Press Brake Hemming Tool Guide: Types, Hemming Process & How to Choose the Right Tool
Last Updated: August 2026
A press brake hemming tool is used to fold a sheet-metal edge back onto itself or nearly onto itself. Hemming is commonly used to remove sharp exposed edges, increase edge stiffness, improve appearance, create safer handling surfaces, and produce finished flanges on cabinets, doors, panels, enclosures, automotive parts, and general sheet-metal components.
Most press brake hemming operations use a two-stage process. First, the sheet is pre-bent to an acute angle using an acute punch and compatible lower die. Second, the pre-bent flange is flattened or partially closed using a hemming or flattening tool.
This YRS Industrial guide explains open hems, closed hems, teardrop hems, acute pre-bending, flattening tools, hemming dies, tonnage, material thickness, tooling materials, springback, CNC programming, common defects, maintenance, and how to choose the right press brake hemming tool.
Press Brake Hemming Tool Quick Answer
Step 1: Pre-bend the flange to an acute angle, commonly with a 30° acute punch and acute die.
Step 2: Move the part to a flattening or hemming station.
Step 3: Close the hem to the required condition: open, closed, or teardrop.
Main benefits: safer edges, greater stiffness, cleaner appearance, and improved part quality.
Main caution: hemming force can be high, especially for thicker material and closed hems. Tool load and press brake capacity must be checked carefully.
Related guides: Acute Punch Guide, Radius Punch Guide, Press Brake Tooling Guide, Multi-V Die Guide, Press Brake Tonnage Calculator, and Sheet Metal Bending Guide.
Table of Contents
- What Is a Press Brake Hemming Tool?
- Why Hem Sheet Metal?
- Types of Sheet Metal Hems
- Open Hem
- Closed Hem
- Teardrop Hem
- How the Hemming Process Works
- Acute Pre-Bending
- Flattening / Hemming Stage
- Types of Hemming Tools
- Spring-Loaded Flattening Tools
- Hydraulic Flattening Tools
- Combined Hemming Tooling
- Material Thickness and Hemming
- Mild Steel
- Stainless Steel
- Aluminum
- Coated and Painted Sheet
- Hemming Tonnage
- Minimum Flange Length
- Inside Radius and Edge Condition
- Tooling Materials
- Heat Treatment and Hardness
- CNC Programming and Delem
- Springback and Hemming
- Common Hemming Defects
- How to Choose the Right Hemming Tool
- Hemming Safety
- Maintenance and Storage
- Frequently Asked Questions
- Related YRS Industrial Guides
- Contact YRS Industrial
1. What Is a Press Brake Hemming Tool?
A press brake hemming tool is a special upper and/or lower tool designed to flatten or partially close a pre-bent sheet-metal flange.
The tool is normally used after the sheet has already been bent to an acute angle.
Hemming tooling can be dedicated to the flattening step or integrated into a combined system that supports both pre-bending and flattening.
2. Why Hem Sheet Metal?
Hemming is used for both functional and cosmetic reasons.
- Remove sharp exposed edges.
- Increase edge stiffness.
- Improve appearance.
- Create safer handling surfaces.
- Improve panel rigidity.
- Create reinforcement without welding.
- Prepare edges for assembly or joining.
- Produce clean finished cabinet and enclosure edges.
3. Main Types of Sheet Metal Hems
| Hem Type | Description | Typical Use |
|---|---|---|
| Open Hem | Folded edge with a controlled gap | Panels, covers, decorative edges |
| Closed Hem | Folded edge pressed nearly flat | Stiff edges, safe edges, cabinets |
| Teardrop Hem | Folded edge with a rounded internal loop | Materials sensitive to cracking or flattening |
4. Open Hem
An open hem leaves a controlled space between the folded flange and the main sheet.
It is often used when full flattening is not required or when the material should not be compressed tightly.
Open hems can provide a finished edge while reducing the risk of cracking or surface damage.
5. Closed Hem
A closed hem folds the flange nearly flat against the base sheet.
This creates a stiff, safe edge and is common on cabinets, enclosures, doors, covers, and sheet-metal housings.
Closed hemming generally requires greater flattening force than an open hem.
6. Teardrop Hem
A teardrop hem leaves a rounded loop inside the folded edge rather than fully flattening the material.
This can reduce extreme local strain and may be useful for materials that are sensitive to cracking.
Teardrop hems can also be used when a specific edge geometry is required for strength or assembly.
7. How the Press Brake Hemming Process Works
- Position the blank against the backgauge.
- Pre-bend the flange using an acute punch and acute V-die.
- Remove or reposition the workpiece.
- Place the pre-bent flange into the flattening or hemming tool.
- Close the hem to the required gap or thickness.
- Inspect angle, edge condition, gap, surface quality, and flange length.
8. Acute Pre-Bending
The first hemming stage normally uses an acute punch, often around 30°, with a compatible acute die.
The purpose is to create enough pre-bend so that the flange can later be flattened without the tool colliding with the sheet.
The exact pre-bend angle depends on material thickness, springback, tooling design, and final hem type.
Read our Acute Punch Guide.
9. Flattening / Hemming Stage
After pre-bending, the part is moved to the flattening tool.
The flange is compressed toward the base sheet until the required gap or closed condition is reached.
The flattening stage can generate high compressive force, especially for thicker sheet and closed hems.
10. Types of Press Brake Hemming Tools
- Dedicated flattening die
- Spring-loaded flattening tool
- Hydraulic flattening system
- Combined pre-bend and flattening tool
- Custom hemming tools
11. Spring-Loaded Flattening Tools
Spring-loaded flattening tools use mechanical spring movement to accommodate the pre-bent flange before the tool closes.
They can reduce setup time and support efficient hemming on suitable material thicknesses.
Tool travel, maximum material thickness, and load rating must be checked before use.
12. Hydraulic Flattening Tools
Hydraulic flattening systems use controlled hydraulic movement within the tooling assembly.
They can provide smooth, controlled flattening and may be suitable for higher production volumes or more demanding hemming applications.
Compatibility with the press brake and machine control must be confirmed.
13. Combined Hemming Tooling
Some systems integrate the acute pre-bend and flattening function into one tooling arrangement.
This can reduce tool changes and setup time.
Combined tooling is particularly attractive for repeated hemming jobs, but it is less universal than a separate punch-and-die setup.
14. Material Thickness and Hemming
As thickness increases, required force rises significantly and the minimum flange length generally becomes larger.
Very thick material may be unsuitable for normal hemming tooling unless the machine and tools are specifically designed for the load.
Always follow the tooling manufacturer’s maximum thickness and tonnage limits.
15. Hemming Mild Steel
Mild steel is relatively easy to hem and is one of the most common materials for press brake hemming.
Closed hems are widely used on cabinets, doors, panels, and enclosures.
Even with mild steel, check edge quality, material thickness, and flattening force.
16. Hemming Stainless Steel
Stainless steel typically requires more force and exhibits more springback than mild steel.
It can also be more sensitive to visible tooling marks.
Use suitable surface protection, correct pre-bend angle, and a tooling system rated for the higher load.
17. Hemming Aluminum
Aluminum can crack during hemming if the alloy or temper has limited ductility.
Teardrop or open hems may be more suitable than fully closed hems for some aluminum grades.
Check grain direction and minimum bend-radius recommendations before production.
18. Hemming Coated, Painted and Cosmetic Sheet
Hemming can damage painted, polished, coated, or pre-finished surfaces if pressure and tool condition are not controlled.
Consider protective film, polished tool surfaces, low-mark tooling, or special inserts when cosmetic finish is important.
19. Press Brake Hemming Tonnage
Hemming force is different from normal air-bending force because the flattening stage compresses multiple sheet thicknesses together.
Closed hems can create very high local pressure.
Always use the hemming-tool manufacturer’s tonnage recommendation rather than assuming a normal air-bending formula applies to the flattening stage.
For the pre-bending stage, use our Press Brake Tonnage Calculator.
20. Minimum Flange Length for Hemming
The flange must be long enough to be supported during the acute pre-bend and flattening stages.
If the flange is too short, it may slip into the V-die or fail to sit correctly in the flattening tool.
Minimum length depends on V-opening, tool design, thickness, and final hem geometry.
21. Hem Radius and Edge Condition
The inside fold radius affects strain and cracking risk.
A very tight pre-bend radius can damage harder materials before the flattening stage begins.
For difficult materials, use a larger radius or teardrop geometry rather than forcing a completely flat closed hem.
Read our Press Brake Radius Guide.
22. Hemming Tool Materials
Common press brake tooling steels include 42CrMo, SKD11, and Cr12MoV.
Hemming tools need good wear resistance because large contact areas slide and compress against the sheet.
Toughness is also important because flattening loads can be high.
Read our Press Brake Tooling Materials Guide.
23. Heat Treatment and Hardness
Heat treatment affects wear resistance, toughness, dimensional stability, and service life.
Ask the tooling supplier for the steel grade, heat-treatment process, hardness, and load rating.
24. Hemming Programming with Delem CNC
Modern Delem controls can store the acute punch, pre-bend die, and flattening tooling in the tool library.
Accurate tool data improves bend-sequence planning and collision checking.
Hemming programs often require two distinct operations, so correct sequencing is essential.
Read the Press Brake Programming Guide →
Explore the Delem CNC Controller Knowledge Center →
25. Springback and Hemming
Springback mainly affects the acute pre-bend stage.
Stainless steel and high-strength materials may require more overbend to reach the correct pre-hem geometry.
The final flattening stage reduces the importance of angular springback but can introduce its own problems if pressure is excessive.
Read our Springback Guide.
26. Common Hemming Defects
| Problem | Possible Cause |
|---|---|
| Hem cracks | Radius too tight, poor ductility, unfavorable grain direction |
| Hem will not close | Insufficient force, wrong pre-bend angle, tool interference |
| Heavy surface marks | Dirty tooling, excessive pressure, sensitive surface |
| Uneven hem | Misalignment, inconsistent pre-bend, tooling wear |
| Flange shifts | Poor support or flange too short |
| Material bows | Excessive force or uneven flattening |
27. How to Choose the Right Press Brake Hemming Tool
- Confirm press brake clamping style.
- Identify material grade and thickness.
- Choose open, closed, or teardrop hem.
- Define finished hem length and gap.
- Select the acute pre-bend punch.
- Select the compatible acute die.
- Choose the flattening or hemming tool.
- Check minimum flange length.
- Check pre-bending tonnage.
- Check flattening-tool tonnage rating.
- Review surface-quality requirements.
- Confirm tooling material and heat treatment.
- Add accurate tool geometry to the CNC library.
- Perform a controlled first-piece test.
28. Hemming Safety
Hemming creates pinch points and high compressive force.
- Keep hands away from the flattening zone.
- Use the machine’s approved safety devices.
- Do not exceed tool or machine load ratings.
- Inspect tooling before use.
- Support large sheets safely.
- Follow the press brake manufacturer’s operating procedures.
29. Hemming Tool Maintenance and Storage
- Keep flattening surfaces clean.
- Inspect for dents, cracks, and wear.
- Protect precision clamping surfaces.
- Keep spring or hydraulic mechanisms clean and serviced.
- Store tooling on a stable rack.
- Apply rust protection during long storage.
- Remove damaged tools from service.
30. Frequently Asked Questions About Press Brake Hemming Tools
What is a press brake hemming tool?
It is tooling used to flatten or partially close a pre-bent sheet-metal flange.
What is the purpose of hemming?
Hemming creates safer, stronger, and cleaner sheet-metal edges.
What are the main hem types?
Open hems, closed hems, and teardrop hems.
What punch is used for hemming?
An acute punch, often around 30°, is commonly used for the pre-bending stage.
What happens after pre-bending?
The flange is moved to a flattening or hemming tool.
Can I make a hem in one press brake stroke?
Some special combined tools can reduce the number of operations, but many hems use separate pre-bending and flattening stages.
Can mild steel be hemmed?
Yes. Mild steel is one of the most common hemming materials.
Can stainless steel be hemmed?
Yes, but it typically requires more force and springback compensation.
Can aluminum be hemmed?
Yes, but alloy and temper must be checked because some aluminum grades crack easily.
What is a closed hem?
It is a folded edge pressed nearly flat against the main sheet.
What is an open hem?
It is a folded edge with a controlled gap between the flange and base sheet.
What is a teardrop hem?
It is a hem with a rounded internal loop rather than a fully flattened fold.
Why does a hem crack?
Common causes include too-small radius, poor ductility, unfavorable grain direction, or excessive flattening.
Why will the hem not close?
The pre-bend angle may be wrong, force may be insufficient, or tooling may interfere.
How much tonnage is needed for hemming?
Use the hemming-tool manufacturer’s rating because flattening force differs from ordinary air-bending force.
Can I use a standard V-die for the pre-bend?
Only if its angle and opening allow the required acute bend. A dedicated acute die is often more suitable.
Can a Multi-V die be used?
Yes if one opening has the required acute angle and correct load rating.
Does hemming increase part stiffness?
Yes. Folding the edge increases local rigidity.
Does hemming remove sharp edges?
Yes. This is one of its most common purposes.
Can Delem program hemming operations?
Yes. Suitable Delem controls can store the tooling and program the required sequence.
What information should I send when ordering hemming tooling?
Send press brake model, clamp type, material, thickness, hem type, flange length, bend length, target gap, part drawings, and production volume.
32. Need Help Choosing a Press Brake Hemming Tool?
YRS Industrial supplies CNC press brakes and press brake tooling for sheet metal fabrication. We can recommend acute pre-bending punches, acute dies, flattening tools, hemming systems, tooling materials, clamping styles, and load capacities according to your parts.
YRS Industrial
Email: info@yrs-industrial.com
Website: https://yrs-industrial.com
WhatsApp: +86 180 0258 8351
Factory: Mingjue Industrial Park, Lishui District, Nanjing, Jiangsu, China
For a tooling recommendation, send your press brake model, upper and lower clamping photos, material grade, thickness range, hem type, flange length, bend length, target hem gap, typical part drawings, and production volume.
Technical Note
Hemming performance depends on material grade, thickness, ductility, grain direction, pre-bend angle, acute punch radius, V-die geometry, final hem type, flattening force, and tool load rating. The flattening stage can generate high local pressure, so always use the tooling manufacturer’s load limits and validate the process with a controlled first-piece test.