• What is Sheet Metal Bending?
• Types of Sheet Metal Bending
• When to Use Sheet Metal Bending in Fabrication
• Sheet Metal Bending Compared to Other Fabrication Processes
• Materials Suitable for Bending
• Design Considerations for Sheet Metal Bending
• In Summary
• FAQ Section

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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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 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”)

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.

Table 2: Sheet Metal Bending Compared to Other Fabrication Processes (“Sheet Metal Fabrication”)

Almost all engineering materials are available in sheet form, and thus can be bent to some degree. There are, however, different processing limitations with different materials because of their different inherent properties.

Sheet metal is available in a selection of sizes, which are commonly referred to as gauges. These range from gauge 50 (or 0.03mm), to gauge 1 (7.62mm). Bending with a brake press can be performed with all these thickness gauges and higher (“Sheet Metal Gauge Conversion Chart”).

Gauge is a traditional term still widely used, despite many materials, such as steel and stainless steel, being specified directly in their millimeter thicknesses. This is especially the case in Europe. One exception is aluminum, which is often still defined in all three dimensions by imperial measurements, i.e. feet and inches, and gauge for thickness.

For the best information on the materials available, refer to our standard materials page.

Fig 7. Sheet Metal Parts (“Sheet Metal Surface Finishing Standard Options”)

Each metal has its own unique characteristics, and the following table outlines some of the factors you should consider when making your choice of materials.

Table 3. Sheet metal materials (“Sheet Metal Standard Options in China”)

Mild Steel - This is available in both hot and cold rolled variants. Both offer excellent cold working performance, with high ductility. Also known as low carbon steel, mild steel is the most commonly used material in the world (“5 Most Popular Types of Metals and Their Uses”).

The largest downside to mild steel is the requirement for coating, which is needed to prevent rust from forming in the presence of moisture. Galvanized steel is available to counter this issue. This comes with a hard wearing pre-applied zinc coating that prevents rust.

Aluminum - First used for aircraft production, various aluminum alloys are available, with a very wide range of applications. Because aluminum alloys with other elements so successfully, an incredibly wide range of types of aluminum alloy can be sourced. These come with a range of different properties.

The most used aluminum alloys for sheet metal applications are the 1000 series alloys, particularly 1060 aluminum. This alloy is widely used due to its high workability and low weight. The 6000 series is also widely used in sheet metal bending. The high level of workability in these metals allows the material to be bent to tight radii without cracking, specifically, and this is often vital for complex parts (“Aluminium / Aluminium 1060 Alloy”).

For general guidelines to material suitability for CNC bending, see the table below:

Table 4: Material Properties (“How Material Properties Impact Air Bending Precision and Tolerances”)

Stainless Steel - Commonly used in the food and medical industries, stainless steel is an alloy of mild steel. To be stainless steel, steel must contain over 10.5% chromium. This gives the material corrosion resistance, with some grades excelling at resistance to acids, alkalis and other chemicals.

Commonly used grades of stainless steel are 301, 304 and 316, with the latter having higher strength and corrosion resistance, 301 having superior flexibility and “spring” and 304 being a good middle of the road material for general use (Burnett).

Parts that are to be processed using bending equipment should be designed from the outset with the characteristics and limitations of the bending process in mind (“Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending”).

We will discuss the main considerations that need to be made below. For even more information, however, you can refer to our sheet metal design guide.

Bend Radius - When a material is formed into a bend, the outer surface is stretched, and the inner surface is compressed. The result is that the part has a rounded corner at the bent edge on both the inside and outside.

The bend radius is a measurement of the curvature of the inside bend edge. The bend radius that is possible with a section of sheet metal will differ depending on the material being bent as well as the tooling geometry and material condition.

It’s good practice to ensure that all bends on a particular part are equal in radius because this greatly simplifies tooling set up, reducing cost.

Bend Length - Another critical pressing variable is bend length. The bend length required will usually depend on the design specifications of a sheet metal part. Bending machines, however, all have maximum widths according to their physical size and configuration. It’s best to seek guidance if your parts are above 2 m as this is a standard sheet and press brake size.

Bend to Bend Distance - When making bends, a physical limit on how close bends can be together is enforced by the size and shape of the tooling being used in the bending machine. Bends on the same side of a metal sheet that are too close will interfere with the tooling, and bends on opposing sides will often be impossible to reach because of the bottom tool.

Common instances of this occurring are where ‘U’ sections are required with the legs or upright flanges being longer than the horizontal section. In some cases, extra deep tooling can be used.

If your bends do need to be close together, it may be possible to find workaround solutions. Alternatively, it may be possible to implement supplementary processes, such as welding or bolting to get to the correct geometry.

Hole to Edge Distance - When bends are produced, the material is stretched. This causes internal stresses that are evenly distributed across the part. If a hole or slot is made too close to a bend, these stresses will be focused on this hole, and this could cause deformation.

Springback - Metals have elasticity and will tend to return towards their original position to a small degree after bending. This effect is called ‘springback’. The exact process is related to metal’s compressive and tensile strength. After bending, sheet metal is compressed on the inside, where the press is applied, and stretched on the outside. Because the material has a higher compressive strength than tensile strength, it springs back towards its original shape.

In practice, springback generally only amounts to 1-2°. This can often be sufficiently compensated for in brake press control because many sheet metal parts don’t need a high level of accuracy. The latest CNC bending machines even incorporate built-in sensors and control to automatically compensate for material variability and other factors to ensure consistent performance.

Where a high level of accuracy is needed, springback can be a challenge because it can be difficult to accurately calculate how much there will be.

Several factors affect springback:

• Materials with higher tensile strength have more springback.
• A sharp bend radius usually has less springback.
• Wider die openings result in more springback.
• The larger the bend radius relative to the material thickness, the more the springback.

Fabricators often use the K-factor to calculate springback and better understand how to compensate and achieve tighter tolerances where accuracy is needed.

Pressing Tolerances - As with any fabrication process, there are tolerances on dimensional accuracy. These often arise due to variations in sheet metal composition, thickness and processing. Variation should be considered when designing parts, and each process should be utilized to its strengths according to the material being used and part specifications.

CNC control has reduced variation in recent years, and most tolerances can be achieved with modern press brake machines. It can, however, still be a pertinent issue, particularly when designing complex or precision parts.

Here are some rules of thumb:

• You can’t typically achieve a true 90° corner.
• Corners will have a radius.
• Bend angles have a standard tolerance of ± 1°.
• Bend length tolerances are typically ±0.20 mm per bend (0.010”).

Consulting with an experienced sheet metal fabricator can help you better understand tolerances in sheet metal bending processes.

Tonnage - Factors such as bend radius, material properties, material type and bend length all contribute to how much pressure is required to make a particular bend. As mentioned, presses have a maximum tonnage capacity, and it may be worth checking that it will be possible to perform the bends you need. Check with your manufacturer before committing to a design you are unsure of.

Heat Affected Zones (HAZ) - Processes such as laser and plasma cutting create heat affected zones in metal. These can sometimes cause issues during bending, such as inconsistent bending near holes and edges. Another issue sometimes seen is cracking due to the increased surface hardness from cutting. If your parts will need other processes that create heat, these issues may need to be taken into consideration.

Table 5: CNC Sheet Metal Bending - Process Tolerances and Techniques

Table 6: Minimum Distance Guidelines for Features Relative to Bends

Sheet metal bending has distinct advantages over alternative sheet metal fabrication processes, including higher output, lower cost and high flexibility in design. It also removes many difficulties associated with assembly techniques such as welding or riveting. With careful consideration during the design process, and with the aid of modern technology, sheet metal parts can be made stronger, lighter and more quickly through bending than through traditional fabrication methods.

The sheet metal specialist at Komaspec are happy to work to review your product design together and to help you select the fabrication process that best suits your product design and application needs.

What is sheet metal bending?

Bending is one of the most common sheet metal fabrication techniques. With bending, metal is deformed with specialist machinery into an angular shape. The bending of sheet metal allows a wide variety of part geometries to be produced and is particularly useful when performed alongside cutting. The most commonly used method for each process is CNC laser cutting and brake press bending.

What is sheet metal bending used for?

Sheet metal typically refers to the use of material under 3mm thick, but bending can be performed on materials far in excess of this, up to 8mm or more depending on material yield strength and the tonnage of the press being used. The process is very flexible regarding the range of materials and thicknesses that can be processed and the complexity of the parts that can be produced.

Bending is ideal for making a wide array of parts, used in every industry from automotive, transport, domestic appliances, furniture, industrial equipment and more.

What are the advantages of sheet metal bending?

Some of the key advantages of sheet metal bending include the speed of manufacture, high accuracy levels, reduced post processing, reduced part weight, low cost, reduction or elimination of tooling and the reduction in the number of parts produced during manufacture.

What are the disadvantages of sheet metal bending?

As with any process, there are some downsides to sheet metal bending. These include thickness limitations, the need for consistent material thicknesses, the extra tolerance (roughly ±0.2mm per bend) and the cost of manufacturing in some circumstances. CNC bending also requires relief cuts to be added to the part in order to prevent material deformation or “mushrooming” due to the bending process.

The specific downsides will differ from one bending method to another. Stamping will have lower cost and higher precision, but there is a requirement for custom tooling. Rolling is limited in the bend geometry it can produce and is often a fairly manual process.

What are some of the bending methods?

There are a variety of bending techniques available. Each has its own advantages. Some are more accurate and others simpler. The major bending techniques include brake press bending and rolling. You may also see several variations of these methods, such as v-bending, bottoming, air bending, coining, u-bending and step bending. Sheet metal bending can also be accomplished by a type of stamping tool.

Which materials are suitable for bending?

Almost all engineering materials are available in sheet form, and thus can be bent to some extent. There are however differences in process limitations caused by the differing material properties. For the best information on the materials available, refer to our standard material page.

How to design a part for bending?

Parts that are to be processed using bending equipment should be designed from the outset with the limitations and characteristics of the process in mind. You will have to consider the bend radius, hole to edge distance, bend to bend distance, springback and processing tolerances. For more information, refer to our sheet metal design guide.

Table 7: Tolerances Guidelines for Sheet Metal Fabrication

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