Company Announcement - One of the unavoidable side effects of press brakes is frame deformation. We're bending steel with other steel, which for physical reasons will have its deflection. In particular, the ram will bow in the center. In that point the punch will be farther away from the die and the resulting angle will be wider. Therefore, the bent profile will have a boat-like shape. We will see how even a small difference can turn metalforming into a process full of pitfalls.

This deformation is compensated with crowning: the table is pushed upwards to keep the die at a constant distance from the ram. There are two types of crowning: the former uses a series of wedges to raise the die mechanically; the latter uses short-stroke hydraulic cylinders embedded into the bench.

But how much crowning is just right? Most manufacturers rely upon tables calculated based on the press brake structure and the sheet metal characteristics as they are declared by the foundry. Everything is fine, apparently. Actually, this method does not work because sheet metal behavior is unpredictable. It has a high variability that depends on a series of factors. Knowing them and, when possible, compensating them is essential in order not to waste time and material in tests and samples. Especially today, when batches are much smaller, getting the right angle at the first try becomes fundamental.

• Not all steels are created equals

What does A36 steel stands for? Sheet metal is labelled according to its yield strength. This value is highly variable and depends on impurities in the casting and imperfections in production techniques. To avoid the risk of building weak structures, regulations require to declare the minimum strength. So any steel with a yield strength higher than 36,000 psi will be labelled as A36.

As a result, 41,000-psi strong steel will still be sold as A36, in spite of being 13% harder. The increased resistance will require a higher force to the press brake, which in turn will cause the ram to bow more. For instance, switching from a 36,000 to a 41,000 psi steel we will get a deformation of roughly 0.002" in depth. Yet, this small curve causes a difference of almost 1° a 0.3" V-die.

V-die opening
Depth for Δ=1°
1/4" (6 mm)
0.016" (0.04 mm)
3/8" (10 mm)
0.024" (0.05 mm)
1/2" (12 mm)
0.027" (0.07 mm)

In other words, a thickness smaller than a sheet of paper (which measures about 0.04") already causes remarkable differences. Bear in mind that narrow openings like these are often used to bend thin plates for very high quality products.

You can't win them hole
Label data of the steel we buy, which may or may not be reliable, become completely worthless when we modify the sheet. Holes change strength along the bending line, both in case of thermal cut and by drilling or milling. And if on the same piece we have some bends on full parts and other on drilled parts, predictions are even more useless. Punched holes, on the other hand, also introduce internal stresses that make even more difficult to consider crowning as a fixed amount of deformation.

• Conditions matter

Steel is a living thing. Rolling directions creates fibers in the structure, so bending across or along the grain will affect both the required bending force and the springback.

A recently pickled sheet is harder than an aged or oxidised one, even after just a few weeks. Moreover, bending a hot plate is not the same thing as bending an ice-cold one. Temperature has an influence on forming conditions and may lead to different results.

Even dimensional figures are extremely variable, especially thickness. For example, EN 10051:1991+A1:1997 regulation divides sheet metal in 5 groups. Let's consider steel roughly between 37,000 and 49,000 psi (260÷340 MPa), up to 0.1" (2 mm), between 4 and 5 feet wide (1201÷1500 mm):

Category
A1
A
B
C
D
Tolerance (in)
± 0.0055"
(0.14 mm)
± 0.0075"
(0.19 mm)
± 0.0087"
(0.22 mm)
± 0.010"
(0.25 mm)
± 0.011"
(0.27 mm)

This means that a sheet with a 0.1" (2 mm) nominal thickness can fall anywhere within a 14% bracket, and within a 31% in the worst-case scenario.

Measure twice, cut once
While shearing alters the grain in sheet metal, plasma, laser, and oxy-fuel cutting all generate localized thermal shocks on sheet edges and hole perimeters that cannot be ignored.

For these reasons, a good press brake cannot and must not rely upon any type of database, and should be based on estimating or forecasting software. For the simple reason that there is no algorithm able to predict sheet metal reaction. To think that steel behaves as a perfect and unchangeable material is a delusion that soon leads to a bad awakening.

If sheet metal forming is not perfect, welding becomes more difficult, painting gets harder and joining becomes a tough job, especially when tolerances are close. Material waste and working hours skyrocket.

These 4 factors can drastically reduce company margins if they are ignored instead than faced. Operators and press brakes must know sheet metal like the back of their hands, they must be able to react to changes and to adapt to it to guarantee the best result.

The only way to manage crowning is to use a system that measures actual deformation and corrects it in real time. Only with such a technology one can be sure that, regardless of the material characteristics, results will be those that the job requires. Any other system will make bending quality worse and will affect finished products.

Gasparini Industries - www.gasparini.it