In sheet metal fabrication, hemming is the process of folding the edge of a workpiece back onto itself to create a smooth, rounded edge. It is widely used on electrical cabinet panels, doors, automotive parts, and other components where structural strength, safety, and aesthetic appearance are critical.
Many fabricators assume hemming can be done by simply “smashing flat” a pre‑bent flange in a standard V‑die. That assumption is incorrect. Hemming is a two‑stage forming process that requires precise geometric guidance—first pre‑bending to an acute angle (20°–30°), then flattening with a dedicated press brake hemming die. Forcing a hem flat in a standard V‑die leads to cracked materials, uncontrolled springback, lifted edges, and even damaged tooling and machinery.
This guide explains the types of press brake hemming dies, key selection criteria, and common mistakes to avoid—helping you understand the essential difference between a dedicated hemming tool and a makeshift V‑die setup.
What Are Press Brake Hemming Dies?
2.1 Definition and Working Principle
Press brake hemming dies are specialized tooling systems designed to fold and flatten sheet metal edges. Unlike standard V‑dies, which concentrate force on two narrow lines and collapse the existing radius rather than closing it smoothly, proper hemming dies capture the flange, support both legs, and progressively close the bend so the material flows instead of fractures.
The hemming process generally consists of two stages:
- Pre‑bending (acute bend): The edge is bent to an acute angle, typically between 20° and 30°
- Flattening: The pre‑bent edge is pressed flat against the base sheet to form a closed hem
2.2 Common Hem Types
| Type | Characteristics | Typical Applications |
| Flat hem | Fully folded and flattened | Door panels, safety edges, appliance housings |
| Teardrop hem | Partially formed, teardrop‑shaped | Decorative edges, interlocking joints |
| Open hem | Not fully flattened, leaves a gap | Interlocking connections, hinge structures |
2.3 SIEOR‘s Hemming Die Offerings
SIEOR provides a full range of standard and custom hemming dies, compatible with Amada, Trumpf, Wila, LVD, Bystronic, and other major systems—with full‑process technical support from die design to press brake parameter matching.
One‑Step vs. Two‑Step Hemming Dies
Press brake hemming dies are divided into two main categories based on the number of forming steps: one‑step hemming dies and two‑step hemming dies. Each serves distinctly different production scenarios.
3.1 Two‑Step Hemming Dies
Pre‑bending and flattening are performed as two separate operations: first using an acute‑angle die for the pre‑bend (20°–30°), then using a hemming die to flatten.
Advantages:
- Wider material adaptability (stainless steel, aluminum, carbon steel, etc.)
- More forgiving to material variations
- Lower tonnage requirements
- Suitable for small‑batch, high‑mix production
Disadvantages:
- Lower efficiency; requires die changes or staged setups
- Higher labor costs
Best for: Small‑to‑medium batches, high‑mix production, prototyping, mixed material thicknesses.
3.2 One‑Step Hemming Dies
Pre‑bending and flattening are completed in a single stroke.
Advantages:
- High production efficiency
- Short setup time
- Suitable for automated production
- Stable batch‑to‑batch repeatability
Disadvantages:
- Limited material flexibility (optimized for specific thickness and hardness)
- Higher initial cost
- Not suitable for prototyping
Best for: High‑volume continuous production (automotive, appliance manufacturing); automated production lines; fixed material specifications.
3.3 Hemming Die Configuration Options
| Configuration | Features |
| Acute punch + flattening die combination | Simplest and most common; can be staged side‑by‑side on the same die holder, no die change required |
| Gooseneck dies | For complex hemming where existing flanges must be cleared |
| Adjustable hemming dies | Adaptable to different material thicknesses and bending geometries |
How to Choose Press Brake Hemming Dies
Selecting the right hemming die requires careful evaluation of several factors. Start by considering the material type and thickness. Different materials—stainless steel, carbon steel, or aluminum—place distinct demands on the die geometry and hardness. The material thickness then determines the required V‑opening and overall die selection.
Production volume plays a key role in determining whether one‑step or two‑step hemming dies are more appropriate. High‑volume continuous runs favor one‑step solutions for speed and consistency, while small‑batch, high‑mix operations benefit from the flexibility of two‑step dies.
Press brake tonnage is another critical factor. One‑step hemming demands significantly higher tonnage than the two‑step approach. Verify that your machine can handle the required load before committing to a one‑step solution.
Precision requirements also influence the choice. Applications requiring tight tolerances demand tooling with higher manufacturing accuracy. SIEOR achieves critical dimensional tolerances of ±0.01mm, ensuring consistent hemming results across production runs.
Compatibility: Different press brake brands use different tooling interface standards. SIEOR provides hemming dies compatible with Amada, Trumpf, Wila, LVD, Bystronic, and other major systems, with full‑process technical support from die design to parameter matching.
Material and hardness: High‑quality hemming dies are manufactured from 42CrMo and similar premium alloy steels, with full quenching and tempering to ensure uniform hardness and extended service life.
Customization: Special workpieces, non‑standard thicknesses, and unusual shapes require custom hemming dies. SIEOR offers tailored solutions based on your specific workpiece and machine requirements.
Hemming Die Usage Recommendations
5.1 Staged Setup for Efficiency
Mount acute‑angle tools and flattening dies side‑by‑side on the same die holder. This allows pre‑bending and flattening to be completed in adjacent stations without changing tools, significantly reducing setup time by combining two operations into one clamping.
5.2 Precise Pre‑Bend Angle Control
The pre‑bend angle must be maintained within a tight 28°–35° window. If the angle is too shallow (<28°), the material cannot fully close during flattening. If too steep (>35°), the material may wrinkle during flattening. Regularly verify pre‑bend angles with an angle gauge and fine‑tune based on material type and thickness.
5.3 Lubrication and Surface Protection
Hemming involves significant sliding friction between material and tooling. Apply appropriate lubricant to the contact areas to reduce scratching and tool wear. For surface‑sensitive materials such as stainless steel or pre‑coated sheets, pair with non‑marking protective film or polyurethane‑coated tooling.
Common Misconceptions
Misconception 1: A standard V‑die can “make do” for hemming
Fact: A standard V‑die cannot provide the geometric guidance needed for rolling closure. The result is workpiece marking, cracking, springback, and lifted edges. This is not forming—it is crushing the material.
Misconception 2: More tonnage fixes the problem
Fact: Hemming success depends not on added pressure but on how die geometry controls material flow and strain distribution. More tonnage only increases localized strain and work‑hardening, making the material more brittle and crack‑prone. When added tonnage fails to correct the issue, the problem typically lies in die geometry, not insufficient force.
Misconception 3: Pre‑bend angle precision doesn‘t matter
Fact: Even slight deviations in pre‑bend angle affect final flatness. The angle must be precisely controlled within the 28°–35° range—too shallow and it won’t close; too steep and it wrinkles. This is not a “close enough” parameter; it’s a critical variable determining hemming success.
Misconception 4: All hemming dies are the same
Fact: One‑step and two‑step hemming dies serve fundamentally different production scenarios. Choosing the wrong type leads to inefficiency or inconsistent quality. High‑volume fixed‑specification production favors one‑step; small‑batch high‑mix favors two‑step.
Conclusion
Hemming is not a simple operation that can be “smashed flat” with a standard V‑die. It is a two‑stage forming process that requires dedicated tooling geometry to guide material flow. The essential difference between a proper press brake hemming die and a makeshift solution is this: the former controls material flow; the latter crushes it.
Choosing the right press brake hemming dies requires comprehensive evaluation of material type, thickness, production volume, and precision requirements. One‑step and two‑step solutions each suit different applications.
With over 20 years of industry experience, ±0.01mm precision, full‑brand compatibility, and full‑process technical support, SIEOR delivers reliable hemming die solutions for every application.
Visit the SIEOR website to explore hemming die product details, or contact SIEOR for professional hemming die selection advice and custom solutions.
Reference
LMRM Tool – Comparing One-Step vs. Two-Step Hemming Dies: https://www.lmrmtool.com/comparing-one-step-vs-two-step-hemming-dies-for-sheet-metal/
ADH Press Brake – The Complete Decision Guide to Press Brake Hemming Dies：https://pressbrake.adhmt.com/the-complete-decision-guide-to-press-brake-hemming-dies-why-two-stage-hemming-is-the-industrial-baseline-for-avoiding-a-25-defect-rate/
ADH Press Brake – Stop Cracking Sheet Metal: The Operator‘s Guide to the Hemming Tool：https://pressbrake.adhmt.com/stop-cracking-sheet-metal-the-operators-guide-to-the-hemming-tool-in-the-press-brake/
The Fabricator – Sheet metal hemming strategies on the press brake：https://www.thefabricator.com/thefabricator/article/bending/sheet-metal-hemming-strategies-on-the-press-brake
