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Understanding sheet-metal bending

In the big picture of manufacturing, sheet-metal bending is just one aspect — but it’s critical.


In its simplest form, sheet-metal bending is when you use an exterior force to alter the external features of the sheet. The metal is stressed to a point between its yield strength and its ultimate tensile strength. It doesn’t change the material’s surface area, and it occurs on one axis only, generally. It can produce an array of shapes through die sets and bend brakes.




When it’s bent, sheet metal has three distinct components:


  • Inside surface: It’s compressed to account for bend

  • Outside surface: It’s stretched to account for bend

  • Neutral axis: it’s a line in the metal that is neither compressed nor stretched


These variants mean that a piece at a 90-degree bend won’t have equal leg measures. Calculating the Bend Allowance will reveal how much should be added and subtracted from legs A and B.


Where is the neutral line, exactly? That depends. Factors include:

  • Ambient temperature

  • Bending method

  • The direction of material grain

  • Material

  • The radius of the bend


This line is located at the K factor, a ratio that indicates where the neutral sheet is in relation to how thick the sheet metal is.


Types of metal bending

There’s more than one way to bend your metal. Air bending, bottoming, and coining are prevalent methods. Here’s how they work.


AIR BENDING

The punch touches the workpiece, but the workpiece doesn’t bottom in the lower cavity. Instead, the workpiece springs back and has less bend than that on the punch. Grain, material, temper, and thickness determine the spring back amount, usually 5-10 degrees. Minimize set-up time by using the same angle in both the punch and die. The bend’s inner radius and the punch’s inner radius should match.


In air bending, you don’t need to change dies or equipment to get different bending angles. Punch stroke determines those. It doesn’t take much force, but the punch stroke control must be accurate to achieve the angle you’re after.


BOTTOMING

As the name implies, the punch and workpiece bottom on the die. It requires more tonnage than air bending, and the controlled angle ends up with a sparse spring back. The inner radius on the workpiece ought to be at least one material thickness. The clearance between die and punch surface is less than the blank thickness. This reduces the spring back, and the material yields slightly. Bottom bending takes 50-60% more force than air bending.


COINING

In coining, the punch and workpiece bottom on the die. You apply compressive stress to the bending region to raise the amount of plastic deformation — and decrease the amount of spring back. The inner radius on the workpiece should be as much as .75 of the material thickness.


Tips for metal bending

No matter the method, here are some factors to keep in mind.

  1. Keep the bend radius the same for all radiuses, so you won’t have to change the setup many times.

  2. An inner radius of at least 1 material thickness is best for most jobs

  3. It’s easier to bend perpendicular to a rolling direction than to bend parallel to one, which can result in hard materials breaking. It’s not ideal for cold-rolled steel greater than Rb 70 (and no bending is good for cold-rolled steel greater than Rb 85.)

  4. You CAN bend hot rolled steel parallel to the rolling direction, however.

  5. Figure the minimum flange width as at least four times the stock thickness, plus the bending radius. Doing so helps prevent distortions in the part or damage to tooling or operator because of slippage.


It helps to understand the basics of sheet-metal bending, and the common pitfalls to avoid. Here’s to efficient work and great results.


Dean Fowell

BluTec Machinery


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