The Basics of Bend Radii
Wondering about K-factors and outside setbacks? Here are a few of the things to know about designing for sheet metal
Brackets, covers, cabinets, chassis, electrical enclosures. These and countless other sheet metal components might seem fairly simple to manufacture, but achieving part accuracy involves some fairly complex bending calculations. That’s because sheet metal elongates when bent. Not a lot, but enough to send designers back to the proverbial drawing board from time to time.
The amount of elongation and, therefore, the necessary "bend allowance" (more on this in a moment) is determined by several factors. Among them are the workpiece material and thickness, the bend angle and inside radius, the method used to bend the metal (air vs. bottom bending), and the oft-misunderstood K-factor, also known as either the neutral factor or the Y-factor.
To illustrate, grab a piece of sliced cheese out of the refrigerator. Maybe a Kraft American single? It measures roughly 3-1/2 in. (88.9mm) square by 0.083 in. (2.1082mm) thick—about the same as a piece of 12-gauge brass or aluminum. Now, bend it evenly over the edge of the countertop. If this staple of school lunches everywhere were made of metal, the surface touching the counter would be compressed, its outer face stretched, and the length of the now-bent “workpiece” would have grown slightly beyond the 3-1/2 in. (88.9mm) mentioned earlier.
Somewhere between these inner and outer surfaces lies an imaginary plane, one located at the transition zone that is neither compressed nor tensioned. This is the neutral axis, and it tends to shift towards the inner surface during bending. The K-factor, therefore, is the ratio between this neutral axis’ location as measured from the bend’s inner face (t) and the overall material thickness (Mt). The Y-factor provides a slightly more involved (and accurate) version of the industry standard K-factor, since it takes certain metallurgical properties into consideration. It is, however, rarely used.
Assuming you used an inside bend radius smaller than the material thickness, the K-factor (or neutral factor, if you prefer) in our cheesy example would be 0.33 for an air bend and 0.42 for a bottom bend, both gradually increasing to 0.5 for larger bend radii. The K-factor also increases with harder materials such as steel and stainless steel but never exceeds the 0.5 just mentioned.
But whether you call it K-factor, Y-factor, or neutral factor, if you’re working with us, there’s one important thing that you should know: Don’t worry about it. Sure, go ahead and accept whatever default values that Solidworks, Siemens NX, or Fusion 360 provides in their sheet metal settings. But the fact remains that K-factors are highly application- and material-specific, and we have spent extensive amounts of time developing values that work with our tooling and equipment. In other words, there’s no need to waste time worrying about it.
Setbacks and Bend Allowances
So what about all that other stuff—the outside setbacks, bend allowances, and bend deductions seen on fabricating websites? These values are A) quite important to anyone making manual bend calculations, and B) needed to produce an accurate "flat" layout of the 3D part models, but the truth is that a sheet metal-aware CAM system (which is most of them) does all the number crunching for you. Regardless, here are some brief descriptions that any sheet metal part designer should be familiar with:
- Pick up that piece of cheese once more and lay it flat on the counter, then lift one edge to form an L-shape. The vertical section is a flange. If making a simple box to contain an entire block of cheese, it would have a bottom face and four flanges, as would any lid that fits over it.
- Outside setback (OSSB). Aside from its location and height, each flange is also defined by a setback amount in the vertical and horizontal (X and Y) axes. On a 90-degree flange, for instance, the OSSB is equal to the outside radius. This, in turn, is equal to the bend radius plus the material thickness.
- Bend allowance. Remember that imaginary neutral line from the K-factor discussion? If you were to “unroll” it or make it flat, that would be the bend allowance. Google “bend allowance” and you will see it described on numerous sites as the “arc length of the bend as measured along the neutral axis of the material.”
- Bend deduction. Those same sites will state that the bend deduction is the difference between the bend allowance (which is itself determined by the K-factor) and two times the OSSB, or outside setback. When flattening a 3D model, this bend deduction is the amount that must be subtracted from the workpiece to account for any stretching.
That last point is an important one, at least for those working with us. Here again, there’s no need to get a headache worrying about K-factors and similar bending minutiae, just as it’s unnecessary to flatten your 3D model and generate a .DXF of the workpiece blank. That might be needed with other fabricators, but not us. Just send us the file and we’ll take care of the rest.
Other Sheet Metal Design Considerations
That said, some caveats exist. For starters, the material thickness in any sheet metal part should be consistent. They all start out as a flat sheet, so don’t attempt to design a part that’s 1/16 in. (1.5875mm) thick in one area and 1/32 in. (0.03125mm) somewhere else. It doesn’t work that way. And consider bend radii, which is, after all, part of the title of this design tip.
Remember that the internal radii (the section formed by the punch) of any flange directly impacts the K-factor, bend allowance, and so on. Here again, though, we’ve dialed in our K-factors and found that a 0.030 in. (0.762mm) bend radius works quite well on 95% of all parts. The only exception to this rule is 6061-T6 aluminum, which thanks to a slight brittleness, might require something a bit larger to prevent cracking.
Yes, you are free to specify different radii—to fit a mating part, for instance, or where a crisp internal corner is required. Our standard tooling accommodates internal radii from 0.010 in. (0.254mm) up to 0.250 in. (6.35mm) in standard increments, but whatever value you choose should then be called out on all of the flanges found on that part. Failure to do so will mean additional setups and a higher part cost.
Speaking of corners, you should also plan on a bend relief anywhere that two flanges come together. These are small notches roughly 0.030 in. (0.762mm) wide that prevent the material from bulging outwards at the junction. Many CAD systems are smart enough to create these bend reliefs, but if not, we’ll take care of it and let you know during design approval. If requested, we can also weld up the corner to provide a sealed seam.
A Final Piece of Sheet Metal Advice
When placing holes, slots, and similar features in your part design, be sure to locate them at least 4x the material thickness away from any edges or inside corners. This goes back to the whole stretching phenomenon described earlier—stick a round hole any closer than this to a bend line and, due to the metal deformation, it will likely come out slightly oval or egg-shaped.
There’s a lot to sheet metal design, far more than might be evident when looking at a basic bracket or cabinet. Fortunately, we have the right equipment, a team of skilled personnel to operate it, and a firm grip on the bending calculations just outlined. That means it’s a simple matter to upload a 3D CAD model to our website and order whatever’s needed for your next project—depending on the urgency, part quantity, and secondary finishing options like powder coating and hardware insertion, the parts can be there in just a few days.