Effect of grain direction on sheet metal bending

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If press brake operators bend a small radius with the grain of the material, that is, the bend line is parallel to the grain direction of the material, they should pay attention to the cracks. Getty Images

Question: One of your previous posts suggests that forming “with” the direction of the grain will manifest cracks. I might be confused on the verbiage. Does this mean the grain is perpendicular or parallel to the bend line?

I was researching this because we bend 0.060″ thick 3003 H14 aluminum (see Figure 1), and my toolmaker wants me to draw the folds parallel to the wire, because the tool will be easier for him to work with. I’m not crazy about this idea, but I think it will be OK. Also note that this is an offset bend that will be made in a coil fed stamping press, not a press brake, but I guess at least some of the fundamentals of metal forming apply. Any further guidance on this subject would be greatly appreciated.

To respond: Before delving into this topic, I would like to start with your comment on verbiage. Confusion over verbiage is one of the biggest problems facing our industry. This statement is true whether you are learning in class or discussing a project at work.

Very few trade-specific terms are interchangeable. One person’s bend allowance cannot be another person’s k-factor, and a k-factor is not a bend deduction – although I visit stores where that is precisely the case. Because these terms have exact meanings and applications, their misuse makes communicating complex ideas complicated and creating quality pieces much harder to achieve. Misuse of terminology is often brutal to correct, and everyone will give the same answer as to why they use the terms the way they do: because that’s how I learned it.

To get everyone on the same page and use terminology correctly, I recommend posting a simple laminated wall chart or handout with all relevant definitions. Here are a few you could include:

k-factor: A multiplier value to determine the location of the moved neutral axis. outer marginor setback: The distance between the tangent point of the flat and the radius to the top of the bend. Curvature Allowance: The length of the neutral axis, which can be added to the indents to determine the flat length of a part.Curvature deduction The value that is subtracted from the total outside dimensions to determine the flat length of a part.Neutral axis: The theoretical area in the material at the ply that is neither expanded nor compressed.

These are just a few relevant definitions; there is more. Nonetheless, when everyone is using the language correctly, well, you get the picture.

grain direction

Now back to the topic at hand: the relationship between grain direction and bend line. In previous articles, I have used “bend with the grain” when the bend line is parallel to the direction of the grain, as shown in Figure 1. Bending “across” or “crosses” with respect to the grain occurs when the bend line is perpendicular to the grain, resulting in a stronger curvature that is less likely to crack (see Figure 2).

A bend parallel to the grain will create a lower bend than a line of bend going against or across the grain. Also, the outer radius of the bend is more prone to cracking when bending parallel to the grain direction. The smaller the inside radius when bending parallel to the direction of the grain, the greater the chances of cracking and the more severe this cracking can be. Using a larger bend radius can help prevent these issues.

It takes more force to bend a piece of material when the bend line crosses the grain, but that same bend across the grain can also contain a much smaller inside bend radius. Additionally, the depth of penetration can change from ply to ply, depending on the orientation of the bend line relative to the grain of the material.

Not all materials have grain direction. Copper has no grain; Pickled and Oiled Hot Rolled (HRP&O) have some; and in cold rolled mild steel the grain can be quite pronounced. In stainless steel, it can be difficult and sometimes impossible to define grain and grain direction. Materials whose grain direction affects the bend angle are considered anisotropic. Materials that do not have this property are considered isotropic.

graphic of a metal part bent on a press brake

FIGURE 1. Creases made with the yarn (i.e. the crease line is parallel to the direction of the yarn) are more likely to crack.

Tips to avoid cracks

One of the best ways to mitigate cracks is to make the inside bend radius as close to the material thickness as possible; that is, make the ratio of inside bend radius to material thickness as close to a one-to-one relationship as possible. A smaller radius pulls the material tightly around the bend and thus separates the grains, manifesting as cracks. You rarely see cracks in bends where the radius is greater than the thickness of the material. Sometimes the grains can be pushed apart by stretching or extending the outer radius too much. Usually this is seen in less ductile or high tempered materials, such as T-6 aluminum. Nevertheless, such cracking is not common.

If you need to bend with the grain and cracking is still an issue, you may be able to use the material in the annealed condition and then temper it as needed. For example, you can form soft aluminum and then temper it to a T-6 temper.

Also consider the types of turns you are doing. Offset bends are tricky devils to start with, as the tooling constrains the center flange. This constraint lets the elongation of the elbow go elsewhere, notably the two outer wings. This shift in elongation makes them dimensionally difficult to predict. Such offsets also work better with a smaller bend radius, which adds to the cracking problem.

If you’re forming this part on a coil-fed stamping press, you’re probably melting (because the stamping process doesn’t lend itself well to air forming), so the options for reducing cracking with the air forming methods are not available. However, adding a little backlash to the die set will help maintain parallelism between the bend flanges. A degree or two is enough, depending on the type of material and the amount of springback inherent in a given material. A one-to-one relationship between material thickness and inside bend radius helps maintain flange parallelism.

Grain size also greatly affects yield strength. Smaller grained materials are less prone to grain separation and cracking, and have a higher yield strength, which justifies purchasing higher quality materials even if they are more expensive. Nevertheless, the additional material expenses will be easily covered by a reduction in scrapped materials and the labor savings resulting from the fight against poor quality.

Grain boundaries also play a role in grain separation and cracking by disrupting what is known as the dislocation movement. The smaller the grain, the larger the total area of ​​the boundary becomes, the greater the disturbance, and the more robust and constant the yield strength.

For more on this topic, you can check out my past columns, including “Material Grain Size Matters in Sheet Metal Bending”, “How Metal Grain Size Affects a Bending Operation”, and ” Material Grain Considerations on a Press Brake,” which you can type into the search bar on thefabricator.com.

Stamping is of course different from forming on a press brake, but it has a lot in common, including grain separation and cracking outside the bend. We often have no choice but to bend with the grain, but there are many things we can do to minimize the adverse effects of forming with the grain.

graphic of a metal part bent on a press brake

FIGURE 2. Bending across the grain (that is, when the direction of the grain is perpendicular to the bend) creates a stronger bend that is less likely to crack.

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