Understand sheet metal bending: bending radius, K factor, etc.

                                  Understand sheet metal bending: bending radius, K factor, etc.
    Brackets, covers, cabinets, chassis, electrical enclosures. The manufacture of these and countless other sheet metal parts seems fairly simple, but achieving part accuracy involves some fairly complex bending calculations. This is because the metal plate elongates when it is bent. The amount of elongation and the necessary "bending margin" are determined by several factors. These include the workpiece material and thickness, the bending angle and inner radius, the method used to bend the metal (air and bottom bending), and the often misunderstood K factor, also known as the neutral factor or Y factor.

    K factor: For example, a piece of No. 12 brass or aluminum is about 3-1/2 inches (88.9 mm) square times 0.083 inches (2.1082 mm) thick. Now, bend it evenly on the edge of the countertop, then the surface contacting the counter will be compressed and its outer surface will be stretched.

    Somewhere between these inner and outer surfaces there is an imaginary plane that lies in a transition zone that is neither compressed nor tensioned. This is the neutral axis, and it tends to move toward the inner surface during bending. Therefore, the K factor is the ratio between the position of the neutral axis measured from the inner surface (t) of the elbow and the total material thickness (Mt). Because the Y factor takes into account certain metallurgical properties, it provides a more complex (and more accurate) version of the industry standard K factor. However, it is rarely used. Assuming that the inner bend radius used is smaller than the material thickness, in our example, the K factor is 0.33 for air bends, 0.42 for bottom bends, and for larger bend radii, both gradually increase to a bend radius of 0.5. The K factor also increases with the increase of harder materials such as steel and stainless steel, but it never exceeds the 0.5 just mentioned.

    Curvature and bend tolerance: What about all the other things (external camber, bend allowance, and bend deduction) that you see on the manufacturing website? These values ​​are very important for anyone doing manual bending calculations and need to generate an accurate "flat" layout of the 3D part model. The following are some brief descriptions that all sheet metal part designers should be familiar with: External curvature (OSSB): In addition to its position and height, each flange is also defined by the indentation on the vertical and horizontal (X and Y) axes definition. For example, on a 90° flange, OSSB is equal to the outer diameter. This in turn is equal to the bending radius plus the material thickness.

    Bending allowance: Remember the hypothetical neutral line in the K-factor discussion? If you want to "unfold" or lay it flat, this will be the bending allowance. Search for "bending allowance" and you will see it described on many websites as "bending arc length measured along the neutral axis of the material"

    Bend discount deduction: These same venues will show that the bend deduction is the difference between the allowable bend (which itself is determined by the K factor) and twice the OSSB or the external intake. When flattening a 3D model, this bend deduction is the amount that must be subtracted from the workpiece to account for any stretching.

     Other sheet metal design considerations: the thickness of the material in any sheet metal part should be the same. They are flat at the beginning, so don't try to design a part with a thickness of 1/16 inch (1.5875mm) in one area and 1/32 inch (0.03125mm) in other areas.

    When placing holes, slots, and similar features in a part design, make sure to place them at least 4 times the material thickness from any edge or inner corner. This can be traced back to the entire stretching phenomenon described above. Pasting a round hole closer to the bending line than this, the round hole may be slightly elliptical due to metal deformation.

    You can freely specify different radii to fit the mating part, to fit the mating part or where a clear inner corner is needed, but any value you choose should be called on all flanges found on the part. Otherwise it will mean additional settings and higher parts costs. Speaking of corners, you should also plan to bend and relieve pressure wherever the two flanges are connected together. These are small notches of approximately 0.030 inches (0.762mm) wide to prevent the material from swelling outward at the joint. Many CAD systems are smart enough to create these bend reliefs.