If the press brake operators bend the material grain in a small radius, i.e. the fold line is parallel to the material grain, they should be aware of the cracks. Getty Images
Question: In one of your previous articles it was said that cracks are formed “following” the direction of the fibers. The wording may confuse me. Does this mean that the fibers are perpendicular or parallel to the fold line?
I’m working on this thread because we’re bending 0.060″ thick 3003 H14 aluminum (see fig. 1) and my toolmaker wants me to design the bend parallel to the grain because it’s easier for him to work with this tool. I’m not thrilled with this idea, but I think it will work. Also note that this is an offset bend that will be done on a roll-fed punching machine, not a press brake, but I’m assuming at least some of the basic metal forming principles apply. Any further guidance on this topic would be greatly appreciated.
Answer: Before diving into this topic, I would like to address your remark about verbosity. Confusion in wording is one of the biggest problems our industry faces. This statement is true whether you are in class or discussing a project at work.
Some trade terms are interchangeable. One person’s kink limit cannot be another person’s k-factor, and the k-factor is not a kink deduction – although the store I went to did. Because these terms have a precise meaning and application, using them incorrectly can complicate complex ideas and make it difficult to create high-quality parts. Misuse of terminology is often difficult to correct, and everyone will give the same answer to the question of why they use a term the way they do: because that’s how I learned it.
To keep everyone on the same page and using terminology correctly, I recommend posting a simple laminated wall chart or handout with all relevant definitions. Here are some things you can enable:
These are just a few of the relevant definitions, there are many more. However, when everyone gets the language right – well, you get it.
Now back to the topic under discussion: the relationship of the direction of the fibers to the lines of the bend. In the previous article, I used a “grained bend” when the fold line was parallel to the direction of the fibers, as shown in Figure 1. “Side” or “lateral” folds are when the fold line is perpendicular to the direction of the fibers, which makes the fold stronger and less prone to to cracking (see Figure 2).
A bend parallel to the fibers gives a weaker bend than a bend line that runs against the fibers or across the fibers. In addition, the outer radius of the bend is more prone to cracking when bent parallel to the direction of the fibers. The smaller the inner radius when bending parallel to the direction of the fibers, the greater the likelihood of cracking and the stronger the cracking will be. Using large bend radii can help prevent these problems.
It takes more effort to bend a piece of material when the fold line crosses the texture, but the same bend through the texture also maintains a smaller inside bend radius. In addition, the penetration depth may change during bending depending on the grain orientation of the material by the bending wire.
Not all materials have grain direction. Copper has no grains; in hot rolled pickled and oiled steel (HRP&O) grains are present, while in mild cold rolled steel grains can be very pronounced. In stainless steels, it is difficult and sometimes impossible to determine the grains and their orientation. Materials with grain orientation that affects bending angles are called anisotropic. Materials that do not have this property are considered isotropic.
Figure 1. Bends of grains (i.e. the bending line is parallel to the grain direction) are more prone to cracking.
One of the best ways to mitigate cracking is to keep the inside bend radius as close to the material thickness as possible, even if the inside bend radius to material thickness ratio is as close to one to one as possible. Smaller radii pull the material tightly into the bend, which pushes the grains apart, appearing as cracks. You will rarely see cracks in bends with radii larger than the thickness of the material. Sometimes the grains can be broken due to excessive stretching or elongation of the outer radius. As a rule, this applies to less ductile or high-temperature materials, such as T-6 aluminum. However, such cracks are rare.
If you must bend with the grain and cracking is still a problem, you can use the material in an annealed state and then temper it if necessary. For example, you can form soft aluminum and then harden it to a T-6 temper.
Also consider the type of bend you are making. Offset bends are tricky to start with because the tool restricts the center flange. This limitation results in an extension of the bend elsewhere, in particular to the two outer flanges. This change in elongation makes them unpredictable in size. This offset also works best with smaller bend radii, which can increase cracking problems.
If you were to form this part on a roll forming machine, it would likely bottom out (because the forming process itself is not suitable for air forming), so you would not be able to use air forming methods to reduce cracking. However, adding a small amount of angular clearance to the die set will help keep the curved flanges parallel. Depending on the type of material and the amount of elasticity inherent in this material, one or two degrees is sufficient. The one-to-one ratio between material thickness and inside bend radius helps keep the flanges parallel.
The grain size also significantly affects the yield strength. Materials with finer grains are less prone to separating and cracking and have a higher yield strength, providing a good reason to buy higher quality materials even if they are more expensive. However, additional material costs are easily offset by reduced waste and labor savings due to quality issues.
Grain boundaries also play a role in separating and cracking grains by disrupting the movement of so-called dislocations. The smaller the grain size, the greater the total area of the boundary, the more obvious the damage and the more stable and constant the yield strength.
For more information on this topic, you can check out my past columns, including “Material Grain Size Matters in Sheet Metal Bending”, “How Metal Grain Size Affects Bending Operations” and “Material Grain Size on a Bending Die” . in thefabricator.com search bar.
Stamping is certainly different from press brake forming, but it has a lot in common, including grain separation and cracking on the outside of the bend. We often have no choice but to obey the grain, but there are a number of things we can do to minimize the detrimental effects of obeying the grain.
Figure 2. Bend along the fibers (that is, when the direction of the fibers is perpendicular to the bend) gives a stronger bend and is less prone to cracking.
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Post time: Apr-27-2023