The tonnage required from a press brake depends on the material being bent, material thickness, bend length, tooling and the bending method being used. Material thickness alone is not enough to specify the capacity of the machine.
For air bending, the die opening has a significant effect on the bending force required. Bend length then determines the total force needed across the component. Material strength also needs to be taken into account when working with materials other than the material used as the basis for the bending data.
The press brake therefore needs to be specified around the range of work being produced rather than a single regular component.
What Determines Press Brake Tonnage?
The main factors affecting the bending force required are:
- Material type and tensile strength
- Material thickness
- Bend length
- Die opening
- Punch and die configuration
- Bending method
Changing any of these can alter the force required for the bend. This is why two components made from the same thickness of material do not necessarily require the same press brake tonnage.
Material Thickness
As material thickness increases, the force required to form it also increases. The relationship is not simply proportional, so doubling the material thickness does not mean that the required bending force merely doubles.
This becomes particularly important when specifying a machine that will process a wide range of sheet and plate thicknesses. The heaviest regular work needs to be considered, together with any less frequent jobs that the machine will still be expected to produce.
Material Type
The type and grade of material affect the force required to bend it.
Mild steel, stainless steel and aluminium do not have the same mechanical properties, and different grades within those material groups can also vary. Bending data should therefore be appropriate to the material being processed rather than applying the same figure to every material of the same thickness.
Where several materials are regularly processed, each needs to be considered when establishing the required machine capacity.
Bend Length
Bending force is normally considered in relation to a specified bend length. As the length of the bend increases, the total force required from the press brake increases.
A short component may therefore be comfortably within the capacity of a machine while a full-length bend in the same material and thickness requires considerably greater total force.
This is one reason why both tonnage and working length need to be considered when choosing a press brake. A machine having sufficient bed length does not automatically mean that it has sufficient capacity to produce a full-length bend in the required material.
Die Opening & Press Brake Tonnage
The V-die opening is an important part of calculating the force required for air bending.
For the same material thickness and bend length, changing the V-opening changes the required bending force. A smaller opening generally requires greater force, while increasing the opening reduces the force required.
The die opening cannot be selected purely to reduce tonnage, however. Material thickness, required inside radius, flange dimensions and the component itself all affect tooling selection.
This is why press brake tooling and machine tonnage need to be considered together rather than as separate specifications.
Bending Method
The bending method also affects the force required.
Air bending, bottom bending and coining do not have the same tonnage requirements. A tonnage figure based on one method should therefore not automatically be applied to another.
The tooling and bending method being used need to be established before the required force can be calculated correctly.
Tonnage Per Metre & Total Bending Force
Bending force may be expressed for a given length of material, but the press brake itself is rated by its total available force.
If bending data gives the required force per metre, this needs to be applied to the actual bend length to establish the total force required for the component.
This distinction is particularly important when comparing short components with long bends. The material and tooling may be identical, but increasing the bend length increases the total load placed on the machine.
Do I Need Additional Press Brake Capacity?
A press brake should not routinely be operated beyond its rated capacity, so the machine needs sufficient capacity for the work it will actually encounter.
At the same time, simply specifying substantially more tonnage than the work requires is not a substitute for calculating the requirement properly. The objective is to establish a machine specification that covers the material range, bend lengths and tooling that will be used.
If occasional work is considerably heavier than normal production, that should be identified when the machine is being specified rather than discovered after installation.
Full-Length & Off-Centre Bending
The total tonnage rating is not the only load limit that needs to be considered.
Press brakes have limits governing how loads can be applied across the machine. Concentrating high bending forces over a short section of the bed or carrying out off-centre work can impose different loads on the frame, ram and tooling compared with a force distributed across a longer bend.
The manufacturer’s loading limits should therefore be checked for concentrated or off-centre bending rather than assuming that the machine’s full rated tonnage can be applied anywhere along the bed.
Replacing an Existing Press Brake
If an existing press brake is being replaced, its tonnage provides useful information but should not automatically determine the capacity of the new machine.
The work may have changed considerably since the existing machine was purchased. Material thicknesses may have increased, different grades may now be used or longer components may have been introduced.
Conversely, a machine originally purchased for work that is no longer produced may have considerably more capacity than is now required.
The current component range should therefore be reviewed before specifying the replacement. Our guide to choosing a press brake covers the other factors that should be considered alongside tonnage.
Does Press Brake Type Affect Available Tonnage?
The drive system does not change the bending force required by the component, but it can affect the range of machine capacities available.
Hydraulic press brakes are available across a broad range of tonnages, including high-capacity machines for heavy plate work. Servo-electric machines are generally available within a more defined capacity range.
If the calculated requirement falls within the available capacity of both technologies, other factors can then be considered. See our hydraulic vs servo-electric press brake guide for a more detailed comparison.
Choosing the Correct Press Brake Capacity
The required tonnage should ultimately be calculated from the actual work being produced. Material type and grade, thickness, maximum bend length, tooling and bending method all need to be known before the required force can be established properly.
Where a machine will produce a wide variety of work, the calculation needs to cover the complete production range rather than one typical component.
Once the required capacity has been established, working length, stroke, daylight, throat depth, CNC control, back gauge and tooling can be considered as part of the complete machine specification.
You can view our range of CNC press brakes covering different tonnages, working lengths and machine configurations.
Frequently Asked Questions
Can press brake tonnage be calculated from material thickness alone?
No. Material thickness is only one factor. Material type and strength, bend length, die opening, tooling and bending method all affect the force required.
Does a longer bend require more press brake tonnage?
Yes. With the other bending conditions unchanged, increasing the bend length increases the total force required. This is why machine tonnage and working length need to be considered together.
Does the V-die opening affect the tonnage required?
Yes. For air bending, the V-opening has a direct effect on the required bending force. A smaller V-opening generally requires greater force, although tooling must be selected around the material and component requirements rather than tonnage alone.
Does stainless steel require the same bending force as mild steel?
Not necessarily. Required force is affected by the mechanical properties of the material, so bending data for one material should not automatically be applied to another. The actual material grade should be considered when calculating the requirement.
Can I use the full tonnage of a press brake on a short bend?
Not automatically. Press brakes have limits for concentrated loads and how force can be applied across the bed. The manufacturer’s permissible loading figures should be checked before applying high tonnage over a short section of the machine.
Should I buy the same tonnage when replacing an old press brake?
Not necessarily. The existing machine provides a useful reference, but the replacement should be specified around the work currently being produced. Changes in material, thickness, bend length, tooling or component range may alter the capacity required.
