Press Brake Tonnage: Definition, Factors, & How to Calculate It

Powerful metal forming machines can achieve some incredible results, but there’s a point where too much power can be a drawback. 

Tonnage is a critical part of the press brake’s specs. Manufacturers must determine the exact amount needed to meet production requirements. Selecting the wrong tonnage can lead to long-term quality issues, including low tool lifespan and cracking or deformation of parts. 

Let’s define press brake tonnage, what determines the required tonnage, and how to calculate it.

What Is Press Brake Tonnage?

Press brake tonnage, measured in tons, is the amount of force a machine applies to form or bend metal. The forming operation needed for the project helps determine tonnage. Each method requires a different amount. 

• Air Bending: The process of using force to bend metal into shape. The tonnage needed depends on the tensile strength and thickness of the material, the size of the tooling, and the length of the part.
  
  
• Bottom Bending and Coining: Demanding 3 to 5 times the tonnage in comparison to air bending, the tool is pressed firmly into the metal (bottom bending), or the metal is pressed into the die’s shape (coining), essentially acting as a stamp. 

In press brakes, “machine capacity” refers to its rated tonnage and bed length. 

The rated tonnage is the machine’s maximum force output. Bed length, also referred to as working length, is the entire distance of the machine bed and ram. More tonnage is necessary for longer bends.

Why Press Brake Tonnage Is Critical

Using the correct tonnage ensures the press brake runs within its limits and does its job correctly without damaging the material or causing mechanical failure. Tonnage is critical for these key reasons: 

• Tool life: Overages on tonnage can lead to damaged dies and punches, and even shortened lifespan of the tooling.
  
  
• Part quality: Improper tonnage will lead to improper production. With under-tonnage, there may be incomplete bends and inconsistent angles. Over-tonnage causes material to thin, crack, and become distorted.
  
  
• Machine health: Using incorrect tonnage depletes the lifespan of the press brake through frame deflection and ram misalignment. Running with proper tonnage adds overall value to your production, helping to avoid unplanned downtime and keeping material waste to a minimum.
  
  
• Safety: Running a press brake operation outside of the tonnage requirements risks damage to the machine and tooling, and puts the operator at risk.
  
  

Key Factors That Affect Required Press Brake Tonnage

Besides the operation itself, there are several key factors that impact the force needed for accurate bending. These include:

• Material type
• Material thickness
• Bend length
• Die opening (v-die width)
• Bend radius and angle
• Tooling condition and type 
  
  

Material Type

The material type directly impacts required tonnage. Different materials have varying levels of hardness and strength.

Softer materials like aluminum require less force than a stronger material, such as stainless steel. Materials with a higher tensile strength, like titanium, require a lot of force to bend compared to materials with a lower tensile strength, like aluminum. 

For example, soft aluminum typically requires 50% of the force of mild steel using the same size tooling. Stainless steel typically requires 50% more force than mild steel using the same size tooling.

Material Thickness

A primary driver for tonnage is the thickness of your working piece. Thicker materials require much more tonnage to bend. While a thinner material is easier to bend with less force, it’s more prone to deformation during the pressing process. 

Small shop machines are best for thinner materials, while industrial shop press brakes can exert much more force for thicker bends. Required tonnage depends on the length of the bend. Gauge thickness materials ranging from 20 ga to 5/16" would require 2.2 to 19.3 tons per linear foot, roughly 30-230 tons for 12'.

Bend Length

Tonnage is usually calculated per foot or per meter. The longer the piece of material, the greater the force that is needed to bend it. Pressure has to be distributed over a greater length, which increases the required tonnage.

Die Opening (V-Die Width)

Die openings in press brakes are the shaped pieces, typically V-shaped, of the tooling that the material is pressed into for its bend. Wider dies require less force to bend the material.

Bend Radius and Angle

A smaller, sharper bend radius requires significantly higher tonnage, especially with harder materials, such as titanium or stainless steel. 

A task requiring a sharper angle will need more tonnage compared to a wider, softer bend.  The tighter the angle, the more force the press brake has to generate to bend the material.

Tooling Condition and Type

A machine with worn tooling can impact tonnage requirements. If the parts of your press brake are worn, chipped, or misaligned, more force is required to complete the job at hand. This can also lead to damaging the material and poor overall results. 

How to Calculate Press Brake Tonnage: Step-by-Step Guide

Now that you know all the factors that contribute to determining the required tonnage, you’re ready to calculate it. Follow these steps to make sure you land on the correct spec for your operation. 

To calculate the required press tonnage (F(lbf)), you’ll need to know:

• Length (W) in.
• Thickness (t) in.
• Ultimate tensile strength (S) psi
• V-die opening (D) in. 

Once you determine these values, you can plug them into this standard calculation that’s widely used across the industry. Then cross-reference in the press brake tonnage chart below.

F(lbf)= 1.33SWt²/D

Common Tonnage Mistakes

Mistakes can happen in manufacturing, but when it comes to press brakes, they can be extra costly. As you’re determining tonnage requirements, avoid these common mistakes:

❌Relying on a rule-of-thumb: Don’t simply fall back on what typically works. Always check your individual machine specifications to get a more accurate estimate.

❌Ignoring material strength differences: Don’t treat aluminum the same as stainless steel.

❌Not factoring in bend length: You need to account for more than just the straight length of the metal. The full length of the bend must be accounted for because longer bends require more force.

❌Exceeding local tonnage limits: Don’t put too much force all in one spot of the machine. If all pressure is concentrated on one side or the middle, you can damage the machine and the material or cause an unsafe condition.

❌Skipping safety margins: Running at maximum tonnage doesn’t leave any room for error in your operation. You may be setting yourself up for increased wear or a safety risk. 

To avoid these common errors, use charts as a reference, not a guarantee. While tonnage charts provide good starting points, actual results may vary based on material type, thickness, and machine setup.  

It’s also helpful to document each of your jobs. Keep records of successful bends, as well as failed attempts. Take note of material, tooling, and tonnage used for each job. If it was unsuccessful, make a note as to why it failed. 

Use the correct bending method to avoid unnecessary damage. Make sure your staff is trained properly to select the right method as well as run the machine. They’ll need to know how to perform preventative maintenance on the machine, as well. 

Press Brake Bending Calculator

When determining exact tonnage requirements, keep in mind the key characteristics of your material, such as type and thickness, as well as bend length and die opening. Use formulas, along with a tonnage chart, to ensure you’re estimating the correct amount of force required for the material and job. 

Calculating accurate tonnage protects your tools, improves product quality, and extends your machine’s life, adding overall value to your business. 

Use the Press Brake Bending Calculator to reveal the exact tonnage required to process a 90° air bend.