What Is Sheet Metal Hemming: Process, Types and Applications

Sheet metal hemming is a crucial process in the metalworking industry, often overlooked but incredibly valuable for enhancing product quality and safety. It involves folding the edge of a metal sheet back on itself, creating a smooth, rounded finish. 

Hemming helps strengthen the metal’s edge, improves its appearance, and reduces sharpness, making the metal product safer to handle. Manufacturers often opt for this process for applications requiring strong and durable metal edges. However, the process can be misunderstood. 

This is why we will be breaking down the sheet metal hemming process and the different types available. This will help you understand its capabilities and applications in metalworking processes.

What is Sheet Metal Hemming?

This metal-forming technique involves rolling or folding the edges of a metal sheet back onto itself to create a double-layered edge. The process helps strengthen the edge and gives it a smooth, rounded finish. Sheet metals come in varying thicknesses and are categorized as either high-gauge or low-gauge. High-gauge metals are thicker, which makes them generally less suited for hemming. Low-gauge metals, being thinner, are usually more compatible with hemming techniques.

Sheet metal hemming process

When done correctly, this sheet metal folding provides strong, clean, and durable edges that meet aesthetic and functional requirements in many industries. The process is a multi-step procedure that follows these essential steps:

• Material Selection: Choose the appropriate sheet metal based on application needs. Knowing the metal properties will make this step much easier. Stainless steel offers a strong and durable option, ideal for heavy-duty uses but challenging to shape. Likewise, cold-rolled steel provides strength and flexibility and is often used without additional coating. Manufacturers also use aluminum as a lightweight and corrosion-resistant material. It is great for many uses but wears quickly due to softness. Copper and brass offer attractive color options for decorative applications, though generally softer and less durable.
• Design and Preparation: Determine the best hem type based on the material’s thickness and project requirements. For instance, a closed hem may be suitable for basic edges, while aluminum might require a teardrop hem to avoid cracking.
• Marking the Bend Line: Mark the bend line where the edge will fold. For optimal results, set the flange length at four times the sheet’s thickness.
• Initial Bending: Bend the sheet to a slight angle (usually acute) using a forming tool or press brake. For roller hemming, this step can be skipped.
• Forming the Hem: After the sheet metal bending, use a press to fully flatten the fold. Insert a shim as needed to create a consistent gap based on the hem type.
• Finishing Touches and Inspection: Polish and smooth the hemmed edge, then inspect for any damage or irregularities. Use a micrometer or gauge to confirm the hem’s thickness and width match specifications.

Features

• Smooth Edge Finish: Hemming removes sharp edges, creating a rounded or flat surface that’s safer to handle and looks polished.
• Increased Durability: By precision metal folding, hemming adds strength to the edge, making it resistant to bending and other deformations.
• Enhanced Compatibility with Sealing: Hemmed edges form tighter seals, which is useful in applications that need secure, closed edges, like in automotive doors.
• Supports Additional Coatings: The rounded, reinforced edge from hemming makes it easier to apply protective coatings, helping prevent corrosion.

Pros

• Increased Strength and Rigidity: Hemming strengthens the metal edge, making it less prone to deformations like warping or bending. The double-layered edge is thicker and sturdier, able to withstand more pressure without losing shape.
• Better Functionality: Many manufacturers use hemming to secure metal edges, enhancing the part’s functionality. This is especially critical in industries where metal parts must endure significant stress and wear.
• Greater Resistance to Damage: It also provides metal parts with better resistance to wear and corrosion. This makes hemmed edges more durable in harsh environments, including exposure to chemicals or extreme weather conditions.
• Enhanced Appearance: Hemming creates a smooth, attractive edge that improves the overall look of the part. The folded edge can align seamlessly with the contours of the surrounding metal, offering a clean, polished finish.

Cons

• Time-Consuming: The hemming process can be time-intensive, especially for large or complex parts. Tool setup and precise hemming steps take time, which can slow down production.
• Risk of Deformation or Damage: Hemming can deform or damage sheet metal if not done properly. Mistakes like under-bending or over-bending can cause the metal to warp, which may lead to structural issues.
• Requires Skilled Labor: Skilled labor is often needed to ensure high-quality hemming. The operator must be well-trained to handle the tools and manage the variations in hemming techniques.

Common Sheet Metal Hemming Procedures

Metal hemming services commonly use two main processes to fold sheet metal edges back onto themselves. Each method has its applications and advantages.

Hemming procedure for sheet metal

Die Hemming Process

The die-hemming process is well-suited for high-volume production. This technique involves folding the flange of a custom sheet metal part along its length using a die and press. The hemming operation usually follows initial forming, drawing, and trimming steps. 

Using a hemming tool, the flange is folded over in stages, based on the specific angle of the flange at each step. Die hemming works best for flat, simple panels. While the initial setup can be costly, this method achieves shorter cycle times, making it efficient for mass production.

Roller Hemming Process

This process uses a hemming roller to fold the edges. Guided by an industrial robot, the roller bends the sheet metal edges gradually over several steps until it reaches the desired angle. This method was developed to add flexibility to hemming operations, allowing manufacturers to handle a range of shapes and sizes.

In roller hemming, the metal sheet can be repositioned to minimize the roller’s travel path, which helps shorten cycle times. This process is ideal for both low- and high-volume production. Additionally, roller hemming requires less expensive tooling than die hemming, though it typically takes longer to complete each cycle.

Types of Sheet Metal Hems

Manufacturers use various types of hems in sheet metal fabrication, each with unique features and ideal applications. Here are the four most common types:

Common types of sheet metal hems 

Open Hems

Open hems feature soft, curved external edges with a small air pocket within the bend, making them ideal for high-touch parts such as handles. They work best with material thicknesses of 0.040 to 0.125 inches and require a minimum flat length four times the thickness of the material.

To create an open hem, bend the sheet metal at an angle of 30–45º, place a shim within the bend, and then flatten the material with the shim in place. Typical gap sizes for open hems include 0.060, 0.090, 0.125, 0.187, and 0.250 inches.

Closed Hems

Also known as crushed hems, they involve rolling or folding the sheet metal edge completely flush onto itself. This type of hem is commonly used because it’s easy to form and provides a secure edge. Closed hems are compatible with material thicknesses from 0.040 to 0.125 inches. For thicker sheets (above 0.125 inches), closed hems can cause fractures, so it’s best to avoid thicker materials.

To create a closed hem, first bend the sheet metal to 90º, then flatten it completely using a press brake. Closed hems are not recommended for aluminum, as the material’s tendency to crack makes it less suitable for this hem type.

Teardrop Hems

These hems involve bending the edge of the sheet metal to 90º, then rolling it into a teardrop shape. This method is useful for materials that may not withstand the tight bends required in closed hems. It offers a durable and resilient edge with excellent resistance to deformation.

Teardrop hems are ideal for fragile metals like aluminum, providing a reinforced edge without risking fractures. However, due to the specialized metal hemming tools and skills required, teardrop hems can be costly to produce.

Rolled Hems

Machinists use rolled hems to eliminate raw edges on sheet metal, creating a smooth, rounded finish that’s safe to handle and visually appealing. Rolling hems are common in products like cabinets, doors, electronics, and fixtures and enhance both appearance and durability.

To form a rolled hem, bend the sheet metal edge to 90º and then roll it over itself with a hemming or rolling machine. This method results in a fine, polished edge, improving the product’s physical properties and longevity.

Rope Hem

A rope hem is formed by bending a flange back at an angle greater than 180 degrees. The flange is then pressed against the inside radius of the sheet metal using a flattening die, resulting in an open shape along the hem’s length. To create a joint, the edge of the hem is shaped similarly to an open hem.

The second piece to be connected fits into the gap between the metal sheet and the bent flange. Further pressing ensures the second piece sits flush, securing the joint tightly.

Applications of Sheet Metal Hemming

This sheet metal forming technique has a wide range of applications to enhance the durability, appearance, and functionality of metal components. Here are some of the primary uses of hemming in different fields:

Sheet metal hemmed parts

Automotive Industry

In the automotive industry, sheet metal hemming is essential for securing and reinforcing parts that undergo frequent stress. Hemming is often used to join exterior panels, like doors, hoods, and trunk lids, creating a smooth, rounded edge that protects against sharp corners. This also improves the strength and stability of the panels, allowing them to withstand vibrations, impacts, and harsh weather conditions.

For instance, the door panels on cars are hemmed along the edges to reinforce their structure and ensure a snug fit around the door frame. This improves the aesthetics and enhances the door’s resistance to bending and warping over time.

Aerospace Industry

This industry requires precise and strong components that can withstand extreme conditions. Hemming is often used to enhance the durability of metal parts in aircraft interiors and structural components. Lightweight, durable sheet metal materials like aluminum are commonly used in aerospace, and hemming helps reinforce these materials without adding significant weight.

For example, hemming is used in the construction of overhead bins, seating components, and floor panels, where a sturdy edge can prevent damage and reduce sharp edges. Additionally, since hemming provides a tight, secure edge, it is crucial for areas where two components need to fit seamlessly. It contributes to the overall functionality & safety of the aircraft.

Electronics and Appliances

This process is also frequently applied to create clean, safe edges on products like refrigerators, washing machines, ovens, and various small devices. This improves safety by eliminating sharp edges and enhances the appearance of the products. Additionally, a well-hemmed edge protects internal components from external damage and provides a robust enclosure for the device.

For example, refrigerator doors often have hemmed edges to provide a smooth, rounded appearance while reinforcing the frame and reducing the risk of accidental cuts or scrapes for users. Similarly, metal housings for washing machines and dryers are hemmed for aesthetic appeal and improved structural integrity.

Industrial Equipment and Machinery

Industrial equipment manufacturers use sheet metal hemming to enhance the durability and safety of various machine parts. Many machine enclosures, guards, and panels incorporate hemmed edges to prevent cuts from sharp metal surfaces and improve the strength of the enclosures. Hemming also helps protect sensitive internal components by reinforcing the metal casing around them. It provides added protection against impact or accidental contact.

For example, hemmed edges are commonly found on the access panels of industrial machinery, such as those used in manufacturing or food processing plants. This ensures that workers can handle and operate the equipment safely while maintaining its integrity over time.

HVAC Systems

Hemming is frequently applied in the HVAC industry to manufacture ducts, vents, and enclosures that require smooth, safe edges. HVAC systems often feature long metal ducts and vent covers that can pose safety risks if left with sharp edges. By hemming these components, manufacturers can provide a safer handling experience during installation and maintenance while also improving the durability of the HVAC system.

For instance, sheet metal ducts used in commercial buildings often have hemmed edges to ensure safe installation, prevent air leaks, and enhance the overall longevity of the ductwork. Additionally, hemmed edges on vent covers create a cleaner look, which is important in visible locations, such as in residential or office spaces.

Medical Equipment and Devices

In the medical industry, where safety and hygiene are critical, hemmed sheet metal is often used to create smooth edges on equipment and devices. This helps prevent accidental injuries and makes surfaces easier to clean and sterilize. Hemmed edges are found in equipment like hospital beds, surgical tables, and medical carts, where safe, rounded edges are essential for both patients and healthcare providers.

For example, medical carts used in hospitals and clinics often feature hemmed edges to minimize sharp surfaces, reducing the risk of injury to healthcare staff and patients. Hemming also makes these surfaces easier to sanitize, supporting the stringent hygiene requirements in medical environments.

Conclusion

Sheet metal hemming is a vital process in metalworking, offering multiple benefits across various industries. From enhancing safety with smooth edges to improving strength and appearance, hemming adds value to metal parts in countless applications. With various types available, this process is adaptable to different needs, materials, and structural requirements.As a result, manufacturers use it to achieve strong, reliable, and visually appealing metal components. Zintilon can help you choose the best hemming technique based on your specific requirements. Our sheet metal fabrication services incorporate sheet metal hemming to produce high-quality metal parts that stand the test of time. Contact us today, and let’s work together!