Grain direction’s effect on sheet metal bending

Question: A previous article of yours suggests that forming “with” the grain direction will manifest cracks. I might be confused over the verbiage. Does this mean the grain runs perpendicular or parallel to the bend line?

I was researching the topic because we are bending 0.060-in.-thick 3003 H14 aluminum (see Figure 1), and my toolmaker wants me to design the bends parallel to the grain, because the tool will be easier for him to work on. I’m not crazy about this idea, but I think it will be OK. Note also that this is an offset bend that will be made in a coil-fed stamping press, not a press brake, but I assume at least some of the metal forming fundamentals apply. Any further guidance on this topic would be much appreciated.

Answer: Before I delve further into this topic, I would like to begin with your comment about verbiage. Confusion over verbiage is one of the biggest problems our industry faces. This statement is true whether you are learning in the classroom or discussing a project on the job.

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—though I visit shops where this is precisely the case. Because these terms have exact meanings and applications, misusing them makes communicating complex ideas complicated and creating quality parts much harder to accomplish. Terminology misuse is often brutal to correct, and everyone will give the same response 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 using the terminology correctly, I recommend posting a simple laminated wall chart or handout with all the relevant definitions. Here are a few you could include:

k-factor: A multiplier value to determine the location of the relocated neutral axis. Outside setback, or setback: The distance from the tangent point of the flat and the radius to the apex of the bend. Bend allowance: The length of the neutral axis, which can be added to the setbacks to determine the flat length of a part.Bend deduction The value that is subtracted from the total of the outside dimensions to determine the flat length of a part.Neutral axis: The theoretical area within the material at the bend that is neither expanded nor compressed.
These are only a few relevant definitions; there are more. Nonetheless, when everyone is using the language correctly—well, you get the picture.

Grain Direction
Now, back to the subject at hand: the grain direction’s relationship to the bend line. In previous articles, I’ve used “bending with the grain” when the bend line is parallel to the grain direction, as shown in Figure 1. Bending “across” or “traverse” to the grain is when the bend line runs perpendicular to the grain, which makes for a stronger bend that’s less likely to crack (see Figure 2).

Bending parallel to the grain will create a weaker bend than a bend line running against or transverse to the grain. Also, the outside radius of the bend is more prone to cracking when bending parallel to the grain direction. The smaller the inside radius is when bending parallel to the grain direction, the greater the chances that cracking will occur, and the more severe that cracking can be. Using a larger bend radius can help prevent these problems.

It takes more force to bend a piece of material when the bend line runs across the grain, but that same across-the-grain bend also can hold a much smaller inside bend radius. Also, the penetration depth can change from bend to bend, depending on the bend line’s orientation to the material grain.

Not all materials have a grain direction. Copper has no grain; hot-rolled pickled and oiled (HRP&O) has some; and in mild cold-rolled steel, the grain can be quite pronounced. In stainless steel, it can be tough and sometimes impossible to define the grain and grain direction. Materials with a grain direction that affect the bend angle are considered anisotropic. Materials that don’t have this property are considered isotropic.

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