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Sheet metal hemming is an important technique which is used in manufacturing. This technique not only gives metal parts a clean finished look but also increases their durability. Therefore in this blogpost we will cover different types of hemming, their benefits, and practical applications. We’ll also give some tips so you can improve your projects.
What is Sheet Metal Hemming?
To put it simply sheet metal hemming is a fabrication technique in which the edge of sheet metal is folded back on itself. This process creates double layer rounded edges which in turn improve appearance, safety and structural integrity of metal components.
How Does Sheet Metal Hemming Work?
Step# 1: Material Preparation
The production procedure starts with selecting and inspecting materials. Some of the common materials are cold rolled steel, aluminum, copper, brass, stainless steel etc. Once the material is selected each sheet is carefully inspected for scratches, surface defects and warps.
When inspection is completed, the sheets are accurately cut to the required dimensions and then all the bend locations are marked on sheets.
Step# 2: Positioning
The sheet produced is then placed firmly on edge or die rolling machine where it is held in place using pins & clamps. This assures proper alignment which is important to get uniform as well as consistent hemming results along the whole edge.
Step# 3: Initial Bending
Once the sheet is properly positioned, a 90 degree bend is then made using press brake and forming tool along the marked line on the edge. This serves as the foundational base for hem. Moreover flange length should be four times the sheet thickness to get accurate and clean bends.
Step# 4: Final Folding
When the initial bend is formed, press or rollers are used to fold the bend edge back onto itself. This step can use different methods based on the needs of particular project. For example die hemming or roller hemming.
Final Step: Flattening
In the last step which is flattening, pressure is applied to the folded hem to smooth out any irregularities and create uniform thickness. This guarantees that hem is completely closed and has smooth accurate finish along with consistent dimensions.
Sheet Metal Hemming Tools & Equipment
Each tool and machine used in sheet metal hemming performs an important part in this process to assure quality and precision. The tools used in this process are as follows
Hemming Dies
These special tools are used to fold sheet metal edges with precision. They apply controlled pressure to create smooth reinforced hems that assure uniform results in complicated designs and large volume production.
Roll Hemming Machines
These machines use rollers to gently bend sheet metal edges. They provide outstanding accuracy and are perfect for making hems on complicated shapes. Besides that their low tooling requirements make them cost effective.
Hand Tools
Sheet metal hemming uses hand tools for precision bending and folding for example hammers, pliers, seamers etc. These tools provide control and adaptability that make them suitable for detail or general tasks.
Press Brakes
These machines perform main function of bending and folding metal sheets. They use die and punch system to clamp sheets between them and create strong, smooth edges as well as precise angles.
Types of Hemming Processes
Die Hemming
In die hemming sheet metal edges are folded by die and press in two stages of operation—pre hemming and final hemming. This process assures uniformity as well as precision. Hence it is used for making flat panels & large volume production in industries such as aerospace and automotive.
Roller Hemming
In this process sheet metal edges are gently folded with robot controlled roller in several passes. This process decreases tooling costs and proficiently handles complicated shapes as well. This method is useful for irregular or curved applications because it provides great proficiency for both big volume & small volume production requirements.
Common Types of Hems in Sheet Metal Fabrication
Rope Hem
A rope hem is made using flattening die which bends flange more than 180° and presses it strongly against sheet metal. This technique creates a secure joint that not only increases edge’s strength but its longevity too. It is generally best suited for applications where reinforcement is important such as in construction & automotive applications.
Open Hem
This is a folded edge with small air gap between layers that provides curved and smooth finish. And it is perfect for material thicknesses ranging from 0.040 to 0.125 inches. Moreover it is suitable for metal roofing, ductwork and siding as well as improving edge safety by reducing sharpness.
Teardrop Hem
A teardrop hem has distinct loop like edge which bends more than 180°. The process begins with 90° bend and continues until the metal forms a teardrop shape. It works particularly well with fragile metals like aluminum and requires diameter of two to three times the thickness of material.
Rolled Hem
Rolled hem is produced by rolling edge of sheet metal onto itself with smooth tubular finish. The metal is passed through series of rollers or dies to form rolled hem. In HVAC and furniture manufacturing, they are mostly used to assure that there are no sharp edges.
Closed or Flat Hem
In closed hem the sheet metal edge is folded completely flat against itself at 180° angle. They are used mostly in electronics and automotive industries. It provides three important advantages including sharp edge elimination, improved durability, and a clean, smooth finish that assures structural integrity.
Advantages & Limitations of Hemming in Sheet Metal Fabrication
Advantages
Better Safety
Hemming process folds edges to remove sharpness and produces safe and smooth surfaces. This in turn minimizes risk of injury during assembly or handling. Thus it is generally best suited for industries such as aerospace & automotive where safety is a priority.
Improved Aesthetics
Hemming makes polished and professional looking edges in order to improve the appearance of product. This smooth, rounded finish not only adheres well to surrounding contours but also aids in better adhesion of surface coatings for premium finishes.
Outstanding Longevity
Hemmed edges resist corrosion, wear and environmental stress because of their double layer structure. In addition to that, this gives them great protection against tough conditions and extends life span of these components in heavy duty applications.
Facilitation of Assembly
Hemming makes uniform edges that facilitate assembly and assures accurate alignment during component integration. This process in addition to decreasing assembly time and errors also improves their compatibility with other parts.
Edge Reinforcement
In sheet metal hemming, edge reinforcement increases strength of material by folding its edges and doubling the thickness of material at important points. This reinforcement besides preventing deformation under stress and increasing structural integrity also assures long term durability.
Limitations
Risk of Deformation or Damage
Improper hemming technique can cause many problems such as cracks, structural failures, warping etc. Also, in some cases over bending or under bending can create too much tensile stress at bend corners. Similarly applying wrong pressure can cause deformation of material during flattening.
Time-Consuming Process
Hemming requires precise execution of several steps—pre hemming and final flattening. Hence tool setup and handling of complex shapes can take more time which makes it slower compared to other alternative techniques.
Requirement for Skilled Labor
Hemming requires operators who are skilled in process execution, tool setup as well as material handling. Because without skilled workers, complex projects can be prone to errors, inconsistent quality and slow production.
Material Thickness Constraints
If a sheet is thicker than 3mm it may be prone to cracking or warping during folding.
Applications of Hemming in Various Industries
Automotive Industry
Hemming is used in automobile industry to join trunk lids, doors, body panels and hoods. This technique provides advantages like corrosion protection and lightweight design which in turn improves vehicles fuel proficiency.
Aerospace Industry
Hemming is used to reinforce lightweight aluminum components in aircraft. These include floor structures, cabin interiors and aircraft panels. Moreover it facilitates accurate assembly of complicated aircraft structures and decreases aerodynamic drag as well.
HVAC Systems
Ducts, vents and enclosures that are used in HVAC systems are made by hemming. This technique not only prevents air leakage but also extends product life and assures clean finishes in visible installations.
Electronics Enclosures
Electronic device manufacturers use hemming to create smooth edges that protect internal components & prevent injuries from cuts. It further improves structural integrity of enclosures and permits accurate assembly of high-performance electronic devices.
Appliance Manufacturing
Hemming is broadly used in ovens, washing machines and refrigerator doors in order to make smooth, durable edges.
Tips for Successful Hemming
Select Appropriate Hem Type
When picking the type of hem, you should consider characteristics of your material, its thickness and application needs. If you want maximum strength, use a closed hem but if you want flexibility, use open hems. In case you are dealing with delicate materials such as aluminum, then rolled or teardrop hem would be best.
Consider Material Thickness & Bend Radius
You must match your bend radius with material thickness in order to obtain best results. Therefore for sheets that are less than 6mm in thickness, use bend radius equal to the thickness. Similarly use 1.5x and 2-3x of bend radius for sheets with thicknesses between 6-12mm and greater than 12 mm respectively.
Apply Progressive Bending Techniques
You should start with a 45 degree pre hem bend, then slowly increase the angle and complete the process at 180° with final flattening. This step by step procedure will help you prevent cracks as well as decrease stress on material.
Conduct Quality Control Test
You should perform systematic inspection with gauges & micrometers to guarantee that dimensions are within ±0.1mm tolerances. In addition you should check for edge smoothness, structural integrity and surface defects at each production stage. You can assure durability and uniform quality by doing regular testing.
Use Proper Tooling & Equipment
You should pick tools according to your hemming needs. Thus choose press brakes for flat hems, rollers for complex shapes, and special dies for aerospace components. Besides that keep your tools properly cleaned and maintained to guarantee steady quality and avoid surface damage.
Sheet Metal Hemming vs Seaming
| Characteristic | Hemming | Seaming |
|---|---|---|
| Process | Folds edge of single sheet onto itself | Joins two separate sheets via interlocking folds |
| Primary Purpose | Edge reinforcement and finishing | Creating strong, sealed joints |
| Tools Used | Rollers, hemming dies | Interlocking rollers, seaming machines |
| Applications | Appliances, automotive panels | HVAC ducts, canned goods, Roofing |
| Edge Profile | Smooth, folded edge | Interlocked or overlapped joint |
To Sum Up
In short sheet metal hemming is adaptable technique that improves edge strength and appearance in variety of industries. Manufacturers can obtain highly-precise hems for applications such as aerospace and automotive by using proper tools and hem types.
If you require sheet metal hemming or fabrication services for any kind of material, then RICHCONN is best option. You can contact us anytime.
Related Questions
How to bend a hem in sheet metal?
It requires two main steps. The first is to create 90 degree fold on the edge using press brake tools and then press it to 180 degrees to get hem with smooth and even finish.
How to hem sheet metal by han?
First you need to make initial bend using a medium straight edge and mallet to create a hem. Then, along with the hand seamers, slowly lower the edge and flatten the hem by hammering it on flat surface.
How to make a rolled hem on sheet metal?
In this process, you will first make a 45 degree bend and then gently curve the edge with roller tools. Then make multiple passes with consistent pressure and continue till you get a cylindrical shape.
How does hemming differ from other edge-finishing techniques like flanging or beading?
Hemming folds edges back on themselves for reinforcement. Flanging and beading, on the other hand, create raised edges and grooves for decorative and structural purposes respectively.
Can hemming be performed on non-metal materials, such as plastics or composites?
Yes you can do hemming on no metallic materials. But this process requires special tools or heat forming to obtain edge reinforcement similar to metal hemming.
