Bending Sheet Metal Processing

Bending, also known as flanging or folding, is among the most commonly utilized techniques in sheet metal fabrication.
Sheet metal bending enables manufacturers to transform a straight sheet metal component into one with angled features. This approach is more cost-effective than welding two separate parts together or fastening them.
The process of sheet metal bending involves applying force to the sheet metal in a manner that results in permanent deformation. However, various bending methods and principles are employed to achieve the desired outcome.

 What is sheet metal bending?

Sheet metal bending is a metal processing technique used to transform flat sheet metal material into V, U-shaped, or grooved forms.
This process is essential and cost-effective because bending a flat metal plate to create a new shape is typically cheaper than manufacturing a solid part with V, U-shaped, or channel-shaped features, or casting it in a die-casting factory. Additionally, bent parts are generally more robust than those created by welding two flat metal plates together.
Various types of sheet metal bending involve the use of a brake press, also known as a bending machine or sheet metal folder. This machine allows for the application of force either manually or with the assistance of a hydraulic pressure device.

 Sheet metal bending equipment

The primary tool used for sheet metal bending is the brake press, which comes in various types.
The cornice brake, a commonly used machine, is a straightforward folding device. It clamps a metal plate in place and then uses a movable bending blade to create straight bends or simple creases.
A press brake is a bending machine equipped with a movable punch head and corresponding molds. In this process, the metal plate is positioned on a mold, and the punch head applies force to bend it into the desired shape. Depending on the shape of the mold, press brakes can create V-shaped, U-shaped, and other configurations.
The box brake, also known as a finger brake, is another type of bending machine that utilizes an array of metal fingers to achieve multiple customized bends. It is commonly used for fabricating custom-sized boxes.
A bar folding machine is a compact and simple bending device with a handle that clamps the metal plate and bends it in a single motion.

Types of sheet metal bending

Various sheet metal bending techniques are employed to achieve different types of bends, utilizing different methods and machinery:

  1. Air bending:
    Air bending involves pressing the metal plate with the punch head into the mold, without fully contacting the walls of the mold. While less precise, it offers flexibility to create U, V, and other shapes, as the mold geometry does not need to exactly match the required bend shape due to the absence of contact between the surfaces.
  2. Bottoming:
    Bottoming is another bending method where the punch head fully presses the metal plate into the mold to create a bend corresponding to the internal geometry of the die. It is commonly used to create V-bends.
  3. Punching:
    Punching is a more costly bending method where the punch head applies higher force to the metal plate and mold to produce permanent deformation and mild rebound.
  4. Folding:
    Folding can be executed using cornice brakes, bar folding machines, and other equipment. Before the clamping beam lifts to bend the metal around the contour, the sheet metal is firmly clamped and folded to obtain V-bends and positive or negative bending angles.
  5. Wiping:
    Wiping, also known as edge bending or wipe bending, is another bending method suitable for mechanisms like cornice brakes and steel bar folding machines. It offers faster bending speeds but may result in more severe damage to the metal surface.
  6. Roll bending:
    Roll bending is perhaps the most distinctive sheet metal bending technology due to its unique mechanics. It employs a three-cylinder roller to bend the metal plate into an arc, making it suitable for creating pipes and other circular parts.
  7. Step bending: also known as incremental bending or uneven bending, is a method utilized with a bending machine to approximate smooth bending similar to roll bending. It involves performing multiple small V-bends consecutively to achieve bends in parts that resemble continuous curves.

Springback of sheet metal after bending

When sheet metal is bent into a new shape, it naturally undergoes a certain degree of rebound after the bending force is removed. This rebound occurs due to the compressive strength of the bent metal plate. During bending, one side of the metal is under tension while the other side is under compression. However, the compressive strength of the material is higher than its tensile strength, allowing the compression side to resist deformation and decompress itself upon removal of the force.

Springback is not the primary concern, as manufacturers must anticipate and compensate for the expected rebound by over-bending the metal plate slightly. This ensures that a small amount of natural rebound will result in the correct angle. Springback can be calculated accurately, and various factors, such as material types and specifications, can significantly influence its extent. Additionally, larger inner radii tend to result in larger spring back.

Allowance of sheet metal bending

When a metal plate bends, the outer side of the bend experiences elongation, resulting in a change in size. For instance, the total length of the two legs in a V-bend will be longer than the original length of the sheet.

So, how do we ensure that the changed size aligns perfectly with other components? Properly designing a part requires considering the bending allowance, which involves factors such as the sheet metal thickness, bending radius, bending angle, and others. Utilizing a bend factor calculator helps determine the coefficients needed for the sheet metal part.

Best sheet metal material and gauge for bending

Some sheet metal materials are better suited for bending, with malleable materials generally preferred over brittle ones.

Common materials used for sheet metal bending include:

  1. Low carbon steel: Can be bent under any temperature conditions.
  2. Spring steel: Requires annealing before bending.
  3. Alloy steel 4140: Requires annealing before bending.
  4. Aluminum 5052: Offers higher flexibility compared to other aluminum alloys.
  5. Copper: Highly flexible and easily bendable.
  6. More challenging materials (though not impossible) to bend include aluminum 6061, titanium, brass, and bronze.

How does grass direction affect sheet metal bending

The orientation of the metal plate’s grain, determined by the original rolling direction, influences its bending behavior.

Grain direction strengthens the metal along one axis while weakening it along another. Bending parallel to the grain (longitudinal) increases the risk of cracking, tearing, or producing an uneven surface texture. Bending perpendicular to the grain reduces these issues.

However, although bending parallel to the grain reduces damage rates, it requires greater force due to the metal’s increased toughness. Consequently, this may result in more springback and necessitate greater compensation in this regard.

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