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In response to the original question

I think it's safe to say that over 1/8 wall thickness you a very likely to damage the bender. I would be worried about both the arms and the die itself where the U-strap attaches. If you are going to try this, first do yourserlve a favor by bracing the rectangular stock that holes the U-strap pin on the main/forming die. You can weld in a gusset that connects it back to the center of the die. At least this way if you damage the bender, the damage will likely only be to the arms.

I have read about bending thick tubing in these, but they are usually accompanied by damage reports.

In response to fabcam regarding larger CLRs for bending larger tubing

Let's see here. First off, I did a VERY simplified approach for determining my bending requirements which I felt errored on the conservative side. My approach was to put the top and bottom halves of the cross section at ultimate stress and determine the bending moment from geometry.

In reality there is quite a bit more going on. There isn't much you could do without iterative or numerical solving and you would have to make a few assumptions. I would start with a tensile test to determine the complete stress-strain relationship of the material (you could also approximate this using the %elongation, ultimate, and yield strengths). Next, I would use a solver to determine the stress throughout the material given a particular deformation. You can't do this easily by hand since it's very difficult to determine the equilibrium state of the material. This is where iteration becomes necessary and a numerical solver or finite element model would come in handy.

I can tell you that yes, a broader radius will require less bending moment, but there's a little more to it than that. If you wrap a thicker/larger tubing around a broader radius die, the springback is going to become more and more of a factor. This is because you may not be deflecting the material enough to significantly yield it, so a significant portion of the cross section will want to spring back.

Another point that may be important to consider is that a curved beam (bent tubing) is slightly stiffer than a streight beam (straight tubing). This also contributes toward higher bending requirements for tighter radii.

A sensativity study would be advisable to determine how much the bend radius actually affects the bending torque requirement. It may be that increasing the radius from 5 to 8 inches doesn't have a significant effect. If I was going to build tube benders for a carreer, I would spend much more time nailing down the specific requirements. Perhaps I will sit down with a professor and develop an FEA model next quarter.
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