Forming with, against, or diagonal to the grain will give you different results in a completed bend.
Question: Over the past few years my company has been striving to become a world-class competitor. We work mostly with light-gauge cold-rolled steel, hot-rolled steel, aluminum sheet, and occasionally stainless. We’ve moved from a 1970s-vintage turret punch to a state-of-the-art punch/laser machine for our flat blank processing. We’re using CAD/CAM software and optimizing our programs.
We’ve moved from a mechanical press brake and bottom bending with planer tooling to air forming with precision-ground tooling and a press brake that supposedly repeats in microns. Yet even with all this leading-edge technology, all that it seems to have done for us is to help us build scrap faster.
The punch/laser has been incredible; it’s fast, and the parts in the flat are dead on the money. But when it comes to forming, that’s another story. We knew that transitioning from bottoming to air forming would be challenging, but we did not expect as many problems as we’re having now.
We are hard-pressed to get two bends to come out the same. The varying bend angles are causing dimensional problems in the part. As I noted already, it seems our employees have just gotten better at building scrap. And they’re generally unhappy. At least once a day I hear, “Can we please just go back to bottom bending?” What are we missing that makes parts so difficult to manufacture?
Answer: Many extenuating circumstances can lead to the inconsistencies your team faces. These include incorrect die widths, less-than-optimal punch nose radii, material thickness variations, and material uniformity issues from batch to batch. Even tolerancing callouts could be a factor.
Although all of these might contribute to your issue, I believe your greatest problem does not begin at the press brake. It starts at the punch press—specifically, how your parts are programmed on the sheet. This has to do with the sheet grain direction.
The grain direction is in the rolling direction of the sheet as it was being manufactured. Two rollers compressing the hot metal cause the polycrystalline material to elongate in the direction of rolling. Once the crystallites are elongated, they appear as the grain that we see in cold-rolled steel.
These microscopic crystalline structures form as the metal cools from its molten state. Rolling the material into sheet aligns this crystalline lattice structure. Grains vary in size and orientation before rolling but then take on the preferred orientation we see as grains running the length of a sheet.
The cold-rolled steel, the inside bend radius can change depending on whether you’re bending with or against the grain. Grain direction also plays into the amount of springback you can expect. To predict the material’s behavior accurately during bending, especially at the press brake, you need to accommodate for the grain direction.
Bending the sheet metal longitudinally (with the grain) allows those grains to separate at the grain boundaries. Bending with the grain also limits how tight of an inside bend radius you can achieve without cracking the outside of the bend. Bend angles also can be less consistent. That said, forming with the grain takes less pressure to make a bend.
The terms “cold rolled” and “hot rolled” refer to the different temperatures at which the steel was formed. The scale on the outside surface of hot-rolled steel is the residue left behind by the steelmaking process. When the steel cools, scarring or scaling makes the steel look like it has a burnt crust. Temperatures reached during hot rolling are so high that it does not allow for recrystallization to occur in the freshly made steel.
Recrystallization is the mechanism whereby grains damaged during processing are replaced by new grains during rolling with the steel in a cold state. Hot rolling occurs above the recrystallization temperature while cold rolling occurs below. Warm rolling occurs at temperatures between hot and cold rolling.
Put another way, hot-rolled steel is rolled at temperatures above the recrystallization point. Cold-rolled steel, on the other hand, is rolled after the material has cooled, below the recrystallization point. Because rolling occurs below the recrystallization point, new grains can form as the old and damaged grains are replaced.
The steel is reheated to the recrystallization temperature, then allowed to cool back to room temperature very slowly. This allows the steel to form a uniform microstructure and resets the grain structure to something similar to an untreated steel microstructure. All this returns the steel close to its original, strong, yet malleable state.
During forming, cold-rolled steel is strongest when a bend is made against the grain, weaker when bent with the grain. Hot-rolled material, on the other hand, doesn’t have grains like cold-rolled steel, so its strength when bent does not vary with the grain direction.
When bending high-strength materials, such as high-strength steels and high-carbon steels, orient those parts on the punched sheet so each part can be bent diagonally to the grain direction.
Because cold-rolled steel has grains—which causes variations in the bend angle, inside radius, springback, and ultimately the bend deduction—it is anisotropic. Hot-rolled steel, on the other hand, is isotropic, and so does not affect the above-listed elements. Stainless steel, titanium, and some aluminums are isotropic as well.
It’s extremely important that your prints have all the information an operator needs about grain direction. No matter where you are in the production chain—engineer, designer, programmer, or press brake technician—you need to keep grain direction in mind throughout the design or build phase, especially if you have forming applications in which anisotropic material properties are involved. Prints with CAD drawings sometimes have only the most basic information, which in turn can lead to serious production problems.
For example, if a part that should be bent across the grain is called with the grain, you’re likely to see cracking on the outside of the bend, especially in thicker material. Many prints don’t specify a grain direction at all, so the parts end up being nested both ways on the sheet. This causes one operator to form with the grain (and cause cracking) and another to bend across the grain without issues.
Whether you experience cracking when bending with the grain depends on your material and the application. Regardless, when it comes to the grain direction in parts destined for bending, consistency is key.
So why, considering all the potential root causes of your situation, did I think about grain direction after reading your question? You stated that you were optimizing the program to place as many parts as possible on a sheet. I also knew that you used to bottom bend, which effectively “stamps out” your grain-direction problems.
Optimizing your nest layout is an excellent practice when your parts will remain flat. But if you have bending operations to think about, the story changes. If you bend some parts with, others against, and still others diagonal to the grain direction, you’ll make each bend angle and dimension different from the last.
Remove such nesting layout optimization from your production process (and/or use the grain restraint option in your nesting software), note the grain direction on the print, and I believe that you will find a marked improvement in quality and production, and, not least, get the most from your new equipment.
2952 Doaks Ferry Road N.W.
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