What is a Hem in Sheet Metal? How to Hem Sheet Metal Properly

Hem sheet metal refers to the act of folding the edge of a piece of metal sheet over onto itself to create a smooth, rounded, or reinforced edge. It serves to improve safety, strength, and appearance while removing the sharp unfinished edges. Knowing what is a hem in sheet metal and, more importantly, understanding how to hem sheet metal correctly, are mandatory skills for any fabricator looking for quality and precision in their work. Hemming adds strength to the material when it's time to assemble or finish the product, regardless if the hem was done by machine or by hand.

What Is a Hem in Sheet Metal?

What is a hem in sheet metal? In manufacturing, a hem refers to a bend at the edge of a sheet of metal that folds back over it, generally by 180 degrees. The reason for this is to remove sharp edges, enhance the structural rigidity, and enhance the appearance of the part. In addition to doubling the thickness at the edge, the hem can help to prevent warping, cracking, or tearing. 

Hem sheet metal processes are frequently employed in automotive panels, enclosures, appliances, and architectural applications. This process is helpful for both functionality and aesthetics as it will create nice clean lines that enhance ease of management and assembly. Depending on application, the hem can be flat, open, teardrop or rolled.

Fabricators often choose thin gauge sheet metal hemp, such as aluminium, stainless steel and steel. Hemming is a versatile process with respect to the link to manual and automated bending processes and delivers a consistent result for low-volume customized applications and mass-produced components.

How Does Sheet Metal Hemming Work?

How to hem sheet metal will depend on the equipment used, what material is needed, and the type of hem needed. The process usually involves bending the edge of the sheet metal over to a set angle using something like a press brake, hemming die or hand tools. The use of the term "hem" implies that we will finish the edge of the sheet metal with something that folds down flat against the base metal or maybe leave a small gap depending on the hem type.

When learning how to hem sheet metal by hand, fabricators often begin with some simple tools like clamps, hammers, and metal brakes. They mark an edge, bend it to roughly 30-45 degrees, gradually folding it down, or making repeated or progressive bends until the full hemming is completed. 

For production purposes, CNC press brakes and automated hemming machines are used. These systems can produce repeatable bends at suggested radii of hem, which will ensure the ending shape dimensions will remain consistent over large production runs.

Types of Hems

Types of hem in sheet metal define how the edges of the metal will be folded to improve strength, safety or aesthetics. Each hem serves a different purpose.

1. Flat Hemming (Closed Hemming)

Types of hem in sheet metal include several variations, with flat hemming - also known as closed hemming - being one of the most common. 

• A flat hem means that the metal has been folded flat against itself to create a double thickness of metal at the edge. 

• The flat hem is preferred when one desired outcome is a smooth and compact edge, for safety and sometimes aesthetics. 

• Common uses of flat hem occur in consumer appliances, vehicle panels, and cabinetry. 

2. Open Hemming

Open hem sheet metal designs exhibit a partial fold that results in a small gap or separation between the folded layers, making this design useful when a fully closed edge is not needed.

• Open hems are frequently used in ducting or HVAC parts or components that need to accommodate other materials/fastening. 

• The little gap allows for less stress and cracking in the material during the bending process.

Open hems are also easier to create if you are doing it by hand and do not involve as many forming steps; therefore, open hems are useful for rapid prototyping or small-scale production when complicated dies/press have not already been built for specific parts.

3. Teardrop Hemming

How to bend a hem in sheet metal into a teardrop shape means instead of a flat fold, a rounded profile or bulb shape. The teardrop hem is a useful design feature as it simultaneously adds strength while producing a smooth finished edge of the metal.

• A teardrop hem, commonly used in automotive and aerospace various applications, offers aerodynamic advantages, while also having safety and look uniformity as priorities. 

• Taking a teardrop hem is often made with specialized tooling that can handle the radius and thickness of the bend accurately and precisely.

4. Rope Hemming

Rope hemming produces a rounded, tubular edge that resembles the shape of a rope. 

• This hem style is mainly used for safety, and offers elegantness to an edge detail as well as functional qualities.

• Rope hems are typically used in sheet metal furniture, signage, and artistic fabrication.

• Rope hemming has the added benefit of being tubular as it can act as a cover for wires or additional strength as an edge condition.

Different Hemming Processes for Sheet Metal Parts

How to hem sheet metal differs according to the method of fabrication used, substrate, and complexity of design. There are three main hemming processes which are: 

• Press Brake Hemming: It consists of bending the sheet metal to a certain angle and then flattening it. It is the most flexible and popular process for both prototyping and product commissioning.

• Die Hemming: This method is mostly found in the car manufacturing industry where special dies are made for certain shapes of parts. It guarantees high accuracy & consistent results in mass production.

• Table Hemming: It is applied to large panels and assemblies. The sheet edge is gradually folded over using a mechanical arm or roller while the sheet is held on a table.

Why is Hemming Used in Sheet Metal Fabrication

How to hem sheet metal by hand or machine, serves the same purpose: to strengthen edges and remove sharp edges, and improve functionality. Hemming is commonplace in manufacturing industries like automotive, aerospace, HVAC, and consumer goods due to the utility and precision it provides. 

Hemming action allows you to safely seal exposed edges preventing injury to operators, adds corrosion resistance avoiding exposed edges, and gives welders or painters a clean surface on the edge. Hemming means part alignment during assembly is assured for better fit and structural integrity.  

Choosing the right hemming technique, fabricators can easily and dramatically improve the performance and durability of their components while avoiding waste and rework.

Choosing the Right Hem for Your Project

Choosing the appropriate hem type depends on material thickness, design specifications, and the intended use. Open hem sheet metal might be a good option for such ventilation components, while closed or teardrop hems may be appropriate for other applications such as automobile and decorative work.

If you are looking for precision bending, hemming or a combination of custom fabrication and work, there are companies that specialize in providing sheet metal fabrication services, such as Norck. They provide services that can handle precision fabricating for military, plumbing, automotive and many industries, or if you are just making one prototype - whatever you need.