Discover the Process and Benefits of Sheet Metal Hemming

What is Hemming in Sheet Metal?

As we mentioned before, sheet metal hemming aims to improve the surface finish and enhance functionality, thereby producing a perfect hemmed edge on components. Manufacturers will utilize the specialized hemming machine to fold the metal sheet edge at a 180-degree angle. Generally, the hemming process mainly involves three steps, including flanging, pre-hemming, and hemming. Thus, this technique commonly finds in the manufacturing of metal enclosures, containers, household appliances, and more.

How to Hem Sheet Metal?

Now that we have a general understanding of the definition of sheet metal hemming, how can we perform this process? Below are the primary steps to hem sheet metal:

• Prepare Tools and Materials: First, choose appropriate sheet metal materials and tools, such as hammer, brake, or specialized metal folding tools.
• Marking and Cutting: Mark the edges on the sheet metal and use cutting tools to precisely cut along the marked lines. If needed, it can use a folding plier to pre-bend and create a clean crease.
• Clean the Edges: Use sandpaper or polishing tools to clean the metal edges, removing any burrs or irregular parts. As a result, it can achieve a smooth and high-quality edge.
• Fold the Edges: This is the most essential step. Manufacturers usually place the sheet metal on a hard, flat surface, using a hammer to gently tap or using a brake to fold along the pre-marked crease. For thicker materials that require more precise control, metal folding tools can also be used to perform the folding operation.
• Inspect and Adjust: After folding, check that the edges are uniform and meet the requirements.
• Secure and Finish: Utilize welding, riveting, or other fastening methods to hold the folded edges in place. Finally, perform any necessary post-processing, from coating to painting.

Moreover, different types and thicknesses of metal may require specific tools and techniques.

Different Types of Sheet Metal Hems

Each hem has its specific uses and advantages. Therefore, the selection of the appropriate hem should be based on the specific application requirements. This section will introduce five common types of hems used in sheet metal fabrication.

Open Hem:

It refers to using mechanical bending or rolling to bend the edge of the metal sheet outward, thereby forming an open hem. Typically, the bending angle ranges from 90º to 180º, and the standard gap dimensions for this application are 0.060″, 0.090″, 0.125″, 0.187″, and 0.250″. Additionally, open hems are well-suited for sheet metal with thicknesses ranging from 0.040″ up to 0.125″.

Closed Hem:

As opposed to an open hem, a closed hem, also referred to as a flat hem, involves the bent edge being fully flush against the metal sheet, forming a precise 180-degree angle. Consequently, there is no gap or space between the hem and the underlying metal surface. Likewise, this type of hem is suitable for materials, except for aluminum, with thicknesses ranging from 0.040″ to 0.125″. Furthermore, in order to avoid breakage, the flat section of the hem must measure at least 4 times the thickness of the material.

Teardrop Hem:

It refers to curling the edge of the metal sheet into a teardrop shape, with the curling angle ranging between 180º and 360º. This hem design is especially well-suited for aluminum and other lightweight, delicate metal materials.

Rolled Hem:

This edge treatment method is formed by curling the edge of the metal sheet into an arc shape, with a curling angle of more than 180º. Rolled hems are a common choice among manufacturers for edges on figures, cabinets, and doors, as they provide an attractive appearance and a safe handling experience.

Rope Hem:

A rope hem features a returning flange that is bent at an angle greater than 180º. After forming the initial open hem shape, a flattening die is applied to press the returning flange down onto the surface of the part. This creates a gap between the metal sheet and the returning flange, then presses them to form a tight joint.

Processes of Sheet Metal Hem

There are two major processes in sheet metal hemming, which are explained below.

Die Hemming Process

This process completes the hemming operation using a stamping die. The stamping die consists of an upper and lower part, with the upper die bending the metal sheet, and the lower die providing support. This method can quickly and efficiently complete hemming and is suitable for large-scale production.

Roller Hemming Process

This process uses rollers to complete the hemming operation. The metal sheet is gradually fed into the rollers, which bend it into the required shape. This method can achieve continuous hemming and is perfect for complex-shaped hemming. However, compared to die hemming, roller hemming can better control the quality and precision of the hemming.

Pros and Cons of Hemming in Sheet Metal

Benefits:

• Increased Structural Strength: Hemming can increase the bending strength and rigidity of metal sheets, improving the overall load-bearing capacity of the structure.
• Aesthetic Appeal: Also, it can improve the appearance design of the product, enhancing its visual attractiveness.
• Edge protection: It can protect the edges of metal sheets from damage and wear.
• Facilitates Post Processing: In addition, hemming can provide convenience for subsequent welding, assembly, and other processes.

Limitations：

• Process Complexity: It requires specialized dies and equipment, making the process relatively complex.
• Higher Cost: Hemming calls for dedicated equipment and dies, resulting in higher manufacturing costs.

Overall, the hemming process can improve product performance, but the complexity and cost factors need to be weighed. Therefore, properly selecting the hemming process is essential for improving product quality.

Tips for Successful Sheet Metal Hem

Hemming offers a great number of benefits in producing sheet metal parts, but how can we manufacture perfect hemmed components? The following tips provide some useful guidance.

1. Maintain Consistent Hem Depth: Ensure the depth of the hem remains consistent throughout the length of the edge.
2. Achieve Proper Bend Radius: Carefully control the bend radius to create a clean, tight hem.  Avoid over-bending, which can lead to cracking or tearing of the material.
3. Use Appropriate Tooling: Select the right hemming tools, such as dies and rollers, that are suited for the thickness and type of sheet metal being worked.
4. Lubricate Surfaces: Apply a thin layer of lubricant to the tooling and metal surfaces to reduce friction and wear.
5. Maintain Clean Tooling: Keep the hemming tools free of debris and burrs to prevent damage to the sheet metal surface.
6. Inspect for Defects: Regularly inspect the hems for any issues like uneven bends, wrinkles, or cracks, and make adjustments as needed.
7. Consider Material Properties: Adjust the hemming process based on the specific characteristics of the sheet metal, such as hardness, ductility, and springback behavior.
8. Allow for Springback: Account for the natural springback of the material when determining the initial bend angle to achieve the desired final hem shape.

Sheet Metal Hemming vs Seaming

As we mentioned above, sheet metal hemming is a process of folding the edge of a single metal sheet back onto itself to create a clean, finished edge. This is typically done by bending the metal over a die or roller, resulting in a 180-degree fold or a tighter, more acute angle. Hemming provides a smooth, rounded edge that is less prone to sharp corners or burrs. This technique is commonly used for edges on products like appliances, furniture, and automotive parts.

In contrast, sheet metal seaming involves joining two separate metal pieces together by folding and interlocking their edges. The seam is created through a specific pattern of bends, such as a Pittsburgh or grooved seam, to form a tight, secure joint without additional fasteners. Seaming often works in applications like roofing and other sheet metal fabrication. Moreover, the seam can provide structural integrity and weather resistance to the assembled sheet metal assembly.

Customize Sheet Metal Parts at Huayi Group

Huayi International Industry Group Limited introduces sheet metal hemming as a vital technique for enhancing the strength and appearance of metal parts. The process works by folding the edges of the metal, increasing its durability and aesthetics. This manufacturing method is widely used in industries such as automotive and electronics. The article explores the definition of sheet metal hem, its functionality, advantages, and drawbacks. It also covers the various applications of the technique. Huayi International Industry Group Limited aims to showcase its expertise in metalworking and its capability to deliver quality products in the market, offering innovative solutions to meet the needs of different industries.