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V-opening size

When air bending on a press brake, the size of the V-opening in the die basically determines the product radius formed in the bend. But which size should you choose and what consequences does this have for bending force, flat pattern, minimum leg length and markings on the product?

Natural product radius

When air bending sheet metal on a press brake, the formed product radius is not equal to the tip radius of the tool. The product radius is formed naturally and is called the natural product radius. How large the radius becomes depends, among other things, on the V-opening of the die.

Choosing the V-opening

WILA measures the V-opening as the distance between the virtual intersections of the bearing faces with the faces of the groove. Note that not every manufacturer measures this the same way.

A general rule of thumb used for air bending is that the V-opening in the case of steel (450kN/mm2) should be about 6-8x the sheet thickness for sheet thicknesses up to about 3mm. A 90° angle creates a natural internal product radius that approximates the sheet thickness.

If a smaller product radius is required, a smaller V-opening can be selected. A larger V-opening creates a larger product radius.

Example: Natural internal product radius by material and angle (source: VM113)

Example: Natural internal product radius by material and angle (source: VM113)

But what about other materials and angles? The formed internal radius is a percentage of the V-opening as shown in the practical examples in the table.

The minimum radius that can be formed during air bending depends on material properties such as tensile strength, ductility and rolling direction. If the radius becomes too small, the material will crack.

High-strength steels

Materials with a much higher tensile strength than steel (450kN/mm2) require larger V-openings. This could easily be more than 12-15x the sheet thickness. More on [link]bending high-strength steel[/link].

When choosing the V-opening, note that this also affects the optimal tip radius of the punch.

V-opening too small

If the V-opening is relatively small compared to the sheet thickness (steel: V<6xS), the bending process becomes less precise. Due to lack of space, the radius cannot be formed properly.

Also, the required bending force increases considerably. Pay attention to the maximum bending force the press brake can deliver but also the maximum load capacity of the tool.

CNC-Deephardened dies will last many years under normal use. If a V-opening smaller than 6x the sheet thickness is chosen, the tool will suffer excessive wear and the flanks of the die will be subjected to greater stress. When bending sharp corners, the tool may even split. This is therefore strongly discouraged.

Less marking

With a larger V-opening to sheet thickness ratio, the bending force required will actually decrease significantly. A larger radius is created and the markings on the sheet due to friction will be less. If the maximum bending force of the press brake is limited or you want to bend damage-free, this is an interesting consideration.

Minimum leg length

Another aspect is the minimum leg length. This is because the V-opening determines the minimum leg length you can bend in a die. The leg length must be a bit more than half the V-opening so that the product does not slide into the V-opening. In the WILA Smart Tooling App check the minimum leg length you can bend with a WILA tool at a given angle. More information on bending short flanges can be found here.

Flat pattern

Finally, a general warning is in order here. A press brake operation is only one of the machining steps of the product. If the V-opening is varied, the product radius changes. This also affects the flat pattern of the sheet and could be an expensive change!


The general rule of thumb for air bending steel is to select a V-opening of 6-8 times the sheet thickness. By deviating from this you can influence the product radius, the required bending force, degree of marking and minimum leg length. But be aware that a modified V-opening will also affect the flat pattern of the sheet.

Use the WILA Tool Advisor to analyze a critical bend or to adjust the V-opening of a standard tool yourself.

For detailed advice and accurate flat pattern calculations, use bending simulation software. It can often even calculate with values that you have entered of the material batch to be used.

Do you need help choosing the right V-opening? Our experts will be happy to help!

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