What Is Sheet Metal Bending? Basics, Allowance, and Tips

The rise in the demand for custom-made products resulted in research that proved sheet metal to be a versatile material that can be transformed into different forms. The transformation occurs by employing simple sheet-forming processes such as metal bending to shape the sheet metal into the desired forms needed for various production purposes. There are many processes involved in making this happen, and knowing how to bend sheet metal comes with having a fair knowledge about them.

This article will explore the importance of sheet metal bending, its importance in sheet metal fabrication processes, and how to bend sheet metal. It also includes some useful bending tips which can guide on bending steel plates.

What’s Sheet Metal Bending?

Do you know most sheet metal parts are made by first cutting into specific sizes, then bending into the desired shape, and assembling them? So, it plays a crucial role in sheet metal fabrication.

Sheet metal bending refers to deforming the flat metal sheet into a specific angle or curvature. It does not change the thickness of the sheet. Instead, the plastic (permanent) deformation is responsible for the formation of the final shape. Typically, a press brake or similar too applies pressure along a straight axis to bend the metal into a specific angle.

You also need to understand the punch-and-die setup to grasp the basic concept. A punch applies force to deform the metal against the die. Meanwhile, the die supports and shapes the metal with the desired bend angle and radius.

How Is Sheet Metal Bent?

Step 1: Initial Design

The metal bending process starts with the creation of a detailed design of the final part. The CNC bending requires 3D files and you can draft the design in software like AutoCAD and Solidworks. Subsequently, the design must include a wide variety of considerations, including allowance, reliefs, springback, etc.

You can use a bending calculator available online to calculate the design variables and considerations.  We will discuss these in further sections. Furthermore, you need to specify the clear dimensions and tolerances in the design. 

Step 2: Preparing Your File

Ensure your file is in the compatible format, and all GD&Ts are manufactured. Next, the bend line indicator is a crucial element in communicating the design between engineers and technicians. It might be denoted by different symbols according to the software and file format, solid, dashed center lines, or even separate colors.

At RapidDirect We support the following file formats;

• 2D: .dxf, .ai, and eps.
• 3D: step, and. stp.

Step 3: The Bending Process

In the bending process, sheet metal is deformed along a straight axis to create a desired angle or curvature. Arrange the tooling (die, punches, press brakes) based on your requirements and specified angle. The process forms complex parts but has limitations: no angles over 130°. Consequently, the bend radius varies by material and thickness.

Step 4: Finishing Processes

The sheet metal operations leave some aesthetic defects on the surface like die marks and ununiformed texture. To enhance this, you can use a suitable surface finishing technique. For example, painting, powder coating, sandblasting, plating, etc. However, you can also leave the surface as it is if it does not influence the performance and aesthetic is not your priority.  

Types of Metal Bending Processes

Sheet metal bending methods are similar in that their end goal is to transform metal structures into the desired forms. However, they differ in operation. Knowing how to bend sheet metal comes with an understanding that factors such as the thickness of the material, size of the bend, bend radius, and intended purpose determine the methods.

The following methods will not only demonstrate how to bend sheet metal but also help you choose the right technique for the best results. The most common sheet metal bending methods are:

V-bending

This is the most common sheet bending method as it is used for most bending projects. It employs an instrument known as the punch and v-die to bend sheet metals at desired angles. During the process, the bending punch presses on the sheet metal placed over the V-die.

The angle formed by the sheet metal is dependent on the pressure point of the punch. This makes this method easy and efficient as it can be employed for bending steel plates without altering their position.

The V-bending method can be classified into three:

Bottoming

You can relate bottoming with the air bending process, but the difference is that the punch presses the sheet into the die until it fully contacts the cavity surface. This mechanism solves the springback risk defect associated with the air method.

Bottoming requires a punch with more tonnage as it applies additional force for the deformation and also holds the sheet for a moment after the completion of the process. It is compatible with V and V dies.

Furthermore, this technique is more accurate, whereas you don’t need precise control over tonnage compared to other processes. So, old and less accurate punches and press brakes are also fit for the execution of bottoming.

Coining

Coining involves, squeezing the sheet between a punch and die under high tonnage. As a result, deformation produces accurate bent angles with almost no spring-back effects.

Although the precision is high, coining requires higher tonnage, and cycle time is also longer than other methods or processes.

Air Bending

Air bending or partial bending is a less accurate method compared to bottoming and coining. However, it is usually employed for its simplicity and ease of manipulation since it doesn’t require tools.

There is also a disadvantage. Air bending is the only method prone to the spring-back of the sheet metal.

In air bending, the punch exerts a force on the sheet metal resting on both points of the die’s opening. A press brake is usually employed during the V bending as the sheet metal does not come in contact with the bottom of the die.

(A press brake is a simple machine employed in sheet metal bending processes, it forms desired bends on the sheet by working to clamp the sheet metal in position for press between the punch and the die)

Roll Bending

The roll bending process uses 2, 3, or 4 rollers to shape metal sheets into desired curves. The most common setup is the 3-roll configuration, where three rollers are arranged in a triangular pattern. The upper roller is adjustable, while the other two remain fixed.

The metal sheet is fed between the upper roller and the two fixed rollers. As the two fixed rollers rotate, they grip the sheet, while the adjustable roller applies downward pressure to achieve the required curvature. The 4-roll setup adds an extra roller for additional support, making it ideal for heavy-duty operations.

This method is mainly used to produce cylindrical and conical shapes in sheet metal fabrication, such as tubes, cylinders, tanks, pressure vessels, and pipes.

Wipe Bending

Wipe or Edge bending uses a wiping die and punch. The sheet is clamped between the die and a holding pad, exposing the portion to be formed. The punch or wiping flange then moves downward, forcing the edge of the part into the desired angle. This method is an excellent alternative to using a press brake for smaller profiles.

With this technique, all sides of the edge can be shaped simultaneously, significantly boosting productivity. Additionally, the risk of surface cracking in the deformed area is minimal.

Rotary Bending

It is more common for tubes and pipes to shape curvatures from 1 to 180 °. However, it is not limited to bent sheet metal. The process included a bend die, a clamp die, and a pressure die. The bend die and clamp die to hold the work, meanwhile, the pressure die applies tangential pressure to the reference position from the free end. Here, the rotary die can rotate according to the desired position and radius. Additionally, a “mandrel” is placed inside the tube or pipe, whereas it is not required for sheet metal worksheets.

This metal-forming process is suitable for creating curvature shapes from flat sheets. Meanwhile, it has numerous applications in tube shaping. 

You get more control over the process to maintain an accurate and precise radius. It can easily achieve a tolerance of ± 0.5 °. Consequently, the surface will be less prone to cracking and other defects as it requires 50 to 80% or less tonnage.

Sheet Metals for Bending

Different varieties of metal and alloys are compatible with the bending fabrication and properties of each material type influence the variables like tonnage and spring back. Subsequently, the large pool of material options allows you to choose the best fit for the required functionality and performance. 

Moreover, the maximum thickness of metal sheets that can be formed varies depending on the material type. For instance, aluminum, with its higher formability, can be shaped into thicker sheets than titanium.

Stainless Steel

Stainless steel is a versatile material with high strength, toughness, and corrosion resistance, making it suitable for forming parts with tight radii. Various grades, such as 304, 316, and 430, are commonly used. Due to its hardness, shaping stainless steel requires greater force, and the spring-back effect needs to be carefully accounted for to ensure accuracy.

Steel

Steel alloy grades like A36, 1018, and 4140 are popular in metal bending due to their high tensile strength, durability, cost-effectiveness, and versatility. While steel may require heat treatment for more complex operations, it is still easier to work with compared to stainless steel. Mild steel, in particular, is relatively easy to shape.

Aluminum

Aluminum is ductile and easy to form in different shapes and curvatures. It offers excellent corrosion resistance and strength-to-weight ratio. The aluminum bent parts are widely used in aerospace, automotive, and electronics. However, it could be susceptible to cracking, especially with the smaller radii.

Brass

Brass is a malleable and conductive material, easier to bend than steel. Various grades, such as CZ129/CW611N, are commonly used for forming sheet metal. Brass is often preferred in electrical, thermal, and plumbing applications due to its ease of forming and excellent conductivity.

Copper

Copper is a soft material and sheets are easy to bend. However, it requires careful handling and controlled force to avoid surface damage or cracking. Additionally, copper’s shiny aesthetic appeal makes it popular in electrical and various other applications.

Key Concepts in Sheet Metal Bending

You can find different concepts in sheet metal bending, which are design considerations to incorporate with the dimensions after the process. Before the key concept, let’s understand some terminologies related to this.

• Neutral Axis: The imaginary line in the sheet metal that doesn’t stretch or compress while exerting the force.
• Tension Zone: The area on the outside of the bend where the metal stretches.
• Compression Zone: The area on the inside of the bend where the metal compresses.
• Bend Line: The line where the sheet metal is bent.
• Flange Length: The length of the straight and flat section extending from the bent.

Next, the key concepts are as follows.

Bend Radius

It is the radius of the curved sheet metal formed after you bend sheet metal. All designs start with this crucial variable. It significantly influences the dimensional accuracy, final strength, shape, and structural integrity. 

Based on the material type and thickness, there is a minimal value for this radius. It means, you cannot bend the sheet metal at a very small radius, there is a threshold. Typically, you have to maintain a radius as low as equal to sheet thickness. 

Minimum Bend Radius(R_min)= Thickness(t)

Bend Deduction

The total length of the flat section slightly decreases after the operations as the bent portion stretches some material. Here, you need to deduct some length to get the total flat length, known as bend deduction. So, it refers to the amount of material that needs to be subtracted from the total length of the flat sheet metal to achieve the desired dimensions. It means you need to deduct a length to determine the correct flat length.

Bend Deduction=2× (Outside Setback−Bend Allowance)

Considering the deduction in design is essential to ensure the correct length and other specifications of parts. Moreover, the sheet metal gauges(thickness), radius, and material type affect the deduction value.

Bend Allowance

Bend allowance is a manufacturing term that refers to the allocation given to accommodate sheet metal’s stretch and bending. When sheet metal is manipulated from its original flat shape, its physical dimensions are also altered. The force employed in the work causes the material to compress and stretch on the inside and outside.

This deformation causes a change in the sheet metal’s overall length due to the exerted force of compression and stretching on the bend. However, the length calculated from the thickness between the inner compressed surface and the exterior under tension remains the same. This is indicated by a line called the “neutral axis.”

The allowance considers the thickness of the sheet metal, the angle, the method employed, and the K-factor (a constant that allows to estimate of the amount of stretch of a material). It is a measure of the ratio of compression on the inside to the tension on the outside of the bend.

While the sheet metal’s inner surface contracts, the exterior expands. Therefore, the K factor remains constant whenever you bend sheet metal. The K-factor (usually between 0.25 to 0.5 max) serves as a control figure in design variable calculation. It aids in the determination of the exact materials required before trimming portions of the sheet metal and is also useful in the sheet metal bend radius chart.

K-Factor

It is another critical aspect of sheet metal bending design. The k-factor characterizes different bent sheet metal geometry and helps to calculate other design variables like required allowance. The K-factor is defined by the “Ratio of length that the neutral axis shifted from the original position to sheet thickness.” The K -factor value ranges from 0 to 1. For example, 0.2 represents that the neutral axis will be shifted by 20% of thickness. Additionally, the recommended value is different for each material type and bend radius.

K factor also gives an idea about the material stretched and expanded inside and outside of the bend. Therefore, it is crucial to calculate the flat length-related design parameters.

Bend Relief

The relief is defined as a small cut at the end of a bend line to prevent the deformation and tearing of material. It is essential for both structural integrity and precision of end parts and products. You can use notches, holes, and cutouts.

However, you don’t need to consider it for a straight bend from one edge to another. Only consider if they are required to separate from flat material other than the edges. The reason behind this is, that if there is immediate material after the compressed material, you need to adjust the flat material.

The rule for calculation;

Minimum width and depth of the relief= Thickness(t)/2, Thickness (t) + bend radius(R) + 0.5 mm.

Another similar concept is corner relief; it is the length that needs to be cut out at the meeting position of the bent line. So, at the corners, you need to consider a cut-out for their perfect alignment and avoid material tearing.

Springback

Sheet metal tends to differ in its shape created under applied force and after releasing the force. It might contract once you bend the metals into a specific curvature of shape, affecting the dimensional accuracy. Therefore, the designs require some compensations to spring back for precision.

To understand this phenomenon, you need to know the permanent and elastic deformations. Elastic deformations try to retain their shape, whereas permanent keeps the deformed shape unchanged. Some elastically deformed material around the bending line attempts to return to its original shape and cause spring back. Additionally, factors like employed method, radius, and material properties also affect the spring back.

Bend Sequence

It is the systematic approach to designing multiple bents in a single sheet without any interference or distortion. Bend sequence involves sequencing them based on their size and complexity. The typical order is first large and simple and more complex one subsequently. The sequence is also related to the die and tooling. It needs to be feasible with the corresponding tooling (dies and press brake).

Grain Direction

All metallic structures are crystalline lattices internally, which are repetitive arrangements of atoms. Consequently, grains are the individual crystalline regions within the metal. The orientation and shape of these grains can be different for each material type and method of forming like forging, casting, etc.

In press braking, considering the grain direction for tighter angles or curvatures helps to reduce the fracture risk. Meanwhile, the grain direction should be perpendicular to the bend to prevent cracking.

Practical Tips for Designing Bent Sheet Metal Parts

Sometimes a simple negligence or error in sheet metal design can bring challenges to bent sheet metal. Therefore, every single feature and detail matters in the quality of the final parts. 

The following are some practical tips for designing;

Maintain Uniform Thickness

The worksheet must have a uniform thickness throughout the cross-section. Otherwise, it results in an inconsistent bend radius and increases the risk of cracking or warping. Typically, you can choose between a uniform thickness of 0.5 to 6mm.

Bend Radius and Orientation

There is a certain limit of minimum bend radius and it differs based on the material type and thickness. A general rule of thumb is that “the minimum radius should be at least equal to the sheet thickness”. Regarding the orientation, maintain a consistent radius throughout the bent line and also keep them in the same plane. 

Avoid Successive Bends

If you place the bends too close to each other in design, it can cause alignment issues and increase the residual stress. Thus, an appropriate distance between them is essential, at least three times of thickness.

Use Bend Relief

If the bends are close to the end, it can tear or crack due to excessive stress. To avoid this, use reliefs, such as small cutouts and notches at the start and end of the line.

Proper Hole and Slot Placement

If your design includes holes and slots, you need to be careful about their placement like their size and distance from the bend. Too close holes from the curvature line can cause material distortion.

• Minimum distance (bend to hole) = 2.5 t + R
• Minimum distance (slot to hole) = 4t + R
• Minimum distance (edge to hole) = 3t
• Minimum hole radius (r min.) = 0.5 t

Above, t is sheet thickness and R is bending radius.

Countersink Design

These features can be achieved with machining or punching with a press brake. There are some rules about their placements in designs;

• Maximum depth= 0.6 t 
• Minimum distance from bend: 3t
• Minimum distance from edge: 4t
• Distance between two countersinks= 8t

Correct Curl Dimensions

Curl refers to bending a circular roll(hollow) at the edge of a metal sheet. It is used to maintain the strength of edges and avoid sharpness. Consider the following factors while designing a curl feature;

• Minimum outer radius= 2t
• Minimum distance (bend to curl) = Curl radius + 6t 
• Minimum distance (hole to curl) = 2 x Curl radius + t
• Lastly no intersection between curl and other features 

 Designing Hems

Hems are folded-back edges on sheet metal parts, which can be opened or closed. Sometimes, joining two hems also acts as fasteners. Bend sheet metal with ensuring the following criteria;

• Minimum inside radius= 0.5 t
• Minimum return length for the closed hem= 4t
• Minimum return length for the open hem= 4t
• Inside edge of bend to hem’s outside edge= 5t + Hem radius 

Flange and Chamfer Design

A flange is an edge that extends from the main body of a sheet metal part, typically at 90°. If you have flanges in the design, consider the following dimensioning limits;

• Minimum flange length = 4t
• Minimum bend radius= t
• Minimum bend-to-flange distance= 2t

Tabs and Notches 

Tabs and notches are the sheet metal features mostly used for joining purposes. A tab is a small extension on the edge, whereas a notch is a small cut-out. They can weaken the material in case of inappropriate positioning. So, consider the following design rules;

• Minimum bend to notch distance= 3t + radius (R)
• Minim distance between notches= 3.18 mm 
• Minimum notch length= 2t 
• Minimum notch width = 1.5 t 
• Maximum tab and notch length= 5 x width of tab (w)
• Notch corner radius= 0.5 t 

5 Tips for Bending Steel Plates

Bending steel plates can seem complex. However, with some tips, it can be easy. Below are some tips that can help you with the process.

Watch out for spring back

When bending a sheet, the material must be bent beyond its required angle. This is because sheet metal possesses a high-bending ability that allows it to spring back to its original position. Hence, an allocation must be made for such occurrence by bending the material slightly above the desired position.

Is the sheet metal malleable enough?

Bending into a sharp corner would crack the sheet metal. Therefore, you should avoid this as much as possible. It would be best if you considered the steel metal gauge, as not every material would be malleable enough to withstand bends into sharp corners.

Always use a press brake

Always use a press brake where applicable as it provides support and guarantees cleaner sheet metal bending and continuous pattern across bent sheets.

Do not forget the process position holes

Process position holes should be created on bending parts to guarantee the sheet metal’s precise positioning in the die. This would eliminate the sheet metal moving during the bending process and ensure precise results across multiple sheet metals.

Bend Allowance

Accounting for bend allowance is important in knowing how to bend sheet metal. This would guarantee more precise numbers ensuring accuracy in finished products.

Conclusion

The demand for custom-built products may never diminish, and custom metallic products, require the knowledge of sheet metal bending. Therefore, this article introduced sheet metal, its importance, and what you need to know about how to bend sheet metal to the right shape you want.

Getting to know about the process is not enough. The process is not that sophisticated since you can’t try it yourself. However, for those who value quality and lead time, RapidDirect’s metal bending services can be your golden fleece. With our engineering support, you can turn your designs into reality and gain a competitive edge in no time.

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