What’s Sheet Metal Hemming? Process, Uses, and Tips

Sheet metal is widely used in the automotive and electronics industries. However, its sharp edges can warp over time and pose a safety risk. To address this, the edges are often folded in a process known as sheet metal hemming.

This article explains what is hemming, how it is carried out, the types of hems, and their major applications across various industries.

what is sheet metal hemming

The stitched clothing has folds on the sides to prevent warping or tearing. Similarly, the thin sheet metal is folded to hide its burs and improve the strength and warping of the metal.

Sheet metal hemming is a type of sheet metal fabrication in which the thin sheet is bent and folded onto itself. The bent can have a closed shape, completely touching the metal surface, or be open with a slight cavity in between.

The primary purpose of hemming is to add structural strength and durability. Since the open edge of the sheet metal is bent, the risk of developing cracks and warping at the ends is minimal. Furthermore, bending the sheet metal enhances aesthetic appeal. The raw edge, which is sharp and difficult to polish, becomes smoother and ideal for polishing when folded into a curve.

Hemming suits only high-gauge sheet metal (thinner sheets <=0.125”). Low-gauge sheets (thicker sheets) are prone to fracturing on bending.

Sheet metal hemming is a multi-step process carried out in the industry as follows:

Step 1: Material Selection

Material selection is critical as not all types of sheet metal are suitable for hemming. Some common options are:

  • Stainless Steel: Known for strength and durability, suited for heavy-duty applications but challenging to work with.
  • Cold-Rolled Steel: Offers strength and flexibility without needing a protective coating.
  • Aluminum: Lightweight and corrosion-resistant, ideal for many applications but softer and wears out quickly.
  • Copper and Brass: Provide unique colors and finishes that add aesthetic appeal but are soft and not suitable for all applications.

Step 2: Design and Preparation

Various hem types are available, each suitable for different material thicknesses and specific requirements. For example, a closed hem may suffice for a simple, aesthetically pleasing edge, but aluminum may require a teardrop hem due to its structural properties.

Step 3: Marking the Bend Line

Mark the line where the edge needs to fold. Typically, the flange length should be four times the sheet thickness.

Step 4: Initial Bending

Prior to hem forming, bend the sheet slightly at an acute angle with a metal forming tool or press brake. When using the roll hemming process, skip this step.

Step 5: Forming the Hem

After the initial bend, use a press to flatten the bend completely. Insert a shim if needed, depending on the hem type.

Step 6: Finishing Touches and Inspection

Polish and smooth the rounded edge. Inspect the final part for any damage or deformations using a micrometer or any other gauge to ensure the proper width and thickness of the hem.

The flange is not always at 180 degrees, touching the sheet metal itself. It can have different geometries. Based on that different types of metal hem exist:

types of hems

Flat or Closed Hem

Closed hem, the most popular type, is also known as crushed hem. In this case, the returning flange is at 180 degrees to the sheet metal, with no gap in between.

The flat profile is easy to make – fold the sheet metal edge 145 degrees first and then flatten it with a press. The common applications are appliance housing and automotive parts where you need a smooth, round hemmed edge.

The flat hem is compatible with a few metal thicknesses and materials. The profile is ideal for materials in between thicknesses: of 0.40 to 0.125 inches.  Beyond 0.125 inches (3mm), the sheet can fracture on bending. Aluminum is also not suitable for closed hemming.

Open Hem

The open hem is slightly similar to the closed hem. The returning flange is 180 degrees to the sheet metal but there is an air gap in between.

To create, the sheet gets bent to about 145 degrees. Then we place a shim, and flatten it with a press, with a shim inside. Typically, the gap is 0.06, 0.09, 0.125, and 0.250 inches.

The open hem is ideal for use as handles and handheld applications, and ideal for materials within 0.125 inches. However, it is also limited to material with thickness within 0.125 inches.  

Teardrop Hem

Teardrop hem has a closed profile that resembles the shape of a tear. The returning flange gets bent over 180 degrees, forming a loop. To form this profile, first bend the sheet to over 90 degrees and then roll it with a metal hemming tool. Here, the diameter of the loop needs to be at least equal to the sheet metal’s thickness.

Teardrop hems are for those fragile and less flexible materials that cannot undergo flat or closed hemming like Aluminum. They are considered excellent for their resistance to deformation. However, they are slightly expensive to manufacture due to customized setup and skilled labor.  

Rope Hem

Rope Hem is similar in appearance to a teardrop hem but the returning flange is flat from one end. To create a rope hem, the hem is first created like an open hem and then pressed. Similar to teardrop hem, this type only suits less ductile materials.  

There are two routes to carry out the metal hemming operation:

Die Hemming Process

The die hemming process is a traditional method of bending the raw sheet metal edges. The method employs a flattening die and press to shape the metal along its entire length at predefined angles.

Die hemming takes place in two steps. They start with pre-hemming, where the edge is bent to an initial angle such as 145 degrees using a press brake. The final hemming completely flattens the sheet and completes the hem shape using a different part of the same sheet metal hem tool.

This process is mostly used for producing flat, simple panels and is not flexible when it comes to handling complex shapes. Although the initial investment in die-hemming equipment is high, it offers low cycle times, which makes it quite suitable for mass production.

Roll Hemming Process

Roll hemming is an advanced process that utilizes robots or manual machines equipped with rollers to bend the sheet metal edges. The rollers travel along the edges, methodically bending them to the desired angles in multiple steps. During this process, the orientation of the sheet can be altered at times to bend it according to the design. This flexibility makes it ideal for varied and complex part geometries.

Although hemming allows for rapid changes between different production tasks, thanks to quick programming adjustments, it’s particularly economical. It can handle both low and high-volume tasks effectively.

Moreover, the tool costs for roll hemming are significantly lower than those for die hemming, but the cycle times are slightly longer due to the incremental nature of the bending with hemming rollers.

Tips for Successful Hemming

sheet metal hem edge

Adding hemming in sheet metal has both aesthetic and structural benefits. However, the process’s complexity requires extreme care. Otherwise, it can harm instead of benefit.

Proper Material Selection

Material selection is crucial as not all materials are ductile enough to withstand bending. Some materials like cast iron could break on bending. Materials like aluminum are generally suitable only for flat hems. Low-gauge (thicker) sheet metals over 0.125 inches are also prone to fracturing.

Accurate Preparation and Measurement

The folding edge needs to have a standard length and radius, otherwise it may fracture. The standard length of the flange should be four times the thickness of the metal (in an open hem), and the diameter (in the case of a closed hem) needs to be at least equal to the material thickness.

Use the Appropriate Tools

The hemming process needs to be done using bending tools, either the press brake or a hemming roller. For simple flat hemming or bulk production, use the press brake. The hemming roller suit bends on complex geometries.

Gradual Bending Technique

Employ a two-step bending technique for effective results. Start by bending the metal at an acute angle in a V-shaped die. Then, complete the process by flattening it with a press. This gradual approach helps in managing the stress on the material, reducing the risk of damage.

Hemming is a popular bending technique due to multiple advantages:

Hemming reinforces the sheet metal by folding its edges, which doubles the material thickness at the critical points. This not only increases the strength and durability of the metal but also reduces the risk of deformation under stress or load.

Polishing and finishing the raw edge of sheet metal can be challenging and sometimes unfeasible. These sharp edges also pose a safety risk. Hemming produces a smooth, rounded edge that is easier to polish, which enhances the overall aesthetic appeal of the metal component.

Raw sheet metal with thin edges is susceptible to warping and wear. When folded, the delicate edges gain twice the thickness, which enhances their resistance to physical damage. This process effectively seals the edges, reducing exposure to environmental factors that can lead to corrosion and wear.

Besides multiple advantages, there are a few downsides to the process as well:

Sheet metal hemming cannot be accomplished with ordinary bending or mechanical tools; it necessitates a specialized hemming press or roller setup. This specialized equipment is costly compared to standard tools.

The hemming process involves a series of steps, from pre-hemming to final flattening, each of which takes time. This multi-step process lengthens production times.

Hemming requires attention to detail at every stage, from the initial design and standard sizing of bends to the final finishing of the metal. This complexity requires skilled operators with specific training and knowledge of metalworking.

Hemming finds its applications in areas where one needs to stiffen sheet metal edges, hide imperfections, or secure two sheets together without a fastener. Some applications are:

IndustryApplicationsAutomotive IndustryHoods, trunk lids, doors, fendersAppliance ManufacturingRefrigerators, ovens, washing machines, dryersFurniture ManufacturingDesks, shelving, cabinetsConstruction and BuildingMetal doors, windows, framesAerospace IndustryAircraft panels, doorsElectronic EnclosuresHousings and enclosures for electronics

Hemming is often confused with two similar metal forming processes, seaming and metal jog. However, it’s different from the two.

Sheet Metal Hemming vs Seaming

In hemming, the sheet metal folds onto itself, with or without a cavity in between. It can have different cross-sections based on the type of hem. The primary purpose of metal hemming is to reinforce metal edges and improve the structural integrity of the material.  

The design of seaming in sheet metal is similar in shape to an open hem – the same U-shaped cross-section. However, its purpose is to seal and interlock the edges of two separate pieces of sheet metal.

Sheet Metal Jog vs. Sheet Metal Hem

Hemming involves folding the edge of sheet metal back onto itself, creating a single curve that strengthens and smooths the edge. On the other hand, a jog (or joggle) involves creating two opposing bends close together in the sheet metal, resulting in a Z-shaped offset that facilitates the assembly of parts.

Hem’s purpose is edge durability and appearance, while jogs improve the assembly and fit of metal components.

Conclusion

Sheet metal is inherently flimsy and prone to bending. The hemming operation, performed using specialized tools, enhances strength by doubling the thickness at the edges. This folding also conceals imperfections and facilitates easier polishing of hemming edges, thus improving the overall aesthetics of the component. Hemming primarily enhances structural integrity and appearance, but open hems can also serve as joints.

If you’re looking to design sheet metal parts and need plasma, laser cutting, or bending services. Then, choose RapidDirect Sheet metal fabrication services for all your needs, from low-volume intricate components to mass production. We can fulfill your requirements with a team of expert fabricators who adhere to the highest quality standards, ensuring precision in every part manufactured. Contact us today to get started!

FAQs

What is the typical length of a hem in sheet metal?

Typically, hemming is performed on sheet metal in thicknesses, 0.040″ to 0.125″.  In open hems, the return length of the hem is typically 4 times the thickness of the hem. Whereas, in closed hems, it’s 6 times the metal thickness. So, its maximum length is 0.75 inches.

What is the recommended length for a hem on sheet metal?

To create an open hem, hem length needs to be within 0.16 to 0.5″, subject to sheet metal thickness. In closed hems, the length needs to be within 0.24 to 0.75″.

What is the purpose of hemming?

The purpose of hemming is to improve the structural integrity and durability of the sheet metal by preventing it from warping or corroding. The bent edge is easy to polish, which improves the overall aesthetics of the component.   

How much length is gained by releasing a hem in sheet metal?

It depends upon the sheet thickness and type of hem. For instance, if you release a flat hem in 0.125″ sheet metal, 0.5″ (4 x 0.125) length is gained.  



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