How To Roll The Edge Of Sheet Metal
Content Menu
● Understanding the Basics of Sheet Metal Edge Rolling
● Materials Suitable for Edge Rolling
● Step-by-Step Process for Rolling Sheet Metal Edges
● Variations and Advanced Techniques
● Common Defects and How to Avoid Them
● Real-World Examples and Case Studies
● Q&A
Introduction
Sheet metal edge rolling, commonly known as hemming, is a fundamental joining technique in manufacturing. It creates strong, clean seams by folding one sheet over another, often used in automotive body panels, appliance casings, and structural assemblies. The process improves rigidity, seals edges against corrosion, and provides a smooth appearance without additional fasteners or welds.
Engineers rely on hemming because it balances cost, weight, and performance. In production lines, a typical car door outer panel hems over the inner reinforcement, distributing loads evenly while keeping the assembly lightweight. Material choice, tool selection, and process control all influence the final quality. Poorly executed hems lead to visible defects, weak joints, or assembly issues.
Research from academic journals highlights how material properties, geometry, and forming methods affect outcomes. High-strength steels, aluminum alloys, and even magnesium sheets have been tested under controlled conditions to optimize hemming parameters. The following sections detail the process, tools, materials, variations, and practical solutions drawn from these studies and industry practice.
Understanding the Basics of Sheet Metal Edge Rolling
Hemming involves bending the edge of a sheet metal blank to 180 degrees or slightly beyond, usually after an initial 90-degree flange. The sequence—flanging, pre-hemming, and final hemming—helps control material flow and reduces defects.
Key Concepts and Terminology
The process begins with a flange bend, typically 90 degrees. Pre-hemming then folds the flange to approximately 135 degrees, and final hemming completes the fold. These stages prevent cracking and wrinkling, especially on curved edges.
Springback is a major concern. Metal tends to rebound after deformation, so the final geometry often differs from the tool shape. Pre-straining or over-bending compensates for this.
Roll-in refers to the inward displacement of the hemmed edge during the final stage. Excessive roll-in causes misalignment in assemblies; insufficient roll-in leaves gaps.
Tools and Equipment Needed
Basic hemming uses a press brake with custom dies or a dedicated hemming press. Production lines often employ robotic roller hemming systems, where a roller follows the edge path under controlled pressure.
For small shops, a manual or hydraulic press with adjustable dies works for flat edges. Advanced setups include electromagnetic forming equipment for contactless bending or laser-assisted systems for localized heating.
Materials Suitable for Edge Rolling
Material selection determines feasibility and quality. Common choices include:
Common Materials and Their Properties
Low-carbon steels (e.g., DC04) offer high ductility and consistent forming behavior. They tolerate large strains without cracking.
Aluminum alloys (e.g., AA6016 or AA5182) provide weight savings and corrosion resistance but require careful control to avoid tearing.
High-strength steels (e.g., DP600) offer superior strength but harden significantly during bending, increasing springback.
Magnesium alloys, used in lightweight applications, need elevated temperatures (around 250–300°C) to improve formability.
Material Thickness and Type Variations
Thickness typically ranges from 0.6 mm to 2.0 mm in automotive panels. Thinner gauges wrinkle more easily on concave (shrink) hems, while thicker sheets demand higher forces and show more hardening.
Combining materials—such as aluminum outer panels with steel inners—creates hybrid assemblies with optimized strength-to-weight ratios.
Step-by-Step Process for Rolling Sheet Metal Edges
Preparation Steps
Clean surfaces thoroughly to remove lubricants, dirt, or oxides. Align sheets precisely, especially when joining two panels. Use fixtures to maintain consistent flange height.
Flanging: The Initial Bend
Bend the edge to 90 degrees using a press brake or flanging die. Apply uniform pressure to avoid uneven deformation.
For curved edges, incremental flanging with a smaller tool radius distributes strain more evenly.
Pre-Hemming: Intermediate Folding
Fold the flange to 130–140 degrees. This step compresses the material and begins to lock the panels together.
Robotic rollers maintain consistent force and speed, improving repeatability.
Final Hemming: Completing the Roll
Complete the fold to 180 degrees or slightly more (flat hem) or 170–175 degrees (rope hem). Monitor for defects such as warp, recoil, or excessive roll-in.
Higher closing force reduces recoil but may increase warp. Adjust parameters based on material and geometry.
Variations and Advanced Techniques
Classical vs. Roller Hemming
Classical hemming uses punches and dies in a press. It suits straight edges but struggles with complex contours.
Roller hemming uses a rolling tool guided by a robot or CNC machine. It follows curved paths, reduces roll-in, and handles variable radii.
Incremental Sheet Forming (ISF) Hemming
ISF uses a single-point tool to form the hem progressively. It excels at small radii and complex curves, compensating for material thickening or thinning.
Laser and Electromagnetic Assisted Hemming
Laser heating localizes softening, improving surface quality on aluminum and magnesium.
Electromagnetic forming applies high-energy pulses for rapid, contactless deformation, suitable for hollow or delicate parts.
Common Defects and How to Avoid Them
Wrinkles and Cracks
Wrinkles occur on concave (shrink) hems due to compressive strain. Cracks appear on convex (stretch) hems from tensile strain.
Mitigation: Use incremental forming, adjust flange height, or apply counter-pressure.
Warp and Recoil
Warp results from uneven force distribution. Recoil (springback) causes the hem to open slightly.
Solutions: Increase closing force, use pre-straining, or compensate in tool design. Finite element simulations predict behavior accurately.
Roll-In Issues
Excessive roll-in misaligns panels. Insufficient roll-in leaves gaps.
Control: Optimize roller path, force, and speed. Simulations forecast roll-in and allow pre-adjustment.
Safety Considerations
Use machine guards and two-hand controls on presses. Wear gloves and eye protection. For heated processes, ensure proper ventilation and thermal protection.
Real-World Examples and Case Studies
In automotive production, roller hemming of aluminum doors achieves tight tolerances with minimal roll-in.
Aircraft manufacturers use laser-assisted hemming on magnesium panels for lightweight structures.
Prototyping shops apply ISF for custom curved hems on high-strength steel brackets.
Electromagnetic forming produces strong hems on hollow steel sections without distortion.
Conclusion
Edge rolling (hemming) remains a critical technique for creating durable, lightweight assemblies. Proper material selection, staged bending, and advanced methods like roller or incremental forming ensure high-quality results. Defects can be minimized through process control and simulation.
Manufacturers continue to refine hemming for new materials and geometries. Understanding the mechanics—strain distribution, springback, and roll-in—allows engineers to produce reliable components efficiently. The process delivers strong joints with clean aesthetics, making it indispensable in modern fabrication.
Q&A
Q: What basic equipment is needed for sheet metal hemming?
A: A press brake with flanging and hemming dies works for small runs; robotic roller systems are better for production.
Q: How does thickness affect hemming?
A: Thinner sheets wrinkle easily; thicker sheets require more force and show greater hardening.
Q: Can curved edges be hemmed successfully?
A: Yes, using roller hemming or incremental sheet forming to control strain.
Q: What’s the main difference between classical and roller hemming?
A: Classical uses punches for straight edges; roller hemming follows curves with less roll-in.
Q: How can warp and recoil be reduced?
A: Increase closing force carefully, use pre-straining, and rely on FEM simulations to predict behavior.