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What Is Sheet Metal Hemming And What Are Its Types

by | Dec 28, 2023

Hemming is a popular metalworking process that is primarily used to strengthen an edge, conceal any rough edges, or enhance the surface quality. It is a joining process for folding sheet metals by bending them to 180°. It’s widely used in the automotive industry for creating body panels. The actual hemming process consists of three stages: flanging, pre-hemming, and hemming. In the classical hemming process, a conventional die-hemming punch is used, while in the roller hemming process, a roller is used.

Another objective of sheet metal hemming is to provide a smooth sheet metal edge on the final products. It strengthens the sheet metal and makes it more resistant to defects like fractures, wrapping, and bending.

Types Of Sheet Metal Hems

Closed or Flat Hem

A closed hem, also known as a crushed hem, is a type of hem that is entirely flush. This is in contrast to an open hem, which allows for a bit of space in the first bend. Closed hems are the most widely used type of hem as they are easy to make. To create a closed hem, the sheet metal is flattened onto itself. However, there are some material limitations to consider. Closed hems are best suited for material thicknesses of 0.040″-0.125″. It is best to avoid forming material over 0.125″ as the sheet metal is likely to fracture.

The flat length of the hem needs to be at least four times the material thickness to prevent it from breaking. To shape closed hems, sheet metal is formed to an acute angle of 145 degrees, then flattened onto itself with a press brake. Closed hems are generally not used for aluminum, as this material is highly susceptible to cracks and fractures.

Open Hem

An open hem is a type of edge that has two layers: a soft, curved outer edge and a small air pocket in the bend. This type of edge is perfect for handles and other applications that will be in contact with people. Like a closed hem, open hems work best with materials that are between 0.040″ and 0.125″ thick, and the flat length should be at least four times the material thickness.

Open hems are created by forming sheet metal to a 145-degree angle and then inserting a shim. The material is then flattened onto itself with the shim in place to form the open hem. The most common gap sizes for open hems are 0.060″, 0.090″, 0.125″, 0.187″, and 0.250″.

Rope Hem

A rope hem is a type of hem that has a returning flange bent at an angle greater than 180 degrees. The returning flange is then pressed onto the inside radius of the part surface using a flattening die, giving the entire length of the hem an open shape. To create a joint, the edge of the hem is bent in a similar shape to an open hem.

The second piece that needs to be connected is then inserted into the gap between the metal sheet and the returning flange. Further pressing is done to make the second stage completely flush between the metal sheet and the returning flange, creating a secure joint.

Teardrop Hem

The teardrop hem is created by bending the edge of sheet metal to 90 degrees and then rolling it into a teardrop form with a hemming tool. Teardrop hems can be used for materials that lack the essential flexibility of closed hems.

This hem gives a strong, long-lasting edge that is resistant to distortion. They are great for delicate metals like aluminum. However, they are expensive to make since they demand specialized labor and complex machinery.

Rolled Hem

Manufacturers and designers frequently use rolled hems for figure edges, cabinets, electronics, and doors. This hem provides a product that is safe to handle and provides a flawless and fine finish that improves the durability and physical properties of sheet metal parts.

Bend the metal part’s edge to 90 degrees first using a hemming or rolling machine, then roll it over to hem sheet metal and make a smooth and rounded edge.

A contract manufacturing company in Chennai provides the necessary guidance to choose the appropriate sheet metal hems for the requirements in different fields.

Types of Sheet metal hems

Sheet Metal Hemming Operation

Folding sheet metal over itself to achieve a neat and finished edge requires specialized equipment and techniques. These variables, however, are dependent on the sort of hemming required for your parts. The following are standard steps to hem sheet metal.

1. Tools Needed

  • Sheet Metal part
  • Hemming tool or Press brake
  • Tool for flattening (plier, press, or roller)
  • Finishing tool ( a hammer or sandpaper)
  • Quality inspection instruments (a micrometer)

2. Planning

To guarantee a good sheet hemming operation, make sure the metal sheet is horizontal and devoid of contaminants that might impede the process. Then, position the hemming tool to get the desired hem width and angle. Insert the sheet metal into the hemming tool, assembling the edge that requires hemming.

3. Hemming Method

The hemming tool or press brake bends the metal sheet’s edge at a 90-degree angle and folds back the first bent edge to make a hem. Following that, grab the hem to achieve a solid fit. You may also use finishing tools to smooth off uneven edges.

Hemming is a two-stage process. The first stage, die hemming, often involves creating an acute bend angle with acute tooling (V die), followed by flattening the return flange with a flattening die. Researchers and sheet metal and stamped parts suppliers mostly use three-dimensional solid elements, shell elements, and their combination for the meshing of sheet metal blanks in the hemming process.

4. Assessment

After finishing the hemming, inspect the completed item for any damage or deformations. A micrometer or gauge, for example, can be used to ensure that the hem is the correct width and thickness. As a result, you may make the required changes to guarantee that the completed product meets all needs or standards.

Various types of high-strength sheets, aluminum alloys, recycle-friendly aluminum alloys, magnesium alloys, and lightweight alloys are used as materials under the assistance of contract manufacturers in the various sheet metal hemming processes.

Conclusion

Some researchers have also developed analytical models for the hemming process. In the past, researchers have devised and adopted new experimental setups and strategies. Additionally, new computational approaches, such as the meta-model strategy, are also applied for the efficient design of the whole hemming tool and process. Speak to our experts for efficient sheet metal hemming process.

Aparna Sushumna | Author

Aparna Sushumna

About the Author

Aparna Sushumna, a mother to a hyperactive toddler who is all over the house. I aspire to be a decent content developer. A Bachelorette of technology says my qualification but I anticipated being a singer. Thanks to the recession, I dwelled into various jobs, from coding to being a tech support executive to a help desk professional, only to conclude that there is something else I wish to do. Here I am to accomplish my profound passion for content writing. Music, nature, jewelry, beauty, mythology, life quotes, celebs, and their life, being my areas of interest.