sheet metal forming

Sheet Metal Forming Techniques

Sheet metal is a very important and popular component of modern-day manufacturing. It can be found in our day-to-day lives. They are found in our domestic appliances, cookware, car doors, door handles, and more sophisticated machine parts. The art of creating these parts, components, and assemblies using sheet metal is known as sheet metal fabrication. Sheet metal fabrication is a manufacturing process that transforms flat sheets into functional parts. This process consists of techniques or methods such as cutting, forming, joining, assembling, and finishing to achieve the desired component. The application of these processes may vary depending on the project requirements. For instance, a steel metal component may only require cutting and forming, whereas more complex assemblies may involve joining and post-processing.  

Sheet metal forming is one of the most essential fabrication processes. It involves deforming the metal without adding or removing material. This article covers all you need to know about sheet metal forming, its definitions, a detailed explanation of its processes, and the considerations before using forming for your project.

What is Sheet Metal Forming?

Sheet metal forming is a fabrication process that involves the reshaping of sheet metals into a part of the desired geometry by applying forces such as tension and compression. This transformative manufacturing process is carried out and completed without cutting or boring any part, maintaining its mass. Sheet metal forming is one of the most important processes in the modern fabrication of parts and components. Through the forming process, the metals' plastic characteristic allows the metals to be deformed into different desired shapes while still maintaining their structure and integrity. The process employs techniques like bending, stretching, and pressing to create components with high precision. 

One of the reasons sheet metal forming has become well utilized in the mechanical industry is because of the favourable properties of materials like steel, aluminium, brass, and copper, which combine strength and malleability. These characteristics enable the creation of lightweight yet durable parts suitable for various uses. Additionally, the process can be cost-effective, particularly for simple designs and standard sizes, compared to alternatives like forging or metal stamping. The choice of forming methods depends on factors such as the metal type, design complexity, and production volume. High tooling and labour costs often make it more efficient for large-scale production, where economies of scale can be achieved. Techniques such as punching, press braking, rolling, and extrusion rely on the plasticity of metals to shape them while maintaining structural integrity.

Types of Materials Used in Sheet Metal Forming

Material selection in sheet metal forming is a critical decision that directly impacts the final product's manufacturability, functionality, and durability. The choice of material must align with the application, mechanical properties, and environmental conditions. Below are the materials commonly used in sheet metal forming, their properties, and applications:

Stainless steel

Stainless steel has high strength, excellent malleability, and superior corrosion resistance, making it suitable for various applications. It is composed of iron, chromium, and other alloying elements. Stainless steel is commonly used in industries requiring hygienic and durable components, such as medical devices, food processing equipment, and kitchenware. Grades like 304 and 316 are widely utilized, with 316 having superior resistance to chemical exposure because of its molybdenum content. Additionally, stainless steel has a sleek appearance, making it perfect for architectural features.

Aluminum 

Aluminum is a lightweight material with a high strength-to-weight ratio and exceptional corrosion resistance. It is also an efficient conductor of electricity and heat. Its resilience makes it appropriate for harsh conditions, including outdoor and marine settings, but its malleability enables the production of intricate designs. 

Hot-rolled steel 

Hot-rolled steel is a budget-friendly choice shaped at elevated temperatures, resulting in a flexible material ideal for construction, automotive frames, and railway tracks. Although it has a rougher surface finish and is less precise than cold-rolled steel, hot-rolled steel is well-suited for situations where extreme accuracy isn't required. Its strength and ease of fabrication make it a common choice in heavy industrial use.

Cold-rolled steel

Cold-rolled steel is created by further processing hot-rolled steel through a cold reduction process, which enhances its strength, improves surface quality, and provides tighter dimensional accuracy. This type of steel is typically used in structural components, household appliances, and aerospace applications where a combination of strength and smooth finishes is crucial.

Galvanized steel  

Galvanized steel is coated with a layer of zinc. The coating improves its resistance to corrosion and is an excellent option for outdoor and industrial settings. It is utilized in roofing, HVAC systems, refrigeration units, and agricultural machinery due to its durability and cost-effectiveness. The zinc coating helps prevent rust and enhances the steel's longevity in harsh environments.

Copper

Copper is a highly malleable metal known for its outstanding electrical and thermal conductivity, which makes it the preferred material for electrical wiring, busbars, and heat exchangers. Its natural antimicrobial properties also make it suitable for medical equipment and surfaces where hygiene is critical. Additionally, copper alloys like brass (composed of copper and zinc) offer improved machinability and visual appeal, often used in decorative items and musical instruments.

High-strength low-alloy (HSLA) steel

HSLA steel is designed to deliver better mechanical properties and corrosion resistance than standard carbon steel. This steel is lighter and way stronger than regular steel. This property makes it perfect for use in the transportation industries like automobiles, bridges, and cranes bridges and cranes), and heavy machinery. Its ability to retain strength in thinner sections contributes significant weight savings and enhances vehicle fuel efficiency.

Sheet Metal Forming Processes

Bending

Bending is one of the most popular metal sheet forming processes. It is done by applying force along the straight axis of the sheet metal, making it bend at an angle. This operation is performed without cutting or punching material, preserving the volume and typically maintaining the sheet's thickness. Bending is usually carried out by a machine called a press brake. It can be operated automatically or manually and is available in sizes 20-200 tons, matching the application. The press brake comprises an upper tool called the punch and a lower called the die, between which the metal sheet is placed. The angle of the bend is determined by how deep the punch forces the metal sheet into the die. Bending applications include the fabrication of brackets, enclosures, automotive components, and various architectural features.

This sheet metal forming process has various techniques. It  can be categorized into several types based on the technique and equipment used:

  1. V-Bendng: This technique can be further divided into two different types:
  • Air Bending: Involves partial contact between the die and workpiece, allowing flexibility in achieving different bend angles using a single toolset.
  • Bottoming (or Bottom Pressing): The sheet metal is pressed entirely into the die, resulting in precise and repeatable bends.
  1. Coining: A high-force bending method where the workpiece is pressed into the die, creating highly accurate bends with minimal spring back.
  2. Roll Bending: This is ideal for forming cylindrical or curved shapes, using a series of rollers to bend the sheet gradually.
  3. Wipe Bending: This uses a punch and die to create bends where the material is clamped along its edges.

Rolling

Rolling is a metal forming process in which a flat sheet continually passes through a series of roll stations. Each station contains paired roller dies, progressively shaping the sheet into the desired profile. The process helps form complex cross-sectional geometries with high dimensional accuracy. Rolling is mainly used to manufacture roofing panels, structural beams, and storage shelves.

Types of rolling processes:

  1. Flat Rolling: This produces uniform sheets and plates by compressing the metal between two rollers.
  2. Shape Rolling: Forms specific cross-sectional shapes such as channels, I-beams, and T-sections.
  3. Ring Rolling: Expands the diameter of a ring-shaped workpiece using rollers, commonly applied in manufacturing flanges and bearings.
  4. Thread Rolling: This creates threads on cylindrical surfaces by rolling the material between dies.
  5. Hot and Cold Rolling: Hot Rolling is performed above the metal's recrystallization temperature, while cold Rolling occurs below it, offering enhanced surface finish and dimensional precision.

Forging

This process shapes metal by applying compressive forces using a die and punch. It is highly versatile, producing components with superior mechanical properties due to the metal's grain structure refinement during deformation.

Classifications of forging

  1. Open-Die Forging: The workpiece is deformed between flat or simple dies, offering flexibility for large and irregular parts like shafts or rings.
  2. Closed-Die Forging (Impression Die): The metal is compressed into a cavity resembling the final shape, which is ideal for producing small, precise components like gears and valve bodies.
  3. Roll Forging: This reduces the cross-section of a cylindrical or rectangular metal rod by passing it through rotating rolls.
  4. Coining: Utilized for creating intricate surface details, commonly used in minting coins

Curling

Curling adds a hollow, smooth, circular roll to the edges of sheet metal, eliminating roughness and sharpness. This process is carried out by feeding the sheets into machines that slowly roll and bend those edges into smooth shapes. Curling not only enhances edge strength but also improves safety and usability. It is commonly applied to parts requiring tubular or rolled edges, such as door frames, can edges, or decorative trims.

Extrusion

This sheet metal forming process involves forcing and compressing the metal through a die to create long pieces with uniform cross-sections. It can be performed using hot or cold methods and is versatile in producing components with complex profiles, including window frames, automotive trim, and lightweight structural components.

Types of extrusion:

  1. Direct Extrusion: The billet is first loaded into a container with die holes and then pushed through the die in the same direction as the punch. This action causes a lot of friction, leading to wear.
  2. Indirect Extrusion: The die compresses the billet, reducing friction and tool wear.
  3. Hydrostatic Extrusion: Uses pressurized fluid to push the metal through the die uniformly.
  4. Tube Extrusion: Produces hollow components like pipes and cylinders.

Stamping

This sheet metal forming process generally produces large volumes of identical metal components. It is a very close, effective, and high-speed process. For this process, sheet metals, called blanks, are loaded into a stamping press, where a tool and die interface exert force to reshape the material into the intended form. Stamping processes can be executed as independent operations or combined with other metal forming methods, allowing them to be effective for short and long production cycles. Stamping presses with capacities that can handle up to 400 tons can produce components as thin as 0.005 inches while maintaining tolerances.

Stamping sheet metal is widely used in mass production of parts. It is valued by many industries for its efficiency, consistency, and ability to produce complex geometries with tight tolerances. This process plays a crucial role in modern manufacturing, whether for small intricate components such as steel plates and door handles or large structural parts like car doors and machine parts. 

Ironing

Ironing achieves uniform wall thickness in components by passing the metal through a narrow clearance between a punch and a die. Commonly used in producing beverage cans and containers, this process enhances strength and reduces weight without compromising volume.

Hydroforming

In this process, the metal sheet is stretched by a highly pressurized fluid over a die to create curved or hollow forms. It's especially effective with malleable metals such as aluminium, producing strong structural components while preserving the material's original qualities. The procedure involves securing a metal sheet over a die and sealing it within a hydraulic chamber. The metal is pressed into the die by pumping fluid at high pressure to achieve its final shape. Hydroforming can efficiently produce parts with consistent thickness and minimal scrap, leading to cost-effective solutions for complex designs in sectors like automotive, medical, and aerospace. However, the initial costs for hydroforming machinery and die creation can be significant.

Spinning

Spin forming is also called the sheet metal forming process, which is utilized to form rotationally symmetric parts by pressing a rotating sheet metal blank against a tool called the mandrel using rollers. It is used for cookware, satellite dishes, and musical instruments. This process has two spinning methods, namely;

  • Conventional Spinning: In conventional spinning, a roller tool presses against a blank, shaping it to fit the contour of the mandrel. The finished spun part will have a smaller diameter than the original blank but keep a consistent thickness. Maintains constant material thickness while shaping.
  • Shear Spinning: the roller curves the blank around the mandrel and exerts a downward force while moving, which stretches the material over the mandrel. As a result, the outer diameter of the spun part stays the same as the original diameter of the blank, but the wall thickness becomes thinner. It reduces wall thickness during forming.

Deep Drawing

Deep drawing stretches sheet metal into deep, cup-shaped components. The process uses a punch and die and is suitable for producing parts with a depth greater than half their diameter. The deep drawing process involves several key components: a blank, a blank holder, a punch, and a die. The blank is a pre-cut piece of sheet metal, usually in the shape of a disc or rectangle, intended for forming into a final part. The blank holder secures the blank over a die with a cavity shaped like the final product. A punch then moves downwards, stretching the material into the die cavity. Hydraulic systems typically power this punch to exert sufficient force on the blank. Both the die and punch undergo wear due to the pressure exerted during the process, which is why they are constructed from durable materials like tool steel or carbon steel. Applications include automotive panels, kitchen sinks, and beverage cans.

Stretch Forming

The sheet metal is stretched and bent over a die, forming wide contour parts. Stretch forming stretches and bends sheet metal over a die to create large, contoured parts. Stretch forming is a stretch press process where a metal sheet is clamped along its edges by gripping jaws. These jaws are connected to a carriage that uses pneumatic or hydraulic power to stretch the metal sheet. A solid, contoured tool known as a forming die acts as a mould for the sheet. Typically, stretch presses are designed vertically; the form die is positioned on a table that rises to press against the sheet, causing it to deform under tension. There are also horizontal stretch presses, where the form die is mounted sideways, and the sheet is pulled around it. It is commonly used in aerospace for fabricating aircraft skins and in automotive industries for door and roof panels.

Sheet metal forming processes in a glance

Process

Description

Common Applications

Bending

Uses a press brake to apply force along a straight axis, bending the sheet at an angle. Its techniques include V-bending, roll bending, and coining.

Brackets, enclosures, automotive components, architectural features.

Rolling

Passes flat sheet through rollers to achieve the desired shape. Can be flat rolling, shape rolling, or thread rolling.

Roofing panels, structural beams, storage shelves.

Forging

Uses compressive forces to shape metal, refining its grain structure for superior mechanical properties. Includes open-die, closed-die, and roll forging.

Gears, shafts, valve bodies, aerospace components.

Curling

Rolls and smooths the edges of a metal sheet to eliminate sharpness and add strength.

Can edges, door frames, decorative trims.

Extrusion

Forces metal through a die to create long parts with a uniform cross-section. Can be direct, indirect, or hydrostatic extrusion.

Window frames, automotive trim, lightweight structural components.

Stamping

Uses a die and press to shape metal into precise, repeatable forms. Suitable for mass production.

Automotive panels, appliance components, machine parts.

Ironing

Reduces material thickness by passing the sheet through narrow clearances, improving uniformity.

Beverage cans, lightweight containers.

Hydroforming

Uses high-pressure fluid to form metal into complex shapes while maintaining strength.

Automotive body panels, aerospace components, medical equipment.

Spinning

Rotates a sheet over a mandrel and presses it into shape using rollers. Can be conventional or shear spinning.

Cookware, satellite dishes, musical instruments.

Deep Drawing

Stretches sheet metal into cup-shaped parts using a punch and die, requiring strong materials to prevent tearing.

Kitchen sinks, automotive fuel tanks, beverage cans.

Stretch Forming

Stretches and bends a sheet over a die, commonly used for large contoured parts.

Aircraft skins, automotive roof panels, structural components.

Important Parameters in Sheet Metal Forming

K-Factor

The K-Factor determines how much steel material is displaced after bending. High K-factor values show that much material is stretched in the bend area. During bending, the outer layers of the sheet stretch while the inner layers compress. The neutral axis is the point where no stretching or compression occurs. The K-Factor varies depending on the material type, thickness, and bend radius and is crucial for producing accurate and consistent angular bends.

Formular; K=tT​

Where; 

t = Distance from the inner bend surface to the neutral axis

T = Sheet thickness

Typical K-Factor Values:

  • Soft materials such as Aluminum, Mild Steel: 0.33
  • Harder materials like Stainless Steel, Titanium: 0.40 - 0.50

Bend Radius (R) 

The bend radius is the inside radius of a bend. It helps determine the amount of bending stress exerted on the material, the chances of cracking (in harder materials), and the spring back effect after bending. Smaller bend radii lead to higher stress concentrations. Mild steel can have small bend radii, but brittle materials must have wider bend radii to avoid failure.

Bend radius recommendation (as a multiple of thickness, T):

  • Mild steel: 1.0T - 2.0T
  • Stainless steel: 2.0T - 4.0T
  • Aluminum alloys: 1.5T - 3.0T

Bend Deduction (BD) and Bend Allowance (BA) 

Bend deduction is the length lost as a result of the curve, whereas bend allowance is the extra length of material required in the flat pattern to accommodate the bend. Both elements are essential to guarantee that the part's final dimensions are precise and that the necessary tolerances are fulfilled. These values can be found using various charts and algorithms based on bend characteristics and material parameters. 

Formular:

  • Bend Deduction (BD):
    BD=2*(R+T)tan2-BA
  • Bend allowance
    BA=(R+K+T) 

Where:
θ = Bend angle (in radians)

R = Inside bend radius

K = K-Factor

T = Material thickness

Springback 

Springback is the tendency for a material to partially return to its original shape after removing the bending force. The amount of springback can depend on the material properties, the bend angle, and the degree of plastic deformation. Springback affects the final part dimensions and must be compensated for during forming. Manufacturers often compensate with an intentional over-bend during the forming process, which requires a deep understanding of the material's behaviour.

Factors affecting springback

  • Higher bending angles increases the springback
  • Thickener materials experiences more springback
  • The smaller the bending radius, the more the springback
  • Higher yield strength of the material leads to higher chances of springback

How to compensate for springback

  • Overbending: Bending beyond the required angle so it settles into the correct shape.
  • Bottoming or Coining: Applying extra pressure to permanently deform the metal.
  • Using stretch bending techniques to reduce elastic recovery.

Die Clearance

Die clearance is the gap between the punch and die during the Sheet Metal Forming processes. Not enough clearance can lead to excessive stress and damage, while too much can cause poor edge quality and increased scrap rates. To get the required results, die clearance must be adjusted according to the thickness and qualities of the material.  

Formula: for (C):
C=P-D2

Where:

C = Die Clearance
P = Punch diameter

D = Die opening diameter

Die Clearance recommendation:

  • Soft materials (aluminum, copper) → 5-10% of material thickness
  • Mild steel → 10-15% of material thickness
  • Stainless steel or high-strength alloys → 15-20% of material thickness

Holding Time 

Holding time is the duration with which pressure is maintained on the material during the sheet metal forming operations. This period allows the material to adapt to the shape of the die and can significantly influence properties such as surface finish and dimensional accuracy. Optimizing holding time can improve the effectiveness of the forming process, reduce defects, and enhance overall product quality. Holding time is particularly important in deep drawing to improve material flow, reduce tearing, coining, and embossing to enhance accuracy and detail, and in hot forming processes to transform uniform grain structure.

Sheet Metal Forming Considerations

When considering the sheet metal forming process to use, you have to consider several key factors that would affect the process's efficiency and the quality of the final product. A clear understanding of these factors is essential for optimizing the process. 

1. Material Considerations

The properties of the material directly impact its behaviour during the forming process. Important considerations include:

Ductility:

For sheet metal forming, ductile materials such as copper, aluminium, and low carbon are more suitable for forming as they can undergo significant deformation without cracking. Meanwhile, brittle materials require other specialized techniques to be adequately fabricated to avoid failure.

Grain Direction (Anisotropy):

The orientation of grains in the metal affects its strength and formability. Be sure to take note of the grain direction before you start the forming process, as working along the grain direction increases the risk of cracking, while working across or at an angle to the grains ensures better performance.

Heat Treatment:

Proper heat-treated materials offer improved ductility and reduced residual stress, making them easier to form. However, hardened or cold-worked metals may require higher forming forces and careful handling.

2. Size Considerations

The dimensions of the sheet metal to be formed are crucial in determining the sheet metal formation. 

Thickness:

Thicker sheets need more force during forming, potentially limiting the complexity of shapes. Therefore, consider using other techniques if you are working with thicker-sized parts. Thinner sheets are easier to shape but are more prone to tearing, wrinkling, or distortion.

Length and Width:

Larger sheets pose challenges for uniform deformation, consistent force application, and handling, often requiring specialized equipment.

Aspect Ratio:

Parts with a high aspect ratio may experience uneven deformation or localized thinning, necessitating additional design or process adjustments to mitigate defects.

3. Load-Bearing Capabilities

Effective management of the applied forces during forming is necessary to maintain the material's integrity and prevent defects:

  • Tensile Strength: Greater forming forces are needed for high-strength materials, which might limit the complexity of parts that can be made and increase tooling wear.
  • Springback: The material's elasticity permits it to partially regain its original shape after forming. Dimensional correctness is ensured by appropriate tooling modifications or compensation during design.
  • Load Distribution: Uniform application of forming forces is crucial to ensure constant component quality and avoid localized thinning or ripping.

4. Design Considerations

The design of the sheet is an important factor in determining how best to go about the sheet metal forming. Taking into account these considerations would ensure reduced defects and increased cost efficiency. Key design aspects include:

Process Selection Based on Part Features:

Selecting the correct forming process is essential, especially for parts with complex features like holes, cutouts, or intricate shapes. For example:

  • Parts with Holes: Punching and laser cutting operations are advisable before forming parts that are meant to have holes. It would prevent misalignment or distortion during deep drawing and bending.
  • Complex Geometries: Progressive dies or multi-step forming processes may be required for intricate shapes.
  • Bend Radius: The minimum bend radius must be higher than the material thickness to avoid cracking. Sharp bends or tight radii can lead to failure, so they should be minimized unless supported by advanced forming techniques. 
  • Hole Placement and Distance from Edges: Holes on the sheet are to be located away from the bends and edges to avoid tearing and stress concentration. Adequate spacing between holes ensures the structural integrity of the part.
  • Relief Features: Relief cuts or notches can minimize stress in high-deformation areas, improving material flow and reducing the risk of cracking.
  • Tolerances: Tight tolerances should be applied only where necessary, as they increase manufacturing difficulty and cost. Designs should account for variations in thickness, spring back, and other factors.
  • Symmetry and Simplification: Symmetrical designs are more straightforward to form and reduce the risk of defects. Simplified geometries help streamline production and minimize waste.
  • Pre-Forming Operations: Pre-forming processes like blanking, laser cutting, or punching should be optimized to ensure clean edges, accurate dimensions, and proper alignment for subsequent forming steps.
  • Material Flow: Designing parts to promote uniform material flow during forming reduces the risk of thinning, wrinkling, or uneven deformation. Features like ribs or beads may be added to strengthen the structure and aid formability.

About the author

Sam Portrait

Sam Al-Mukhtar

Mechanical Engineer, Founder and CEO of Geomiq

Mechanical Engineer, Founder and CEO of Geomiq, an online manufacturing platform for CNC Machining, 3D Printing, Injection Moulding and Sheet Metal fabrication. Our mission is to automate custom manufacturing, to deliver industry-leading service levels that enable engineers to innovate faster.

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