure and increases springback, while a decrease in B increases bending pressure and reduces springback.
Therefore, under certain bending machine conditions, the factors most influencing the bending radius are the punch radius r, die slot width B, and sheet thickness t.
The following formula can be used to calculate the press brake bend radius:
6. Min Internal Edge Size
The minimum internal edge is the shortest side that can be bent without the sheet metal slipping into the vee during bending.
In fact, the sheet metal must lie on both sides of the vee while reaching the angle required, otherwise it will slip into the vee with subsequent unsatisfactory results.
The minimum internal edge can be calculated with the following formula:
If the angle required is 90°, minimum internal edge = V x 0.67
This formula derives from a geometric calculation, since the minimum internal edge is the diagonal of a square with side=V/2. Then by taking the radius into consideration, the result is approximated to V x 0.67.
Where the angle required is other than 90°, the minimum internal edge will also be different, as the shortest side that can lie on the vee depends on the angle.
In fact, if a profile has an acute angle, the sheet metal will be pushed further into the die vee and therefore, the side has to be longer.
On the other hand, if a profile has an obtuse angle, it requires a shorter side to lie on a die. For this reason, correction factors must be used to calculate the proper minimum internal edge.
|30°||B = (V x 0.67) x 1.6|
|60°||B = (V x 0.67) x 1.1|
|90°||B = (V x0.67) x 1.0|
|120°||B = (V x 0.67) x 0.9|
|150°||B = (Vx 0.67) x 0.7|
(1) Minimum bending edge calculation formula
The calculation formula for the minimum bending edge is different for different bending angles, which can be found in the table below.
(2) Minimum bending height reference table
Reference table for inner bending radius R and minimum bending height of cold-rolled thin steel plate materials:
|Serial number||Material thickness||Width of concave groove||Punch R||Minimum bending height|
|5||1.2||8 (or 6)||1/0.2||5.5/4.5|
|6||1.5||10 (or 8)||1/0.2||6.8/5.8|
The minimum bending dimension L for Z-bending of sheet metal with different thicknesses is shown in the table below:
Minimum height of z-bend:
|Serial number||Material thickness||Width of concave groove||Punch R||Z-bend height L|
7. Bending Rebound
Bending rebound angle:
Δα = b – a
b – Actual angle of the workpiece after the rebound
a – Angle of the die
Size of rebound angle:
The rebound angles for 90° single angle air bending are shown in the table below.
|Low carbon steel||<1||4°||2°||0°|
|Medium carbon steel, σb=400-500MPa||<1||5°||2°||0°|
|Hard brass, σb=350-400MPa||1~5||6°||3°||1°|
|Hard copper, σb=350-400MPa||>5||8°||5°||3°|
|High carbon steel, σb＞550Mpa||<1||7°||4°||2°|
Factors affecting rebound and measures to reduce rebound:
- Material strength: The rebound angle is proportional to the yield point of the material and inversely proportional to its elastic modulus E. For sheet metal parts with high precision requirements, low carbon steel should be selected as much as possible to reduce rebound, and high carbon steel, stainless steel, etc. should be avoided.
- Relative bending radius r/t: The larger the relative bending radius r/t, the smaller the deformation and the greater the rebound angle Δα. This is a very important concept. The bending radius of sheet metal should be as small as possible, considering the material performance, which is conducive to improving accuracy. It should be noted to avoid designing large arcs, such as the example shown below, which can cause difficulty in production and quality control.