An Engineers' Guide to Sheet Metal Bending

Table of Contents

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What is Sheet Metal Bending?

CNC sheet metal bending is one of the most underrated processes available for sheet metal part production. With bending, it's possible to produce a wide variety of part geometries without tooling, at fast lead times, with high levels of repeatability and through automated processes. Bending is especially useful for low and medium volume production, where the reduced quantities (such as, several hundred to several thousand per lot) don't justify the creation of costly, difficult to maintain stamping tools, or where production costs for other methods are otherwise high for the volume of production required.

Bending techniques are a key tool in the arsenal of product developers, engineers and business owners who are looking to manufacture metal parts. Often, bending is paired with laser cutting as a series of processes to handle low to medium volume production.

It's good to understand the possibilities with sheet metal bending at the design phase. Bending is a tool that gives engineers the ability to create a wide variety of shapes and designs. In many cases, bending also allows a part to be created from one piece of material. This can have benefits over producing parts from multiple pieces joined together with hardware or welding. These include reducing cost and allowing for improved strength, simplified assembly and little-to-no tooling.

This Komaspec guide provides an overview of the main sheet metal bending processes, the advantages and disadvantages of each, basic design considerations with sheet metal bending and material selection information. This guide, along with our other articles exploring sheet metal fabrication will help you gain a grounding in sheet metal fabrication. The overall aim is to provide you with the information you need to understand how sheet metal parts are manufactured. With this information, you can better discuss the fabrication of your products with sheet metal manufacturers such as ourselves.

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Types of Sheet Metal Bending

There are multiple ways in which sheet metal parts can be bent during fabrication. However, the two main basic methods are:

• Brake press bending
• Rolling

The exact process followed with each method will depend on the material being bent as well as the part being produced. Less commonly used methods are employed when bends can't be achieved through simpler means.

Brake Press

A brake press is a tool that has been in use for many years in traditional fabrication shops all over the world. In its simplest form, a work piece is formed between two dies, as seen in the image below.

Figure 1: CNC Sheet Metal Brake Press (Bystronic Inc.)

Brake presses can be used for a very wide range of sheet and plate materials. Material thicknesses from 0.5mm up to 20mm can be accommodated due to the flexibility of the tooling and the high power levels of hydraulic machinery.

Brake presses are specified by two general parameters: Tonnage and width. The capacity or tonnage of a brake press refers to the maximum amount of force it can exert. The material thickness, type and bend radius dictate how many tons of force are needed when fabricating a part. Width refers to the maximum bend length the press can achieve. A typical brake press, for example, could be 100T x 3m (press brakes).

Brake press bending processes are categorized into two main types: Air bending and bottom bending.

Air Bending

The most commonly used brake press bending method is air bending. This involves using a brake press with a bottom tool that is a v-shape and a top punching tool of narrow shape with a rounded point. To create a bend, the press pushes the top tool downwards a set distance, bending the material inwards into the v-shaped bottom tool. Air bending is called air bending because a gap is left between the sheet metal being bent and the bottom tool when the sheet metal is at its full bend depth.

Bottom Bending

Bottom bending also uses a punch and bottom v-shaped die in a brake press. The difference is that the punching tool pushes the sheet metal fully into the die to form a bend that is the shape of the die. The specified bend angle determines the specific die to be used, and so it is necessary to select the correct die for each bend being performed.

Bottom bending generates less springback and creates more accurate angles. However, each bend radius will require a different bottom die, and the process requires more machine pressure. With air bending, many different bend angles can be produced with the same die, less pressure is needed, and the process is faster.

For a full insight into both methods, check out our guide here: Bottom Bending Vs Air Bending.

Figure 2: - Air and Bottom Bending (Skill-Lync)

Rolling

When a cylinder or curved part is required, sheet metal or plate can be rolled to the required curvature. This is achieved with a machine called a roller. Rollers range in size from around 3 feet/1 meter wide to over 5 meters. The thickness of material being bent can range from 1mm to 50mm+.

Figure 3: Bending Rollers (Barnshaws)

The most common rolling machines have 3 rolls, arranged as seen below in figure 4. The middle or top roll is moved closer to the bottom rolls (in some cases vice versa), and the material is then moved through the rollers as they spin. The material deforms as it moves through the rollers, obtaining a curved shape.

As with all bending processes, some springback will occur with rolling. As such, sheet metal parts are generally rolled to a slightly tighter radius than required to compensate for this.

Figure 4: Bending Rollers (Barnshaws)

Once the rolling process is complete, the bottom roller can be adjusted downwards to release the bent section of sheet metal. Otherwise, most rolling machines also have the provision to open the top end yoke as seen below.

One disadvantage, when using rolling to produce a cylinder, is that a pre-bend operation may be required to ensure each end of the cylinder meets after rolling is complete.

Figure 5: End Yoke Removal (A Rundown on Rolling Machines)

When to Use Sheet Metal Bending in Fabrication

Sheet metal bending offers a great deal of flexibility in terms of the type and thickness of metals that can be bent. Complex parts can also be produced. Bending processes can be used to create sheet metal parts and assemblies in every industry, including automotive, transport, domestic appliances, furniture, industrial equipment and more.

A wide range of metal types can be bent, including common metals such as steel and aluminum, as well as less common metals, such as copper and titanium. Thick materials can also be bent as well as thin materials. Note that the term sheet metal is typically used to refer to materials that are under 3mm in thickness. Sheet metal bending processes, however, can be used on materials that are as thick as 20mm.

In many cases, and with the advent of modern CNC machines that can do both cutting and bending, complete parts can be produced from one piece of sheet metal. Previously, welding or other joining techniques were required where they are now unnecessary. Being able to produce whole parts from one piece of sheet metal can cut costs and production times.

Where it's not possible to produce a complete part from one piece of material, sheet metal bending can often be combined with other value-adding operations without difficulty. Other fabrication processes, on the other hand, can present issues at this stage.

Mechanical fasteners, such as bolts or more permanent fixings such as rivets or welding, can be used to join bent parts to other parts, for example. Parts of different thicknesses can also be attached to one another as well as parts of the same thickness. Other processes, such as threading, chamfering, countersinking or boring, can also further increase the flexibility and versatility of sheet metal components.

Our article about Value Added Operations for Sheet Metal Components provides more information.

Figure 6: Sheet Metal Parts (Precision Sheet Metal Fabrication and Assembly in China)

Advantages

Speed of Manufacture - Once designed and programmed, due to the lack of tooling required and the high levels of automation available (many shops are able to run 24/7 with a handful of personnel monitoring production), sheet metal parts can be produced very quickly.

Accuracy - If the considerations that need to be made in the design phase are made adequately, sheet metal parts can be manufactured to a high level of accuracy. Advancements in fabrication techniques and equipment have made it possible to achieve accuracy levels of ±0.05 mm in some cases. As well as bending being accurate in the first place, accuracy can also be repeated consistently. This is particularly true with CNC bending machines with modern software and equipment.

Reduced Post-Processing  Other fabrication processes require post processing before a part is complete. Heat used in welding, for example, can cause dimensional distortion in a sheet metal part. Straightening may be required to correct this. Alternatively, with welding, weld spatter may need to be removed through time-consuming and labor-intensive grinding and polishing. Issues such as these usually aren't present with bending. Bent sheet metal parts are often ready to go, straight from production.

Less Weight  With sheet metal bending techniques, stiffness and strength can often be achieved in parts without using additional material during manufacture. This reduces part weight and can be beneficial to in-use part performance. This can also help to reduce issues associated with the transport of parts after production.

Low Cost, and Little-to-No Tooling Due to the advances in technology, using CNC bending processes often cuts down the manual labor required to produce sheet metal parts. As well as less labor being needed, work can also often be performed by unskilled workers rather than more expensive specialist workers.

Because most manufacturers carry a line of common tools (such as punches and dies) that can produce most standard bends, using sheet metal bending processes often eliminates the need for specialized tooling. This means no tooling investment and significantly shorter lead times, as there is no need to wait for complex tooling to be produced, tested or adjusted.

Reduction in Part Complexity - With bending, it's often possible to create relatively complex components from one piece of material instead of from multiple parts with joints. This reduces time, the potential for errors, failure points and procurement complexity.

Disadvantages

As with all fabrication processes, there are some downsides to using sheet metal bending, as detailed below.

Thickness Limitations A rule of thumb in sheet metal bending is that thicker materials have higher bend radiuses (Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending). As a result, tight bends are usually better performed on thinner sections of sheet metal rather than thicker ones.

This can mean that some complex parts can become limited to relatively lightweight materials, suitable for low-load or no-load applications. Bending excessively thick material can also result in the material bulging outward post bend (How Material Properties Impact Air Bending Precision and Tolerances), the material to crack if it is too rigid, or the need to move to a higher tonnage (and more expensive) press.

Need for Consistent Thickness Because it's optimal to produce parts from one piece of material instead of by joining different pieces, it's better if the thickness of separate flanges on a part does not change. This means that it may be necessary to design a part to have the same thickness throughout.

Cost of Manufacturing - Sheet metal bending is most competitively priced at low to medium volumes. Part volumes from the 100s to 1,000s are usually best. When volumes increase further, stamping is generally considered to be more cost effective, although this can depend on part geometry and other design specifications. This is because CNC bending requires components to be processed one bend at a time, while multiple bends can be produced at the same time through progressive stamping. Even roboticized bending (generally used for volumes of parts in the thousands) cannot compete with high-volume stamping costs.

While labor costs are usually reduced with machine assisted bending processes, in some cases they can be labor intensive. When this is the case, costs will be increased. Some specialist bending projects may also require custom tooling, which while significantly lower than custom stamping tooling, can still be a capital expense.

Production Issues In some cases, bending will cause indentations or scratches to occur on products during processing, due to the pressure exerted on the part through the narrow bending tool these types of bending marks are often visible depending on placement in the part. Fractures may also occur if hard metals are bent parallel to the direction sheet metal has been rolled in during production. Holes, slots and other features close to bends can also become distorted during bending. Finally, bends need to be in a position on the sheet metal where there is enough material for it to fit into the equipment without slipping during bending. These issues may all arise during production.

Sheet Metal Bending Compared to Other Fabrication Processes

 

 Process Best used for Process Precision Level Thickness (mm) Custom tooling required Minimum order quantity Lead Time from CAD to 1st production Laser cutting Small to large parts with every geometry possible ± 0.10mm 0.5mm to 20.0mm No 1 to 10,000 units Less than 1 hour CNC sheet bending Small to large parts with straight anglegeometry, multiple bend possible ± 0.18mm 0.5mm to 20.0mm No 1 to 10,000 units Less than 1 hour CNC Punching Small to large parts with most geometry available, good for parts with multiple holes and embossed ± 0.12mm 0.5mm to 4.0mm* No unless special form required 1 to 10,000 units Less than 1 hour Stamping  High volume production with tight tolerances, restricted geometry ± 0.05 to 0.10mm 0.5mm to 4.0mm* Yes from 250 USD to 100,000 USD+ 5,000 units 25 days to 40 days