There are several metalworking techniques that product manufacturers use to reinforce the functionality and overall appearance appeal of metal parts. Sheet metal hemming is a standard method for improving the quality and appearance of your sheet metal products.
This article discusses the principle behind sheet metal hemming, its purpose, and how you can get the best from it. You’ll also learn the types of sheet hems alongside the advantages and disadvantages of the process.
What is Sheet Metal Hemming?
Sheet metal hemming is a metal-forming technology that involves rolling or folding sheet metal edges over and onto itself. You have to fold over the edges of sheet metal to create an area with two layers. Sheet metals are of varying thicknesses and are usually categorized into high-gauge and low-gauge sheet metals.
The high-gauge category comprises thicker metals than the low-gauge category, making them not very suitable for hemming processes. It is essential always to ensure the accuracy of the sheet hemming process because it impacts the appearance and quality of the part’s surface.
Purpose of Sheet Metal Hemming
The primary purpose of sheet metal hemming is to enhance the metal sheet’s structural strength and durability. It makes the sheet metal stronger and more resistant to defects such as cracks, wrapping, and bending. Folding the edge of a metal sheet onto itself makes it stiffer and thicker, ensuring it doesn’t deform under pressure.
Another purpose of sheet metal hemming is to create a neatly hemmed edge on the final parts. You can align the folded edge with the curves of the surrounding material for an ideal and polished appearance. The automotive sector utilizes this sheet metal fabrication technique in applications where a product’s appearance is critical to its general value.
Therefore, the overall purpose of sheet metal hemming lies in the aesthetic and functional advantages.
How Does Sheet Metal Hemming Work?
The process of folding or bending the edges of a metal sheet onto itself to form a clean and finished edge works with specific tools and steps. However, these factors depend on the hemming type that your parts require. Below are typical steps involved in sheet metal hemming.
1. Tools Required
Here are some of the standard tools needed for hemming.
- Sheet metal
- Sheet metal hemming tool or Press brake
- Flattening tool (i.e., plier, press, or roller)
- Finishing tool ( i.e., hammer or sandpaper)
- Quality inspection tools (i.e., micrometer)
To successfully perform the sheet hemming operation, ensure the metal sheet is flat without any contaminant that can hinder the process. Then, set up the hemming tool per the preferred hem width and angle. Place the sheet metal in the hemming tool, assembling the edge that needs hemming with the hemming tool.
3. Hemming Procedure
The hemming tool or press brake bends the metal sheet’s edge at a 90º angle and folds the initially bent edge back onto the metal sheet to form a hem. After, clutch the hem to ensure a secure and firm fit. Also, you may use finishing tools to improve uneven or rough edges.
There are two standard procedure options involved in sheet metal. They include:
Conventional Die Hemming
The conventional die hemming method involves using a punch and a die to make a hem on the edge of metal parts. Usually, you have to position the die below the sheet metal while you bend the metal over the die to form the hem using the punch.
This sheet metal hemming process uses a hemming roller to fold the sheet metal’s edge onto itself gently. This method is ideal for parts that need a more gentle and moderate bend or thin sheet metal.
After completing the hemming procedure, you must examine the finished part for any damage or deformations. There are various tools, such as a micrometer or gauge, to confirm that the hem has the appropriate width and thickness. Hence, you can make necessary corrections to ensure the finished product fulfills all requirements or standards.
4 Types of Sheet Metal Hems
This section discusses the typical hemming types of sheet metal hems.
1. Open Hems
Open hems usually have soft, curved external edges and an air pocket in their bends. It is best for handles and several other high-touch applications and handheld parts. The open hems offer the best results with a material thickness of .040-.125 inches and a minimum flat length four times the material’s thickness.
Bend sheet metal to 30 – 45 º to make open hems, then place a shim in it and flatten the material onto itself with the shim in position. The standard gap sizes include .060, .090, .125, .187, and .250 inches.
2. Closed Hems
Closed hems or crushed hems are standard because they are easy to create. A closed hem is a total flush in which the sheet metal is rolled or bent on itself. Although there may be some downsides with materials. This sheet hem type is highly compatible with the material bearing thickness range of 0.040 – 0.125 inches.
Do not attempt forming sheet metal with thicknesses over 2-3mm (.125″), as it is vulnerable to fracture. More importantly, ensure the hem’s flat length is at least four times the material’s thickness to prevent it from breaking.
You can also form sheet metal to 90 º to shape closed hems, then use a press brake to flatten the material onto itself. As a general thumb rule, a closed hem is not ideal for aluminum because of its high vulnerability to fractures and cracks.
3. Teardrop Hems
The teardrop hem involves bending a sheet metal’s edge to 90º and then rolling it into a teardrop shape with a hemming tool. You can consider using teardrop hems for materials without the required closed hems flexibility.
This hem provides a robust, long-lasting edge that ensures excellent resistance to deformation. Teardrop hems are ideal for various fragile metals such as aluminum. However, teardrop hem is costly because it requires skilled labor and sophisticated tools to create.
4. Rolled Hems
Manufacturers use rolled hems to eradicate raw sheet metal edges in metal parts. Product manufacturers and designers often utilize rolled hem for figure edges, cabinets, electronics, and doors. This hem offers an appealing appearance and a safe-to-handle product.
Bend the edge of the metal part to 90º at first, then roll it over the sheet metal, forming a smooth and rounded edge. You can use a hemming or rolling machine to create a rolled hem. Rolled hem offers an impeccable and fine finish improving the durability and physical qualities of sheet metal products.
Different Hemming Processes for Sheet Metal Parts
Product manufacturers often use two standard processes to fold sheet metal onto itself in different applications. Below are the processes involved in sheet metal hemming:
Die Hemming Process
The die-hemming process is perfect for high-volume forming operations. This process involves folding the flange over the length using a die and press. Generally, the actual hemming occurs due to the forming, which involves forming the flange with a hemming tool upon completing the drawing and trimming operations.
The hemming of the formed flange involves different steps, which are usually determined by the respective opening angles of the flange. This process is more suitable for the production of flat, non-complex panels. Furthermore, setting up a production tool for die hemming is a little expensive but offers low cycle times.
Roller Hemming Process
The roll or roller hemming process is usually executed with a hemming roller. An industrial robot guides the hemming roller’s bending of the edges, enabling more efficiency. Initially, this process was developed to improve the flexibility of hemming operations. The roller moves along the sheet metal’s edges in several steps to bend it to the preferred angles. The roller hemming process can be categorized into different pre-hemming and final hemming procedures.
However, you can modify the metal sheet’s orientation to reduce the hemming roller’s travel while increasing cycle time. The roller hemming technique is flexible and suitable for low and high-volume production. More importantly, the required tools cost considerably less than those in the die-hemming process. Nevertheless, its cycle times are higher than die hemming.
Advantages of Sheet Metal Hemming
Here are some of the common benefits of the sheet metal hemming technique:
Increased Strength and Rigidity
Metal hemming strengthens the edge of metal sheets used for parts, making it more resistant to deformations like warping and bending. When you fold the edge of metal onto itself, it becomes stronger and thicker. As a result, it won’t be susceptible to deformation under pressure.
This metalworking technique offers parts with a clean and aesthetically appealing edge. Moreover, you can make the folded edge match the curves of the surrounding material, forming a smooth and brilliant appearance.
Manufacturers across industries often use hemming to create a secure and solid edge, enhancing the functionality of various metal products. Sheet metal hemming improves the functionality of metal parts, especially in the automotive industry, where many components are required to withstand extreme stress and wear.
Greater Resistance to Damage
Sheet metal hemming offers metal parts excellent resistance to damage and wear from exposure to chemicals and other corrosive compounds, harsh environments or weather conditions.
Limitations of Sheet Metal Hemming
Although most product manufacturers and designers widely embrace sheet metal hemming for its advantages, here are some of the downsides of the technique that you need to understand:
The sheet metal hemming technique is time-consuming, especially when handling large or complex metal parts. The devising, tool setup, and hemming steps can take longer.
Requires Skilled Labor
You may sometimes need to hire skilled labor to execute the hemming procedure efficiently for the best results. The hemming tools operator must possess adequate training and experience to operate the equipment and fully understand the variations of the hemming technique.
Can Cause Deformation or Damage to Sheet Metal
Hemming is a critical process that can deform or damage sheet metal if not done right. Complications such as under-bending or over-bending can result in metal sheet warping, causing structural problems.
Considerations and Tips for Hemming Sheet Metal
Here are some tips and considerations and tips that can provide the most effective solutions for your hemming needs:
Choosing a sheet metal material is advisable based on what you expect from your final parts, including its durability, strength, and resistance to corrosion. However, different materials, such as magnesium and aluminum, may demand varying hemming processes.
Another critical factor to consider in the hemming process is maintaining proper alignment of the metal sheet in the hemming tool with the correct orientation. It helps to ensure that the hemming tool hems the sheet metal’s edge in the preferred direction. Ensure you position the sheet metal in the center and align it with the hemming for a consistent and regular hem.
Hemming Method Selection
Ensure you perform the hemming technique carefully and correctly to avoid under-bending or over-bending the sheet metal. More importantly, always close the hemming tool slowly to prevent damage or defects to the metal sheet.
Available Finishing Option
Consider improving the functionality and finished appearance of your final parts after hemming. Using essential finishing tools like sandpaper and a hammer, you can easily smoothen out any rough and uneven edges.
Conduct quality control checks at every stage of the hemming process to guarantee that the final part meets all requirements. To control quality, you can use inspection equipment such as a gauge or micrometer to confirm that the hem has the correct width and thickness.
This article has comprehensively discussed the sheet metal hemming process and all you need to know about the technique. So, you should be able to identify the hem types and the methods manufacturers use to make a hem on the edge of sheet metal and determine the right one for your metal parts.
However, if you need the assistance of professionals, our team of professional engineers and technicians is at your disposal. At WayKen, we can offer ideal sheet metal fabrication services including bending, stamping, forming, and hemming for your custom sheet metal projects.
What are the common applications of sheet metal hemming?
The sheet metal hemming process is commonly used to create metal parts with complex shapes and structures in industries including Heating, automotive, metal furniture, aerospace, and electronics.
How is sheet metal hemming different from seaming?
Although these methods involve rolling sheet metal edges onto themselves, they differ. Hemming consists of rolling the sheet metal edge to flush onto itself while seaming connects the edges of two materials to form a solid joint.
Is sheet metal hemming expensive?
Sheet metal hemming costs vary depending on the material used, design complexity, production volume, and sheet metal thickness. However, this process is cheaper than other metal bending or forming methods like casting, forging, or stamping. It doesn’t require expensive tooling as these listed methods.