CNC bending (paired with laser cutting) is one of the most underrated processes for both low and medium volume sheet metal production available, especially where quantities (several hundred to several thousand per lot or more) don’t justify the creation of costly, difficult to maintain stamping tools, or where speed and flexibility of production come at a premium. The capability to produce a wide variety of part geometries without tooling, the fast lead times, high levels of repeatability and automation mean that sheet metal bending is a key tool in the arsenal of product developers, engineers and business owners when looking to manufacture metal parts.

It’s important to understand the possibilities of sheet metal bending even at the design phase, as it is a tool which gives engineers enormous flexibility to create a wide variety of shapes, and in many cases, allows a part to be created from one piece of material, instead of multiple pieces joined together via hardware or welding, reducing overall costs and allowing for improved strength, simplified assembly and little to no tooling.

This guide will provide an overview into the sheet metal process, the advantages and disadvantages of each, basic design and material selection concerns and other information. This guide, paired with our other articles exploring the sheet metal and bending processes will give you a grounding to understand and discuss your product’s needs with sheet metal manufacturers such as ourselves.

There are multiple ways in which a part can be formed. Some are less commonly used than others, but offer bending that cannot be achieved with competing processes.

Brake Press

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

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

The brake press can be used for a very wide range of sheet and plate materials. From 0.5mm sheet up to 20mm plate and beyond. This is due to the flexibility of the tooling, and the high power of hydraulic machinery. Brake presses are specified by two general parameters: Tonnes 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 tonnes of force are needed. 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 operation can be categorized into two methods of operation: Air Bending and Bottom Bending.

The former and more commonly used method involves a bottom tool that is a 90° ‘V’ shape and a top tool of narrow shape with a rounded point. When bending, the press pushes the top tool downwards a set distance, bending the material into the bottom ‘V’. ‘Air’ bending refers to the gap left above and below the material at full bend depth.

Bottom bending also uses a punch and bottom v-shaped die but bends the metal by bringing the die and punch together.

Because the material is pressed into the bottom of the die, the desired bend angle determines the specific die to be used.

Bottom bending requires more pressure, generates less spring back, and creates more accurate angles. However, each bend radius will require a different bottom die

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 a specific curvature. This is achieved with a machine called a Roller. They range in size from around 3 feet/1 metre wide, to over 5 metres. The thickness of material 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), the material is then moved through the rollers as they spin. This deformation of the material causes it to retain this shape.

As with all bending processes, some spring back is seen, and the part is generally rolled to a slightly tighter radius than required.

Figure 4: Bending Rollers (Barnshaws)

Once the rolling process is complete, the bottom roller can be adjusted downwards to release, with most rolling machines also having provision to open the top end yoke as seen below, to remove the part. A disadvantage can be that a pre-bend operation is required prior to rolling to ensure each end of the formed cylinder meets after rolling.

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

Sheet metal typically refers to the use of material under 3mm thick, but laser cutting and bending can be used on materials in excess of this with ease. The flexibility of the process regarding range of materials, thicknesses and complexity of the parts it can produce makes it ideal for making a wide array of parts, used in every industry from automotive, transport, domestic appliances, furniture, industrial equipment and more.

Sheet metal bending is often combined with mechanical fasteners such as bolts or more permanent fixings such as rivets or welding. This gives even more flexibility, as parts of different thicknesses can be attached to one another depending on the particular use of each. There are a variety of other value-added operations such as threading, chamfering, countersinking, boring, etc. that can further increase the flexibility and versatility of sheet metal components. Our article about value-add operations for sheet metal provides more details about it (“Value Added Operations for Sheet Metal Components”).

In many cases, and with the advent of modern CNC cutting and bending machines, parts can be produced in one component, whereas previously welding or other joining techniques would be required.

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, and the high levels of automation possible (many shops are able to run 24/7 with a handful of personnel monitoring production), sheet metal parts can be produced very quickly, enabling large quantities of components to be produced in relatively short time.

Accuracy - if design considerations are made adequately, sheet metal parts can be extremely accurate, with laser cut holes being within ±0.1mm. There is a high level of repeatability here, as programmed laser cutters and CNC bending machines with the proper software and equipment produce with low levels of variability.

Less post-processing - welding often requires multiple processes to complete a part; often the heat can distort the material and requires straightening, and weld spatter needs to be removed via time consuming and labor-intensive grinding and polishing. Neither of these issues are present with bending – the part is ready to go straight from production.

Less weight - due to the complexity of bends available, stiffness and strength can be achieved whilst using relatively little material, thus also reducing the part weight. This is beneficial for every step in the supply chain including transport.

Low cost & Little to No Tooling - Due to the advances in technology, CNC cutting and bending cuts down the manual labour required to produce parts, the sheet metal bending process has benefited from this tech more than most, with CNC controlled tooling, parts can be produced by less-skilled workers in less time, all resulting in a lower end cost.

The laser cutting and sheet metal bending processes often eliminate the need for specialized tooling, as most manufacturers will carry a line of common tools that can produce most standard bends. 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 parts - Making a component from one piece of material instead of multiple parts with joints, reduces time, potential errors, failure points and procurement complexity

Disadvantages

As with any process, there are some downsides to sheet metal bending, as detailed below.

Thickness Limitations - the thicker the material, the higher the bend radius is a rule of thumb in sheet metal bending (“Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending”), so tight bends are usually performed on thinner sheet metal, meaning that some complex parts are 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”).

Need for Consistent Thickness - Due to the parts being made from one piece of material, the thickness of separate flanges cannot be changed, meaning the entire part should be designed with the same thickness.

Cost of Manufacturing - Sheet metal bending is most competitive at low to medium volumes, from the 100s to 10k part volumes. When volumes increase further, though this can depend on part geometry and needs, stamping is generally considered to be more cost competitive, as CNC bending requires components to be processed one bend at a time, while progressive stamping can have a higher rate of throughput and automation.

Advantages

• Highly accurate process ideal when high precision is required
• Can produce large volumes in a short time
  Low cost for production and minimal tooling costs
• Suitable for high or low volume production
• Can create multiple, custom shapes through a series of bending processes
• Standard punches and dies available including in V and U shapes

DISADVANTAGES

• The process can cause indentations or scratches on the product
• Can be labour intensive
• Custom tooling is required for specialized bending projects
• Bends need to be in a position on the sheet metal where there is enough material to fit into the equipment without slipping
• Fractures can occur when hard metals are bent parallel to the rolling direction of the sheet metal
• Holes, slots, or other features close to the bend can become

Table 2: When to use sheet metal bending (“Sheet Metal Fabrication”)

Designers need to be aware of the limitations and considerations of the process before completing designs for manufacture. Further design information can be found here.

Tonnage - as mentioned, presses have a maximum tonnage capacity, factors such as bend radius, material properties, material type and bend length all contribute to how much pressure is required to make the bend. Check with our engineering team before committing to a design you are unsure of.

Bend Length - another critical pressing variable, bend length is set at a maximum by the physical machine size and configuration. It’s best to seek guidance if your parts are above 2m as this is a standard sheet and press brake size.

Heat Affected Zones (HAZ) - processes such as laser and plasma cutting create heat affected zones in the material. These can sometimes cause issues with forming, such as inconsistent bending near holes and edges. Another issue sometimes seen is cracking due to the increased surface hardness from cutting.

Springback - is the “bounce” back of the metal after the press has been applied and removed. The sheet metal is compressed on the inside, where the press is applied, and stretched on the outside. Because the material has a higher compression strength than tensile strength, it springs back towards its original shape. It is difficult to calculate springback accurately, but it needs to be considered when calculating the bend.

Fabricators use the K-factor to calculate the springback factor and better understand how to compensate and achieve tighter tolerances. The material’s plasticity is also a factor for springback. Plasticity is a measure of the material’s ability to deform without breaking and retain that shape. Higher plasticity metals are often better choices for forming and bending. 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

CNC Sheet Bending Tolerances - the process of bending does require manual operators and different types of equipment can achieve different bend radius tolerances.

• You can’t typically achieve a true 90° corner.
• Corners will have radius
• Bend angles standard tolerance of ± 1°
• Bend length tolerance are typically ±0.25 mm (0.010”)

It’s important to take these types of factors into consideration when designing your sheet metal components – consulting with an experienced sheet metal fabricator can help ensure your part is ready for production.

Almost all engineering materials are available in sheet form, and thus can be bent in some form. There are however differences in process limitations caused by the differing material properties.

Sheet metal is available in a selection of sizes commonly referred to as gauges. These range from 50 or 0.03mm, to 1 gauge which is 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 millimetre thicknesses. This is especially the case in Europe. One exception is Aluminium, 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 material available, refer to our standard material page.

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

Each Metal has its own unique characteristics, the following table outlines some of the factors that 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, it 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, as rust will form in the presence of moisture. Galvanised steel is available to counter this issue, providing a hard wearing pre-applied zinc coating to prevent rust.

Aluminium - first used for aircraft production, various Aluminium alloys are available, with a very wide range of applications. Because Aluminium alloys with other elements so successfully, an incredibly wide range of material properties can be sourced.

The most used ones for sheet metal applications are the 1000 series, namely 1060. This is due to its high workability and, and low weight. The 6000 series is also widely used in sheet metal bending. Workability specifically allows the material to be bent to tight radii without cracking, 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, namely containing over 10.5% Chromium. This gives the material corrosion resistance, with some grades excelling at resistance to various acids, alkali and other chemicals.

Commonly used grades of Stainless Steel are 301, 304 and 316, with the latter having higher strength and corrosion resistance, 301 have 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 limitations and characteristics of the process in mind (“Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending”). We will discuss these below, but for even more information, refer to our sheet metal design guide.

Bend Radius - When a material is formed into bends, the outer surface is stretched, and the inner compressed. This gives the bend a rounded corner. This is termed as the bend radius and differs from one material to another. Other factors that determine the size of this radius are tooling geometry and material condition.

It is good practice to ensure that all bend radii on a particular part are equal, as this greatly simplifies tooling set up and reduces cost.

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 the bend, these stresses will be focussed on this hole, and deform it.

Bend to Bend Distance - when making bends, there is a physical limit enforced by the size and shape of the tooling, on how close bends can be together. Bends on the same side of the sheet that are too close will interfere with the tooling, and bends on opposing sides will be impossible to reach due to the width of the bottom tool.

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

If your bends need to be close together workarounds can be used to enable this, as well as implementing supplementary processes such as welding or bolting to get to the correct geometry. Get in touch for more information if this is required.

Spring Back - due to the elasticity of metals, bends will tend to return to their original position to a small degree, this is termed ‘spring back’. In practice, this generally only amounts to 1-2° and is accounted for in the brake press control. It does however mean that highly acute angles should generally be avoided when designing a part.

Pressing Tolerances - as with any process there are tolerances on dimensional accuracy, CNC control has reduced these in recent years, but they are still pertinent, particularly when designing complex or precision parts.

Due to the variation in sheet metal composition, thickness and processing, bending always has some degree of variation. These should be considered when designing parts and each process should be utilised to its strengths. For example, if a critical pattern of holes is required, try to use a single flat, laser cut pattern for these, rather than relying on holes on bent flanges.

This is however only for extreme circumstances, and most tolerances can be achieved with modern press brake machines, contact us for more information on process tolerance.

Sheet Metal Fabrication has distinct advantages over the alternative 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 than traditional fabrication.

What is sheet metal bending?

Bending is one of the most common sheet metal fabrication techniques. It is used to deform a material to an angular shape. The bending of sheet metal allows a wide variety of part geometries to be produced using two processes: cutting and bending. The most common methods for each process respectively being CNC Laser Cutting and bending on a Brake Press.

What is sheet metal bending used for?

Sheet metal typically refers to the use of material under 3mm thick, but laser cutting and bending can be used on materials in excess of this with ease. The flexibility of the process regarding range of materials, thicknesses and complexity of the parts it can produce makes it 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, accuracy, less post processing, less weight, low cost, little to no tooling, and reduction in the number of parts.

What are the disadvantages of sheet metal bending?

As with any process, there are some downsides to sheet metal bending. These include thickness limitations, need for consistent thickness, and the cost of manufacturing. The specific downsides will differ from one bending method to another.

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 and rolling. You may also see several variations such as v-bending, bottoming, air bending, coining, u-bending, and step bending.

Which materials are suitable for bending?

Almost all engineering materials are available in sheet form, and thus can be bent in some form. There are however differences in process limitations caused by the differing material properties. For the best information on the material 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, spring back and processing tolerances. For more information, refer to our sheet metal design guide.

• “An Engineer’s Guide to Laser Cutting.” Komaspec, www.komaspec.com/about-us/blog/an-engineer-s-guide-to-laser-cutting. Accessed 24 Dec. 2021.
• “Differentiating Air Bending and Bottom Bending for Sheet Metal Fabrication.” Komaspec, www.komaspec.com/about-us/blog/differentiating-air-bending-and-bottom-bending-for-sheet-metal-fabrication. Accessed 24 Dec. 2021.
• Bystronic Inc. “The Rules of Press Brake Tool Selection.” Bystronic Inc., www.bystronicusa.com/en/news/technical-articles/190502-rules-of-press-brake-tool-selection.php. Accessed 24 Dec. 2021.
• “Press Brakes.” LVD Group, www.lvdgroup.com/en/products/press-brakes. Accessed 11 Jan. 2022.
• Skill-Lync. “Week 3 Sheet Metal Bending Challenge.” Skill-Lync, www.skill-lync.com/student-projects/week-3-sheet-metal-bending-challenge-49 Accessed 24 Dec. 2021.
• Benders, Barnsahws Steel. “The Recent History of Design and Selection of Plate Bending Machines – Part 3.” Barnshaws, 12 Sept. 2018, www.barnshaws.com/information/articles/the-recent-history-of-design-and-selection-of-plate-bending-machines-part-3.
• “A Rundown on Rolling Machines.” Thefabricator, 4 Oct. 2010, www.thefabricator.com/thefabricator/article/bending/a-rundown-on-rolling-machines#gallery-3.
• “Value Added Operations for Sheet Metal Components.” Komaspec, www.komaspec.com/about-us/blog/value-add-operations-for-sheet-metal-components. Accessed 24 Dec. 2021.
• “Precision Sheet Metal Fabrication and Assembly in China.” Komaspec, www.komaspec.com/materials-metals-and-plastics. Accessed 24 Dec. 2021.
• “Designing Sheet Metal Components Using Laser Cutting and CNC Sheet Bending” Komacut, https://kmc-portal-files.s3.ap-southeast-1.amazonaws.com/media/zbzjnm0z/komacut_chapter-3.pdf. Accessed 24 Dec. 2021
• “How Material Properties Impact Air Bending Precision and Tolerances” Komacut, https://kmc-portal-files.s3.ap-southeast-1.amazonaws.com/media/gj5f3o5x/komacut_chapter-6.pdf. Accessed 24 Dec. 2021
• “Sheet Metal Fabrication” Komacut, https://kmc-portal-files.s3.ap-southeast-1.amazonaws.com/media/mxtlsmbg/komacut_chapter-1.pdf. Accessed 24 Dec. 2021
• “Sheet Metal Gauge Conversion Chart.” Metalsheets, 14 July 2021, www.metalsheets.co.uk/sheet-metal-gauge-chart.
• “Sheet Metal Standard Options in China: Q235, Q345, AL6061, AL5052H32, SS303, SS304, MN65.” Komacut, www.komacut.com/materials/sheet-metal-materials. Accessed 24 Dec. 2021.
• “5 Most Popular Types of Metals and Their Uses” Texas Iron and Metal, 2 Apr. 2021, www.texasironandmetal.com/most-popular-metal-types.
• AZoM. “Aluminium / Aluminum 1060 Alloy” AZoM.Com, 12 June 2013, www.azom.com/article.aspx?ArticleID=6587.
• “Differentiating Air Bending and Bottom Bending for Sheet Metal Fabrication” Komaspec, www.komaspec.com/about-us/blog/differentiating-air-bending-and-bottom-bending-for-sheet-metal-fabrication. Accessed 24 Dec. 2021.
• Burnett, Chris. “What Is Stainless Steel? Part I.” Analyzing Metals, 18 Feb. 2014, 
• “Sheet Metal Design Guidelines: Component Design.” Komaspec, www.komaspec.com/about-us/blog/sheet-metal-design-guidelines-designing-components. Accessed 24 Dec. 2021.