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Forming tonnage =
[575 × (Material thickness2) / Die width /12] ×
Length of bend × Material factor × Method factor × Multiple-bend-tooling factor
A point of note, in the formula above, the material thickness must be squared
before multiplying it by the 575 constant if you want the answer to be correct.
Also note that this tonnage calculation produces only an estimate of the tonnage that may be required.
With and Against the Grain in the U.S.
Question: I have been reading some of your articles on metal bending, and I was
wondering about your expert recommendation on bending relative to the grain
direction. From what I read, it is recommended to bend perpendicular to the grain
for material 0.25 in. and thicker. I just need a good technical explanation of when to
bend against the grain direction, when it’s not necessary, and why.
Answer: The answer to your question is … there is no absolute answer. It’s more
of a general rule. When bending plate 0.25 in. or thicker, grain direction is an issue
particularly in low-quality steels in which you need to form a small inside bend
radius. The tighter the radius, the greater the stress on the outside surface, which
in turn pulls the grains in the material apart and causes cracking on the surface.
The grain direction issue is more of a poor-quality material issue. Generally,
you will not have such problems when bending materials like stainless and high-quality steels. Still, even with high-quality materials, grain direction can be an
issue at the press brake if it varies from part to part—that is, if you form the same
flanges but each has a di;erent grain direction, which in turn changes your bend
angle from part to part.
On thin material and aluminums, grain splitting on the outside of the bend
can be the result of the finish grain. If it’s deep, the grains tend to pull apart.
My advice: Do not bend with the grain in low-quality steels. Instead, try to
bend across or diagonal to the grain, and try to keep the grain direction consistent across the run of parts.
Steve Benson is a member and former chair of the Precision Sheet Metal Technology
Council of the Fabricators & Manufacturers Association International®. He is the president of ASMA LLC, firstname.lastname@example.org. Benson also conducts FMA’s Precision
Press Brake Certificate Program, which is held at locations across the country. For more
information, visit www.fmanet.org/training, or call 888-394-4362. The author’s latest
book, Bending Basics, is now available at the FMA bookstore, www.fmanet.org/store.