Roller Leveling Metal Parts, Sheets, and Plates

Metal Leveling

Precision leveler machines are a great equalizer for metal fabricators and assembly processes. It’s possible that you will win business based on your ability to guarantee a specific flatness by running a sheet, plate, or part through a leveler. 

The roller leveling process is a metal processing method, that aims to make the metal sheet or strip reach a state of flatness, no corrugation, no distortion, and release internal stress. This process is achieved by going through a series of rolling and bending operations.

What is the Metal Roller Leveling

In the roller leveling process, the metal material is fed into a special machine called a leveler, which contains a set of rollers arranged in an orderly manner. These rollers are arranged in two sets above and below, and the material passes between them.

Working Principle

Under the condition of relatively large elastic-plastic bending, no matter how different the original bending degree is, the remaining bending degree difference after springback will be significantly reduced, and even tend to be the same. As the degree of repeated bending decreases, the residual bending after springback will inevitably approach zero to achieve the purpose of correction. The roller-type steel plate leveler is mainly used for straightening various specifications of plates, using the "Bauschinger effect" of the material to repeatedly bend the plates, gradually reducing the bending deflection, and gradually changing various original curvatures into a single curvature. And finally level it to achieve the leveling accuracy required by the process. Bauschinger Effect: After a metal material undergoes plastic deformation, it exhibits a change in plastic properties under a reverse load. This effect is of great significance in the engineering application and processing of metal materials.

Under the action of the roller, the material will be under controlled pressure and bending force. These compressive and bending actions gradually remove or reduce material unevenness, bowing, wrinkles, and internal stresses. Each time the material passes through the rollers, it is flexed appropriately and then strives to return to its original state with elastic recovery.
This process can be repeated many times, each time gradually reducing the bending and stress of the metal material. With the repeated bending and recovery process, the surface of the metal material will be flattened, and the internal stress will be gradually released so that the material can achieve the desired flatness and stability.

Elastic-plastic Bending and Elastic Recovery Effect

Elastic-plastic bending and elastic recovery effect: After a metal material undergoes large elastic-plastic bending, its bending degree will be significantly reduced after elastic recovery, and even tend to be the same. As the degree of repeated bending decreases, the residual bending after springback will approach zero, achieving the purpose of correction.

Elastoplastic bending

When a metallic material undergoes bending, it undergoes bending deformation. In the initial stage, when the applied bending force is not enough to exceed the yield strength of the material, the metal material will show elastic deformation. This means that after a force is applied, the material bends, but once the force is removed, the material returns to its original state without permanent deformation. This is because in the elastic range, the molecular structure of the material can still return to its original state without permanent changes.

Application of elastic-plastic bending in leveling: In the leveling process, the metal material is reduced in its unevenness and internal stresses by passing through a series of rolling and bending operations. This involves bending the material to a degree that eliminates or reduces uneven bending and internal stresses. Metallic materials may undergo elastic and plastic deformation under the application of large stresses. Elastic deformation causes the material to deform reversibly under small stresses, while plastic deformation results in permanent deformation.

In leveling, elastic deformation helps to temporarily reduce the bending of the material, while plastic deformation causes a permanent change in the shape of the material, reducing inhomogeneities. Through repeated bending operations, the elastic and plastic deformations of the metallic material interact, eventually causing the material to become flatter, eliminating bending and stress.

Springback Effect

The springback effect is the phenomenon that occurs when a metallic material returns to its original shape from a bent state. When an external force is applied to a material to bend it, the molecular structure of the material changes. During bending, the material’s molecules are stretched or compressed, causing deformations in its surface and interior. However, once the external force is removed, the material tries to return to its original unbent state. This recovery is the result of elastic deformation, which means that the molecular structure of the material is able to return to its original state to some extent, thereby reducing or eliminating the deformation caused by bending.

The Application of Springback Effect in Leveling: In the leveling process, when the metal material undergoes a bending operation, a certain degree of spring-back effect will occur. This means that once the external bending stress is removed, the material will strive to return to its original unbent state. The spring-back effect is of great importance in leveling as it helps to correct the shape of the material to some extent and reduces permanent deformation due to bending operations.
Utilizing the spring-back effect, the leveling process can, with proper adjustment and control, enable the elastic recovery of the material to help eliminate bowing. This means that after the material has been bent, it will try to return to its original state to some extent, thereby reducing inhomogeneity and residual stress.

Why Roller Leveling is Needed: 5 Key Factors

The roller leveling process plays an important role in the field of metal processing and manufacturing, mainly because it can effectively solve the shape non-uniformity, bending and stress problems of many metal materials. Here are some of the main 5 key factors why you need a roller leveling process:

• Flatness and Surface Quality Requirements: Many industrial fields (such as automotive, aerospace, electronics, construction, etc.) place high requirements on the flatness and surface quality of metal parts. Roller leveling eliminates uneven bends and wrinkles in metallic materials, resulting in a flat and wavier-free surface that meets high-quality manufacturing standards.
• Internal Stress Relief: Metallic materials may accumulate internal stresses during the manufacturing process, which may cause the part to deform, bend or crack during subsequent processing. Roller leveling removes or reduces these internal stresses through a bending and releasing process, resulting in a more stable and reliable part.
• Preventing Machining Problems: Many metalworking processes, such as stamping, welding, and assembly, are critical to flat material. These machining processes can become difficult and inefficient if the metal material is subject to bending, twisting or stress, resulting in reduced product quality. Roller leveling can pre-treat materials prior to processing, ensuring they achieve the desired flatness and stability.
• Improve Production Efficiency: The use of roller leveling process can quickly make metal materials reach the required flatness in a short time. This saves time and resources, increases productivity and reduces production costs.
• Optimizing Material Properties: Some metallic materials may experience changes in material properties, such as hardening, after undergoing large deformations. Roller leveling allows the degree of deformation to be controlled, thereby optimizing the properties of the material in some cases.

Roller Leveling Process: Bending Process and Alternate Bending

Roller leveling is essentially a bending process in which uneven parts, sheets or plates are deformed by alternating bending. The leveling rolls are offset by half the roll pitch in the direction of material travel, causing the metal sheet to pass through the precision leveler in an undulating path. Alternating elastoplastic bending and decreasing bending strength yields flat and virtually stress-free parts, sheets and plates.

Bending Process

Bending is a mechanical phenomenon that involves bending an object into an arc or curved shape. In the roll leveling process, metallic material is forced to bend through a series of rolling and bending operations, reducing its unevenness and internal stresses. Every time the material passes through the rolling process, a certain bending force is applied, causing the material to bend and deform. These bending forces and deformations cause elastic and plastic deformation of the material, helping to reduce inhomogeneities and stresses.

Alternate bending

During the roller leveling process, the metal material is subjected to several alternating bending operations. This means that after a material has been bent once, it is then bent in the opposite direction. This alternating bending process is achieved by an arrangement of leveling rollers. The leveling rollers are arranged in two sets of upper and lower sets, and each set of rollers bends the material in a wavy path through the leveler. Alternating bending results in deformations in the metallic material that help eliminate uneven bending and stress.

Elastic-plastic alternating bending

In the roller leveling process, the metal material undergoes an alternating process of elastic deformation (temporary bending) and plastic deformation (permanent bending). After each bend, the material strives to return to its original state under the action of elastic recovery, thereby reducing the bend. With repeated bending and restoring operations, the plastic deformation of metallic materials is gradually reduced, eventually resulting in a part, sheet or plate that is flat and nearly stress-free.

Through this process of bending and alternating bending, the roller leveling process can effectively eliminate unevenness, bending and stress, resulting in a flat and high-quality metal material.

Number and Effect of Leveling Rollers

In the roller leveling process, the number of leveling rollers does affect the leveling effect. Typically, better leveling results can be achieved with more leveling rollers. This is because each leveling roller applies some amount of bending, and by using multiple rollers, the metal undergoes more bending and alternating bends, reducing unevenness and stress.

Alternate bending times: The more the number of alternating bending, the better the deformation effect of the metal material. The bending of each leveling roller will produce a certain degree of deformation of the material, and these deformations will be superimposed gradually, resulting in a gradual reduction of inhomogeneity and stress in the metal material. Experience has shown that in order to achieve good tolerances and flatness, usually at least 11 to 13 leveling rollers are required, so that the desired leveling effect can be achieved after multiple alternating bends.

It should be noted that the number of leveling rollers is only one of the factors affecting the leveling effect. Other factors, such as the layout of the leveling rolls, the nature of the material, the settings of the leveler, etc., also have an impact on the final result. In order to obtain the best leveling effect, it needs to be properly adjusted and optimized according to specific materials and requirements.

How Many Rollers Do I Need?

There’s a correlation between how many rollers you need and how much residual stress there is. Research indicates that 21 is the maximum number of rollers the machine should be equipped with. Beyond that, stress relief becomes negligible.

As a rule, thinner materials tend to contain more internal stresses than thicker materials. If you try to bend thin sheet metal, it can just snap back, referred to as springback. It has so much memory, it just doesn’t want to bend or stay flat. So, with thin-gauge metal, you may use up to 21 rollers. You need a lot of those alternating bends to achieve the best result.

However, the thicker and larger the roller gets, the fewer rollers are needed. Industry experts can help you determine how many and which size rollers you need based on several factors.

What material do you need to level? What’s the yield strength? What’s the size? What’s the thickness? Once they know the answers to these questions, they can do calculations and say, “Based on these values, we see this leveler as the most appropriate.”

You could have a piece of 4- by 4-ft. aluminum with ripples in it. Or you could have the exact same size piece with ripples and a bunch of cutouts in it. Each will both behave differently during leveling. One might need a different setting than the other because the one with cutouts has more material missing, so the material strength is not as high; you might not have to dive as deep.

How Big Should My Rollers Be?

Sometimes a leveler can’t handle a material because it’s too thick or too wide. Ideally, as the material thickness increases, you would also want a larger leveling roller diameter.

As the leveling roller gets larger, you will also lose some of the capability to level thinner material.

Ultimately, if you can buy only one machine, look at 90% of your parts. If only 10% of them are made of thick material, it may not be worth buying a really big a machine if 90% of your bread-and-butter parts are made of thin material.

Applicability and material properties

Roller leveling is a versatile metalworking method that works on a wide variety of metal materials including steel, aluminum, stainless steel, copper, and more. However, the performance characteristics of the material are crucial in choosing whether it is suitable for leveling.

Two important material properties parameters

Specifically, focus on the following two important material performance parameters:

• Elongation: Elongation is how much a material stretches before breaking in a tensile test. Materials suitable for leveling usually need to have a certain elongation, which ensures that they will not crack or break during bending. The higher the elongation, the more malleable the material is during the leveling process.
• Yield Strength: Yield strength is the stress level at which a material begins to undergo visible plastic deformation. The yield strength of the material will affect the leveling effect. Too high yield strength may lead to greater bending force during the leveling process, while too low yield strength may cause excessive deformation of the material. Therefore, a suitable yield strength range is important for leveling.

Leveling test

Leveling testing is a method of determining whether a material is suitable for roller leveling. In this test, a piece of sample material is bent alternately several times to see if the material can stay flat during the bend without cracking or breaking. With the leveling test, it can be determined whether the material has the proper plasticity and formability for roller leveling.

Hardening problem

In some metal materials, especially stainless steel, etc., it may cause hardening of the material after roller leveling. This means that after repeated leveling, the hardness of the material may increase, which affects its subsequent processing properties. Engineers need to take this into consideration and make appropriate adjustments during the design and process to ensure that the hardening of the material does not affect the quality and performance of the product.

In conclusion, the applicability of roller leveling is closely related to factors such as elongation, yield strength and possible hardening of the material.

Changes in Material Properties

In most cases, the mechanical properties of ferrous metals do not change. Stainless steel may harden. For non-ferrous and soft metals such as aluminum and magnesium, there may be a risk of material wear or a lowering of the yield point.

• Mechanical properties of ferrous metals: In most cases, ferrous metals, such as general steel, do not undergo significant changes in their mechanical properties, such as tensile strength and yield strength, after roller leveling. This means that for these materials, roller leveling is primarily intended to remove inhomogeneities, bows and stresses without significantly affecting their mechanical properties.
• Hardening of stainless steel: Stainless steel is a special case that may undergo hardening after multiple roll leveling. Hardening refers to an increase in the hardness of a material, usually due to changes in the lattice structure. This may affect the processability and deformability of stainless steel. Therefore, when using stainless steel for roller leveling, it is necessary to pay attention to possible hardening problems and make appropriate process adjustments as needed.
• Risks of non-ferrous and soft metals: Non-ferrous metals (such as copper, aluminum, etc.) and soft metals (such as aluminum and magnesium) may face different risks during the roller leveling process. Because these materials are soft, roller leveling can cause abrasion of the material or lower the yield point (the point of stress at which the material begins to deform plastically). Therefore, when handling these materials, caution is required and process parameters may need to be adjusted to avoid damage or degradation of material properties.

Summarize

From the uniformity of shape to the relief of internal stress, the roller leveling process has become an indispensable tool in the field of metal manufacturing through its unique elastic-plastic bending and spring-back effect. Whether it is to meet the high-quality demands of the aerospace, automotive, construction or electronics industries, mastering the principles and applications of roller leveling is essential for modern manufacturers. From bending to alternating bending, from the elastic-plastic deformation of the material to the action of the spring-back effect, the roller leveling process shapes the material at each step and finally presents a flat and stable finished product. Underpinned by 5 key factors, the roller leveling process has forged its mark of precision and high quality in the world of metalworking, opening up broad avenues for future manufacturing innovations.