Generally speaking, the harder the tube and the smaller the bend’s centerline radius, the greater the springback and resulting radial growth. Like in press brake forming, tubes experience springback after bending, producing a bend that undergoes radial growth. More specifically, it’s the distance between two tangent points, where a straight section begins to curve and the bend starts or finishes. The distance between bends (DBB) is just what it says. So if a tube is bent to “45 degrees,” that’s 45 degrees complementary, or a 135-degree included bend angle (see Figure 2). The bend angle refers to the complementary angle of bend. Other bending variables include the inside bend radius (sometimes called the intrados) the outside bend radius (or extrados) and the centerline radius or the neutral line, where neither compression nor stretching occurs. This variation should be accounted for, especially for bending processes using precise, tight-fitting tooling on small bend radii. There’s a tolerance, and the wall thickness can vary slightly. But when you’re specifying a bending machine, the centerline radius, the outside diameter, and wall thickness are critical variables.Īlso, every pipe schedule has a nominal wall thickness. Pipe, usually used to transport fluid or air, is specified by its nominal pipe size (see Figure 1). Fundamental Termsīending starts with knowing the properties of the tube or pipe you’re working with. Regardless of whether you’re working with tube or pipe, and regardless of the bending process, making the perfect bend boils down to just four factors: the material, machine, tooling, and lubrication. The technology used to bend tubular workpieces has evolved significantly, but all the mechanical magic can’t alter physics. But in reality, the basic principles have remained the same for decades.
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Many call tube bending a black art, a mysterious process with unavoidable trial and error. Editor’ Note: This article is based on the Tube Bending 101 FabCast, facilitated by the Fabricators & Manufacturers Association International (FMA) and presented by Danie Jacobs, president of i-Fab LLC.
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