Nella mia esperienza con press brake bending, selecting the right V-die opening is crucial for achieving precise and consistent bends. The V-die plays a significant role in determining the angle and quality of the bend, and making the right choice can greatly influence the final product. Over time, I have learned how factors such as material thickness, type, and bend radius affect the selection process. In this article, I will share my insights on selecting a V-die opening for press brake bending, offering practical tips and considerations that can help operators optimize their bending operations and improve overall results.

Experience & Steel Specifications:

It is very common that press brake operators have their own “rule of thumb” to choose the V opening for a certain job application. Although these rules come from experience, they do not take into account how commercial sheet metals “behave” in most cases, and can sometimes lead to imperfect final pieces.

Steel is an alloy of iron, carbon, and other minerals mixed together under heat. Because the proportions of each mineral can be different, the mechanical specifications of that steel can change. Therefore no steel is exactly the same and will not “behave” exactly the same. As far as fabricating and bending, commercial steels often have a standard composition, therefore behave alike (not equally, but close).

Before we come to our own “Empiric rule” on how to determine a V-die opening for a certain application, let’s consider what affects this rule and how the V has chosen will affect other parameters of our bending process.

Thickness, Material, & Pressure

Because steel is an alloy, there are some steels that have a higher resistance than others. In other words, some steels are stronger than others, and that needs to be accounted for when bending those steels.

We will not get into how the elements in the alloy determine higher or lower resistance, nor will we illustrate here how that resistance is calculated, but we will say that such resistance is called:  Resistance to traction, and is it expressed as UTS (Ultimate Tensile Strength), a specification we should all be aware of when purchasing from steel suppliers.

What do we do when the UTS is higher?

In most cases, we do nothing at all. Mild Steel has an average UTS of 42 Kg/mm2 while Stainless Steel is about 70Kkg/mm2. But we must remember that the higher the UTS, the higher the pressure (tonnage) required to bend a material.

Remember, pressure for bending on press brakes is expressed and calculated as t/meter or t/ft. Also keep in mind that the thicker the material, the larger the V-die opening needed. This is simply because otherwise, our sheet metal will not fit into a small V opening.

Radius of Profiles

One of the most critical aspects that are affected by the V-Die opening is the radius of the bend.

For some shop operators, it is hard to imagine the radius of the part being determined by the  V opening rather than the punch. To get an idea of this principle, imagine a hanging bridge over a cliff. The farther away both sides of the cliff are, the further the bridge will hang, creating a larger radius.

The larger the V-Die opening, the larger the radius will be on the part. But the UTS will affect the radius, too. Why? Because the stronger material will be like a hanging bridge made of wood in comparison to one made of rope only. We can conclude that the stronger the material, the larger the radius will be when using the same V opening.

From empirical experience, it is determined that the resulting radius R is normally 1/8 of a V opening. This is the case, at least, for thicknesses up to ½” when bending Mild Steel. This determines the formula most operators use as a rule of thumb:

R=V/8

Which can be then adjusted for other material such as:

R x 0.8 for aluminum (lower UTS)

R x 1.4 for stainless steel (higher UTS)

Minimum Leg (or flange)

Last but not least, the easiest aspect to identify when selecting the correct V-Die opening is the leg or flange length that the part requires.

Don’t forget we must have our sheet metal in direct contact with the die shoulders at all times during the bending process. If we fail to do this, the smaller-than-required leg will fall into the V opening and our bending process will be interrupted.

So, the larger the V opening, the larger the minimum leg or flange we must have on a profile.

There is a geometrical formula to determine this minimum length for a leg.

Define b as the internal minimal leg length, V as the opening of the die and consider a 90° bend, we can say that the minimum internal leg is:
minimum internal leg = V x 0.67

this can then be adjusted for different angle bends as follows:

b x 1.6  for bends at 30°

b x 1.1 for bends at 60°

b x 0.9 for bends at 120°

b x 0.7 for bends at 150°

Selecting the correct V-die opening for a bending process

As you can see, the simple selection of a die greatly affects our bending process. It affects the radius, the pressure required, the minimum leg and, why not, the looks of the entire part. Think about how a small V opening will leave more marks on the metal, and how these marks will be seen in many cases.

So, how do we determine the correct V-Die opening? First by understanding that there is not only one correct V-Die opening for a certain material or thickness. There are ideal V-Die opening for different thicknesses but the correct one is only the correct one for OUR bending process.

If empiric experience shows that R (radius on our part) is normally 1/8 of a V opening, then the ideal V opening for a certain thickness is Vx8, since with such a V-Die opening the R (radius on our part) will be equal to the thickness we are bending.

Why does R=Thickness?

Keep in mind that when bending steel, no material is lost, and no material is gained. So when folding sheet metal, if the resulting radius is less than the thickness we are bending, that extra material has to go somewhere. This is one of the most common causes of undesirable deformations. A radius that is smaller than the material’s thickness will cause the material to overboard on the sides of our bend, causing not only an aesthetical issue but a decrease in strength of the part as well.

On the other hand, a radius that is larger than the thickness of the material has no aesthetical issues.

So, avoiding radius deformation and still keeping radius as small as possible is our starting point. From there we can determine:

R= V/8 and R=T (thickness)

then:

V= Tx8

We can say that at least for thickness up to ½” the ideal V opening is 8xT. From there we should check if:

the minimum leg on the ideal V is enough for our project

the pressure required does not exceed the press brake specifications

When T is higher than ½” the resulting radius becomes larger and therefore:

V=Tx10 for  T<1/2”

What next?

If the V=Tx8 is compatible with point a & b (minimum leg and pressure) then we can select that die and start working.

However, if that V-Die opening affects the minimum leg, we will need to decide whether to increase the leg on our part or use a smaller V.

If the resulting V-Die opening requires pressure that is too much for our press brake or our clamping style (yes, this matters), we will need to make another decision because the only opportunity we have is to use a larger V opening in order to decrease pressure required.

At this point, there are a few decisions that need to be made. But it is better to have come to this conclusion before beginning the bending process. Using these principles, your company can produce high-quality parts that your customers will appreciate.