Bending force is a consideration that comes before the selection of a press brake, lower die, or a bending method. If the capacity is too low, the workpiece will not achieve the desired angle. If the capacity is too high, the punch could be overloaded, as well as the die, the clamping system, the ram, and the frame of the machine.
Knowing how to calculate press brake tonnage will allow the fabricator to utilize the correct capacity of the machine for the application, safeguard the tooling, and achieve consistent performance when bending. One must understand that the bending force calculated is a rough estimate and should be cross-referenced with the machine specifications and the limits of the tooling load, as well as the material data and test bends.
What Is Press Brake Tonnage?
Press brake tonnage is the force applied by the ram during bending. It may be expressed as kilonewtons, metric tons, US tons, or force per meter.
| Parameter | Meaning | Effect on Calculation |
| Machine tonnage | Maximum rated machine force | Defines total press brake capacity |
| Bending length | Length actually being formed | Longer bends require more force |
| Tonnage per meter | Force required per unit length | Helps compare tooling loads |
| Tool load limit | Maximum punch or die load | Prevents tooling damage |
| Off-center load | Force applied away from the machine center | May reduce usable machine capacity |
A machine’s rated tonnage does not necessarily mean that full force can be applied over every tool length or at every position. Short-length, concentrated, and off-center bending must be checked separately.
Standard Press Brake Tonnage Calculation Formula
For a typical air-bending application, one practical metric bending force formula is:
F = (1.33 × L × Rm × T²) ÷ [1,000 × (V − Rp)]
Where:
•F = required bending force in kN
•L = bending length in mm
•Rm = material tensile strength in N/mm²
•T = material thickness in mm
•V = V-die opening in mm
•Rp = punch tip radius in mm
This relationship shows that bending force rises with bending length, tensile strength, and the square of material thickness. A larger die opening generally reduces the required force. TRUMPF’s tooling documentation similarly calculates press force from bending length, tensile strength, sheet thickness, die width, punch radius, and a process factor.
Different press brake tonnage charts may use slightly different constants, material references, or tool assumptions. Always confirm whether the result is in kN, metric tons, US tons, tons per foot, or tons per meter.
Step-by-Step Process to Calculate Press Brake Tonnage
Step 1: Identify the Material
Identify the grade of material and its tensile strength. When calculating tonnage, do not assume that the different material factors for mild steel, stainless steel, aluminum, high-strength steel, copper and brass are all the same.
A material certificate is generally more reliable than using only a broad material name.
Step 2: Measure the Actual Thickness
Use actual thickness measurements rather than solely nominal specifications. This includes:
•Tolerance of sheet thickness
•Coatings or additional surface layers
•Variation from one material batch to the next
•Uneven thickness which may occur in heavier plates
Because thickness is squared in the formula, even small increases can noticeably affect the bending force for sheet metal.
Step 3: Establish the Bending Length
For this step, the length that contacts the tooling (the press brake and the bending mechanism) should be used. The length of the sheet, as well as the length of the working pass, are not critical for this step.
Step 4: Select the V- Die Opening
The selection of the V- Die opening has a significant impact on tonnage, the inside radius, the minimum length of the flange, and surface marking.
The V- Die opening has an inverse relationship with the radius and the length of the flange.
Step 5: Apply the Formula
Make sure to keep the units consistent for all the inputs. A common mistake when load calculating for a press brake is mixing inches and millimeters or kilonewtons and tons (US).
Step 6: Include Capacity Margin
Include a reasonable margin to account for variation in the material, wear of the tooling, thickness, rolling, and other tolerances. This margin should not account for an incorrect opening of the die or for improper machine configuration.
Example of Calculation of Press Brake Tonnage
Let us consider the following example of an air-bending operation:
•Material – Mild steel
•Ultimate tensile strength – 450 N/mm²
•Thickness – 3 mm
•Length – 2,000 mm
•V-die opening – 24 mm
•Punch radius – 2 mm
Using the values given, the calculation will be:
F = (1.33 × 2,000 × 450 × 3²) ÷ [1,000 × (24 − 2)]
F ≈ 490 kN
Therefore, the value of approximate theoretical bending force will be 50 metric tons.
The selected machine should provide sufficient capacity above this result without exceeding the permissible load of the punch, die, clamping system, or frame. For a long production run, operating continuously at the machine’s absolute limit is generally less desirable than maintaining an appropriate capacity margin.
How Material Type Changes Required Tonnage
| Material | Relative Force Trend | Calculation Consideration |
| Mild steel | Reference level | Common basis for tonnage charts |
| Stainless steel | Higher | Usually has greater strength and springback |
| Aluminum | Often lower | Grade and temper create major differences |
| High-strength steel | Significantly higher | Check radius and tooling strength carefully |
| Copper or brass | Alloy-dependent | Use actual tensile-strength data |
Material correction should be based on mechanical properties rather than a fixed multiplier whenever reliable data is available.
How the Bending Method Affects Tonnage
Air bending requires relatively low force because the sheet contacts mainly the punch tip and die shoulders. The final angle is controlled by ram penetration.
Bottom bending creates more tooling-to-material interface. It is typically more forceful and more influenced by the tool angle and the compensation for springback.
Coining significantly deforms the material due to the material being forcefully pressed into the die. It requires several times the bending tonnage of the air bending system, and therefore, the air bending system cannot be used without proper modifications.
Key Factors That Increase Press Brake Tonnage
Required tonnage rises with:
•Greater material thickness
•Longer bending length
•Higher tensile strength
•Smaller V-die opening
•Smaller target inside radius
•Bottom bending or coining
•Multiple bends formed at once
•Hemming and flattening operations
•Special tools or complex profiles
•Concentrated or off-center loading
Tooling suppliers recommend calculating bending force according to material type and thickness and checking the result against both press brake and tooling limits.
Press Brake Tonnage Chart vs. Formula
| Method | Advantage | Limitation |
| Tonnage Chart | Quick for routine jobs | Stiff conditions |
| Calculation Formula | Custom for conditions | Coefficients must be exact |
| CNC calculator | Ready at the shop | Quality depends on input |
| Test Bending | Establishes true Material Characteristics | Requires machine time and setup |
Digital tools such as TRUMPF BendGuide and Wilson Tool App provide recommendations for press force and related calculations for bending. The outputs depend on accurate material and tool inputs.
Common Mistakes in Press Brake Tonnage Calculation
Common calculation errors include utilizing the entire working length of the machine as opposed to the actual length of the bend, treating stainless steel as mild steel, selection of an inappropriate V opening, mixing metric and imperial units, and ignoring the load limit of the tooling.
It should also be avoided assuming that the full rated tonnage will be effective for every short, concentrated, or off-center bend.
Conclusion: Determine Tonnage First
Learning how to calculate Press Brake Tonnage consists of five primary inputs: material strength, thickness, bending length, V-die opening, and bending method. The effect of the thickness is greatest, while smaller die openings and stronger materials require a higher force.
An accurate calculation supports the informed selection and configuration for the machine. APEX CNC develops CNC systems for press brakes with varying capacities and options for working length, backgauges, electro-hydraulic synchronization and CNC crowning. Please provide your material grade, thickness, bending length, and die opening, along with part sketches and production details for us to recommend a bending configuration.
FAQs
Q1. How much safety margin is appropriate for the estimated tonnage?
Variances in materials, tooling, part geometry, and production requirements affect the margin. The machine selected should offer adequate additional capacity, and the margin should not be used to offset the margin of error.
Q2. What details are required for a press brake tonnage calculation?
The grade, tensile strength, thickness, bending length, V-die, punch, and method of bend are required. If special operations are to be performed, for example, hemming or offset bending, additional calculations may be required.
Q3. How does the thickness of the plate affect required press brake tonnage?
Due to the nature of the bending force formula in regard to the square of the plate thickness, the thickness of the plate has a considerable influence on the required press brake capacity.
Q4. Does a smaller V-die require more tonnage?
Generally, yes, although a smaller V-die may facilitate a smaller radius, it requires forming a greater bending force over a smaller distance and thus increases the stress on the tooling and the machine.
Q5. Does bending stainless steel require more tonnage than bending mild steel?
Generally, yes, bending stainless steel requires more tonnage than bending mild steel, due to the greater tensile strength of most grades of stainless steel and their greater springback. It is therefore recommended to use the lower of the two values.