Metal Bending Basics

Bending Process

Sheet Metal Bending Basics: A comprehensive introduction to 12 types of metal bending processes.
The Bending process(aka sheet/profile metal bending) involves the shaping of metal plates or profiles to a predetermined shape by applying force to them.

There are 9 common basic types of cold sheet metal bending processes: Air Bending, Bottom Bending/Bottoming, Coining, Folding, Wiping, Rotary Bending, Roll Bending, Stretch Bending, and Roll Forming. These typifications bending manufacturing process are based on the relationship of the end tool's position to the thickness of the material.

What is the Process of Metal Bending?

What is the Force of Bending?

In the most simple terms, a bending moment is basically a force that causes something to bend.

A bending force is a load that is applied to a portion of material at a certain length from a fixed position, it is a force normally measured in a force x length (e.g. kNm). Therefore, the units used to quantify a bending force are typically a unit of length multiplied by a unit of load. Common units used to measure a bending force include pound-foot or Newton-meter.

Bending moments occur when a force is applied at a given distance away from a point of reference, causing a bending effect. An excessive bending force can cause material failure, especially for materials that have corroded.

What is the Bending Strength of Materials?

Flexural strength, also known as bending strength, or transverse rupture strength, is a material property, the bending strength or flexural strength of a material is defined as its ability to resist deformation under load. Essentially, tensile strength is the measure of how much tension the metal can resist. It serves as a good point of reference for how a metal part will perform in an application.

During a bending test described in ASTM D790, the maximum achieved flexural stress value is noted as flexural strength. For materials that deform significantly but do not break, the load at yield, typically measured at 5% deformation/strain of the outer surface, is reported as the flexural strength or flexural yield strength. The test beam is under compressive stress at the concave surface and tensile stress at the convex surface.

3 types of tensile strength

• Yield strength is the stress point at which metal begins to deform plastically.
• Ultimate strength describes the maximum amount of stress a metal can endure.
• Breakable strength is the stress coordinate on the stress-strain curve at the point of failure.

A metal’s plasticity refers to the deformation of the material while it undergoes permanent changes as a result of applied forces. In the case of metal, applied forces’ can include bending or pounding actions.
Once the yield point is passed, some of the resulting deformations are permanent and non-reversible. Prior to yield, there is elasticity deformation where the material is deformed under stress but returns to its original state once the stress is removed.

Click for the chart of tensile strengths for aluminum and stainless steel to compare with mild steel

What are the Types of Bending Processes?

There are two types of metal bending processes: cold bending and hot bending. Usually, the most common metal bending process is cold bending.

There are 9 common basic types of metal cold bending processes: Air bending, Bottom Bending/Bottoming, Coining, Folding, Wiping, Rotary bending, Roll bending, Stretch bending, and Roll forming. These typifications are based on the relationship of the end tool’s position to the thickness of the material.

Types of sheet metal bending machine

Different types of machines are used for different metal bending processes. They are Brake Press/Metal Sheet Bending Machines, Metal Shearing Machines, Sheet Metal Folding Machines, Section Bending Machine/Profile Bending Machines, Plate Rolling Machines, Pipe/Tube Bending Machines, Stretch Forming Machines, Roll Forming Machine, and Punch Press. Read More: 10 Types Of Bending Machine

Cold bending process

Air Bending

Air bending is the most common type of bending used in sheet metal shops today. It is the process of forming materials by pressing a punch into the sheet metal, thereby forcing it into a bottom V-die to be mounted on the press. This enables the distance between the punch and the sidewall of the V to be greater than the material thickness.

The v opening is typically deeper than the angle which is sought in the workpiece. This allows for overbending to compensate for the Spring Back of the workpiece. Typically Acute Angle Tooling can be used to fully air bend and 90 ° or 88 ° tooling can be used to partially air bend. There has recently been the introduction of 75 ° tooling to allow for full Air Bending, without the tooling restrictions of acute punches.

This is the most preferred type of metal bending due to its benefits – such as less need for the punch tip to be pushed past the surface of the metal and less weight required for bending.

Bottoming

Bottoming is a bending process in which sheet metal is pressed against a bottom die featuring a V shape. While other bending processes typically support the use of both U- and V-shaped dies, bottoming only uses a V-shaped die.

In bottoming, the sheet is forced against the V opening in the bottom tool. U-shaped openings cannot be used.

Space is left between the sheet and the bottom of the V opening. The optimum width of the V opening is 6 T (T stands for material thickness) for sheets about 3 mm thick, up to about 12 T for 12 mm thick sheets. The bending radius must be at least 0.8 T to 2 T for sheet steel. A larger bend radius requires about the same force as larger radii in air bending, however, smaller radii require greater force—up to five times as much—than air bending.

It’s often preferred over air bending because of its higher level of accuracy as well as less recoil with the finished sheet metal.

The advantages of bottoming include better accuracy and fewer spring-backs, while the disadvantages are that a different tool is usually needed for each bend angle, sheet thickness, and material.

Coining

The term ‘coining’ comes from coin-making. To put Lincoln’s profile on a penny, machines using extremely high tonnage compressed a metal disc with enough force to make it conform to the image inscribed on the die set.

Coining is a bending process in which the punch and the workpiece bottom are on the die. This produces a controlled angle, which leads to little spring back. There is more tonnage required for this type of bending than in air bending and bottom bending.

In coining, the top tool forces the material into the bottom die with 5 to 30 times the force of air bending, causing permanent deformation through the sheet. There is little, if any, spring back. Coining can produce an inside radius as low as 0.4 T, with a 5 T width of the V opening. While coining can attain high precision, higher costs mean that it is not often used.

The advantages of coining are that, in order to produce outstanding results, accuracy is essential and that is exactly what the coining method can provide. Along with being accurate, repeating the results is also an easy task when it comes to using this technique. Spring back is also less common when using coining, meaning that the metal is less likely to return to its original state.

Three-point Bending

Three-point bending is a relatively new bending process that requires the use of an adjustable die. Unlike with other bending processes, the bottom die isn’t fixed in a stationary position. Rather, the bottom dies used in three-point bending feature an adjustable height. It can be raised or lowered, allowing for a greater level of versatility.

Three-point bending uses a die with an adjustable-height bottom tool, moved by a servo motor. The height can be set to 0.01 mm. Adjustments between the ram and the upper tool are made using a hydraulic cushion, which accommodates deviations in sheet thickness. Three-point bending can achieve bend angles with 0.25 deg. precision.

While three-point bending permits high flexibility and precision, it also entails high costs and there are fewer tools readily available. Like coining, three-point bending is a more costly bending process compared to other bending processes. It is being used mostly in high-value niche markets.

Folding

Changes the shape of sheet metal parts by cold forming along straight lines, simultaneously over the entire length. The bending process is carried out simultaneously along the entire length of the bending line and it is a manufacturing process for the machining of sheet metal, i.e. thin, semi-finished metal products. Folding is used to produce profiles, more precisely, folded profiles.

Folding sheet metal is part of the manufacturing technologies of bending and forming. It involves the folding of a surface part of sheet metal. In folding, clamping beams hold the longer side of the sheet. The beam rises and folds the sheet around a bend profile. The bend beam can move the sheet up or down, permitting the fabricating of parts with positive and negative bend angles. The resulting bend angle is influenced by the folding angle of the beam, tool geometry, and material properties. Large sheets can be handled in this process, making the operation easily automated. There is little risk of surface damage to the sheet.

Wiping

Wiping die bending, also known as edge bending, is performed by holding the sheet between a pad and die then sliding the wiping flange across the face pushing and bending the sheet metal which protrudes from the pad and die. One of the most common methods used, but not always the most effective, is simple wipe bending.

In wiping, the longest end of the sheet is clamped, then the tool moves up and down, bending the sheet around the bend profile. Though faster than folding, wiping has a higher risk of producing scratches or otherwise damaging the sheet, because the tool is moving over the sheet surface. The risk increases if sharp angles are produced.

The wipe-bending method does not allow for much overbending other than the very slight acute angle that can be achieved by wiping the side extremely tight. Even though wipe bending effectively creates a bend, controlling the bend angle is very difficult.

This method will typically bottom or coin the material to set the edge to help overcome spring-back. In this bending method, the radius of the bottom die determines the final bending radius.

Wipe bending is not well-suited to bending high-strength metals or for parts requiring precision bend angle tolerances. Wipe bending can be improved by capturing the outside profile of the radius with the forming die section.

Rotary Bending

Rotary bending is similar to wiping but the top die is made of a freely rotating cylinder with the final formed shape cut into it and a matching bottom die. On contact with the sheet, the roll contacts two points and it rotates as the forming process bends the sheet.

For precision work, rotary draw bending dominates the tube bending landscape, especially for those applications involving tight radii—sometimes down to a CLR that’s just 0.7 times the tube OD (or as tube processors call it, less than 1×D).

This bending method is typically considered a “non-marking” forming process suitable for pre-painted or easily marred surfaces. This bending process can produce angles greater than 90° in a single hit on the standard press brakes process.

A rotary draw setup entails a pressure die that holds the straight section (sometimes called the tangent) of the tube; a clamp dies that rotates the workpiece around a round bend die; a mandrel, sometimes with a series of articulating balls on the end to support the tube interior around the bend; and a wiper dies that contacts the workpiece just before the tangent point of the inside radius, wiping against the material to prevent wrinkles that can form on the bend’s inside radius.

Example of a Rotary Bending Machine: PBT Pipe Bending Machine

Compression Bending

Compression bending is an older method of bending in which the tube is clamped against a stationary bend die and the pressure die sweeps the tube around the bend die to form the bend. This differs importantly from rotary-draw bending in that the point of bend is the point of contact between the pressure die and bend die. Therefore the point of bend moves through space, which makes the use of a mandrel impossible. Compare rotary-draw bending.

Compression bending is cheaper than rotary draw bending due to its simpler setup. However, it is limited to circular hollow sections. The setup does not allow the use of a mandrel to support the inner diameter and may cause the outside surface to flatten slightly. It cannot be used for bending tubes to a small CLR because the tube may break or buckle. This method is commonly used in bending symmetrical workpieces and electrical conduits for structural application.

For speed and economy, fabricators often use compression bending. This bending mode is used when the roundness of the bend is not critical, and when the objective is more about high output to get the lowest price per tube.

Roll Bending

The roll bending process induces a curve into bar or plate workpieces. There should be a proper pre-punching allowance.

Roll-bending is a process whereby we obtain cold process deformation with a wider bend radius that theoretically can range from 5 times the cross-section to infinity. To achieve this process, the equipment used consists of plate bending machines.

Commonly used for large workpieces in construction, roll bending generally entails three rolls positioned in a pyramid, oriented either vertically or, for larger sections, horizontally. The rolls move to produce specific, usually very large radii. Which rolls move where depends on the machine. On some, the top roll moves up and down to produce the desired angle; on others, the two bottom rolls move and the top roll remains stationary.

Another machine type is the two-roll, pinch-style roll bender. For this system, the tube feeds between an upper and lower roll, while on either side two adjustable guides move to produce the desired bend angle.

Examples of a Roll Bending Machines: Profile Bending Machine, PBA Aluminum Bending Machine

Elastomer Bending

In this method, the bottom V-die is replaced by a flat pad of urethane or rubber. As the punch forms the part, the urethane deflects and allows the material to form around the punch. This bending method has a number of advantages. The urethane will wrap the material around the punch and the end bend radius will be very close to the actual radius on the punch. It provides a non-marring bend and is suitable for pre-painted or sensitive materials.

Using a special punch called a radius ruler with relieved areas on the urethane U-bends greater than 180° can be achieved in one hit, something that is not possible with conventional press tooling.

Urethane tooling should be considered a consumable item and while they are not cheap, they are a fraction of the cost of dedicated steel. It also has some drawbacks, this method requires tonnage similar to bottoming and coining and does not do well on flanges that are irregular in shape, that is where the edge of the bent flange is not parallel to the bend and is short enough to engage the urethane pad.

Stretch Forming

Stretch forming is a very intricate process that has evolved in many ways over the years.

Stretch forming is part of metal forming where the metal is bent or “stretched” to create perfectly shaped parts. A machine called a stretch press usually performs this process, shaping multiple different types of metal.

When used in manufacturing, stretch bending helps to create automotive parts. These parts are often stronger as well as lighter than ever before, as a result of using very strong materials and working with higher-quality machines.

Roll Forming

Roll forming, also spelled roll-forming or roll forming, is a reliable, proven approach to metal shaping that is ideal for modern applications. Roll forming is a type of rolling involving the continuous bending of a long strip of sheet metal (typically coiled steel) into a desired cross-section. The strip passes through sets of rolls mounted on consecutive stands, each set performing only an incremental part of the bend until the desired cross-section (profile) is obtained.

Unlike other metal shaping methods, the roll-forming process is inherently flexible. Secondary processes can also be integrated into a single production line. Roll forming increases efficiency while reducing operational and capital costs by eliminating unnecessary handling and equipment.

Roll forming is ideal for producing constant-profile parts with long lengths and in large quantities.

Freeform Bending

The freeform bending process differs from traditional bending methods, such as press bending or roll bending, which typically produce straight or simple curved shapes. Freeform bending machines employ advanced technologies, such as CNC (Computer Numerical Control) systems and servo-controlled hydraulic systems, to achieve precise and flexible bending operations.

Read More

Hot bending process

Induction hot bending: The induction bending process, also known as high-frequency bending, incremental bending, or hot bending, uses inductors to locally heat steel by induction. This results in a narrow heat band in the shape to be bent. The shape is firmly held by a clamp at the desired radius, which is mounted on a free-pivoting arm. The shape is pushed through the inductor by an accurate drive system which causes the hot section to form the induction bend at the set radius. The bent part is then cooled by water, forced or still air to fix the bent shape.

Final Words

In the above, we have summarized 12 kinds of cold bending processes of metal profiles, plus a hot bending process, then there are 13 methods of the metal bending process, so will there be 13 kinds of bending machines? The answer is no, read for details: Beginner’s Guide To 10 Types Of Bending Machine

Recommended Reading

• Click for a 2 Advice on how to bend aluminum without breaking it?
• Click for a Bend Radius Chart for aluminum and stainless steel for recommended minimum bend radii.
• Click for the chart of tensile strengths for aluminum and stainless steel to compare with mild steel