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What is the tightest radius for a bend for any given thickness of sheet metal?
http://sheetmetal.me/air-bend-force-chart/
From: http://www.thefabricator.com/article/bending/bending-basics-the-fundamentals-of-heavy-bending
"A Simple Rule of Thumb
There’s a rule of thumb to determine a steel’s minimum bend radius, and this generally works for aluminum too: Divide 50 by the material’s tensile reduction percentage as specified by your supplier. This value will vary by grade.
If the steel has a tensile reduction value of 10 percent, divide 50 by that value: 50/10 = 5. Next, subtract 1 from that answer: 5 – 1 = 4. Now, multiply that answer by the plate thickness. If the material is 0.5 in. thick: 4 × 0.5 = 2. So in this case, the minimum inside bend radius is 2 times the material thickness.
Note that this is just a rule of thumb. Finding the true minimum bend radius for steel or aluminum plate requires a little research. This should include data from your material supplier, whether you are bending with or against the grain, as well as information specific to the application. Nonetheless, the answers are there, waiting for you to find them."