Press brake hemming is a two-stage sheet metal bending process that folds an edge back onto itself to create a smooth safety edge, improve stiffness, hide sharp burrs, or prepare a part for assembly. For most thin-gauge steel, aluminum and stainless parts, the best setup is a CNC press brake with acute-angle pre-bending, a flattening or hemming tool, correct hem allowance, and a first-piece check for cracks, marks and final edge thickness.
What Is Press Brake Hemming?
Press brake hemming is the process of bending a sheet metal edge over and pressing it nearly flat or fully flat against the part. The hem can remove a sharp exposed edge, increase local stiffness, improve appearance, or create a folded lip for doors, covers, panels, trays and enclosures.
A hem is different from a normal 90-degree flange. In a standard flange, the material remains open at an angle. In hemming, the edge is folded back toward the sheet. The operation usually needs two hits: first an acute bend, commonly around 25-35 degrees, then a flattening stroke with a hemming die or flattening tool.
Open Hem vs Closed Hem vs Teardrop Hem
The right hem type depends on material thickness, part function, surface finish and whether the edge must be completely closed. Closed hems look clean, but they are not always the best choice for thicker or crack-sensitive material.
| Hem Type | Shape | Best Use | Watch Point |
|---|---|---|---|
| Open hem | Folded edge with a small gap | Aluminum, stainless, painted parts and crack-sensitive materials | Gap must be controlled so the edge still looks intentional |
| Closed hem | Edge folded flat against the sheet | Mild steel covers, cabinet doors, guards and low-risk materials | Higher risk of cracking or surface marks on hard material |
| Teardrop hem | Folded edge with a rounded internal loop | Stainless, aluminum and parts needing a softer fold | Needs correct radius and consistent flattening pressure |
| Rope or wired edge | Hem formed around wire or round insert | HVAC, guards, bins and reinforced edges | Requires extra clearance, special tooling or multiple operations |
The Two-Step Press Brake Hemming Process
Most press brake hems are made in two operations. The first operation creates an acute pre-bend. The second operation flattens that pre-bend into the final hem. Some special hemming tools combine both operations in one tool station, but the physics are still the same: bend first, flatten second.
- Confirm hem allowance: add enough flat length for the fold, material thickness, bend radius and final edge condition.
- Pre-bend the edge: use an acute punch and die to form the edge to about 25-35 degrees, depending on tooling and material.
- Move to the hemming station: position the pre-bent edge under a flattening tool or hemming die.
- Flatten with controlled pressure: press the edge to the required open, closed or teardrop condition without crushing the part.
- Inspect the first piece: check edge straightness, cracks, marks, final thickness, flange length and whether the part still fits the assembly.
Hem Design Rules: Allowance, Radius and Minimum Size
A good hem starts in the drawing. If the hem is too short, the edge may slip, look uneven or fail to flatten. If the hem is too tight for the material, the outside bend can crack. If the hem is added after the flat pattern is already released, the finished part size can shift.
A practical starting point is to make the hem length at least 4 times material thickness for thin sheet, then adjust for material, tool nose radius, finish and part size. For thicker stainless or aluminum, use an open or teardrop hem instead of forcing a dead-flat closed hem. Always validate the flat pattern on the actual press brake and tooling before production.
Which Materials Can Be Hemmed?
Mild steel is usually the easiest material to hem. Aluminum and stainless steel can also be hemmed, but they need more attention to cracking, grain direction and surface marks. Galvanized or pre-painted sheet can show coating damage if the hem is too tight or the tool surface is rough.
The harder the material and the thicker the sheet, the more cautious the hem design should be. For 0.8-1.5 mm mild steel, closed hems are common. For 2.0 mm stainless or hard aluminum, an open hem or larger loop may be more reliable than a fully closed edge.
| Material | Hem Suitability | Common Thickness | Setup Advice |
|---|---|---|---|
| Mild steel | Very good | 0.8-2.0 mm | Closed hems are common; check burr direction and paint needs |
| Stainless steel | Good with care | 0.8-1.5 mm for most cosmetic hems | Use open or teardrop hems, polish tools and protect visible faces |
| Aluminum | Depends on grade and temper | 0.8-2.0 mm | Avoid sharp closed hems on crack-sensitive tempers |
| Galvanized or pre-painted sheet | Good if finish is protected | 0.6-1.5 mm | Use clean tooling, film and test for coating cracks |
Tooling for Press Brake Hemming
Hemming tooling can be simple or specialized. For prototypes and low-volume work, a shop may pre-bend with an acute punch and then flatten with a standard flattening tool. For repeat production, a dedicated hemming die improves speed, consistency and operator workflow.
The tool must support the material without sliding or marking the part. A flattening tool can push the pre-bent edge forward if the setup is wrong, which creates uneven hems and unsafe handling. Segmented tooling is useful for box-shaped parts, and mark-free tooling is useful when the hem remains visible after finishing.
Best Press Brake Configuration for Hemming Work
Hemming can be done on many press brakes, but CNC control helps when the shop runs multiple hem lengths, materials and repeat orders. The machine should have enough open height, stroke, tooling clearance and back gauge reliability for both the acute bend and the flattening operation.
| Production Need | Recommended Configuration | Why It Helps | Watch Point |
|---|---|---|---|
| Occasional prototypes | NC or CNC press brake with acute tooling | Lower tooling investment and flexible setup | Operator skill matters; first-piece inspection is essential |
| Cabinet doors and covers | CNC press brake with dedicated hemming die | Repeatable edge quality and faster cycle time | Validate hem allowance before nesting blanks |
| Stainless or painted cosmetic panels | CNC press brake with mark-free tooling | Protects visible faces during flattening | Clean tools and part handling are mandatory |
| High-mix enclosure production | Y1/Y2 CNC, X/R back gauge and tool library | Stores hem programs and reduces setup variation | Use standard program names and tool records |
Common Applications for Hemmed Sheet Metal Edges
- Electrical cabinets: door edges, removable covers and operator-safe access panels.
- Kitchen and food equipment: trays, hoods, guards and stainless covers with safer finished edges.
- HVAC parts: duct edges, covers and transitions that need stiffness and safe handling.
- Machine guards: folded edges reduce cut risk and increase panel rigidity.
- Elevator and architectural panels: cosmetic hems hide raw edges and improve stiffness.
- Metal furniture: lockers, shelves, drawers and panels that need safe user-facing edges.
Quality Checks After Hemming
A hem can look acceptable at first glance but still cause assembly problems. A closed hem that is too thick may interfere with a hinge, gasket or overlapping panel. A hem that is not straight can create a wavy visible edge. Build inspection around the final function of the part.
- Final hem thickness: confirm it fits mating parts, hinges, clips and gasket areas.
- Edge straightness: check long hems for waviness after flattening.
- Cracks: inspect the outside radius, especially on stainless, aluminum and pre-painted sheet.
- Tool marks: check both visible faces and the flattened edge.
- Part size: verify that the hem allowance did not shift finished dimensions.
Safety Notes for Hemming on a Press Brake
Hemming creates extra pinch and handling risks because the part moves during both the acute bend and the flattening stroke. Operators may be tempted to hold a narrow edge close to the tooling. This should be controlled with proper back gauge setup, support arms, safe work procedures and guarding.
OSHA powered press brake guidance highlights point-of-operation hazards, foot control hazards and risks from large workpieces moving during bending. For hemming, also watch sharp burrs before the hem is closed, sudden part rotation during pre-bending and hand placement during flattening.
Useful Technical References
SheetMetal.Me explains common sheet metal hem types and notes that flattening requires special attention because the angled sheet can move during the second stage. Hemming and seaming are broader metalworking processes where a sheet edge is folded over onto itself.
For operator safety, review OSHA powered press brake machine guarding guidance before setting up hemming jobs, especially on long panels or narrow edges.
Common Press Brake Hemming Mistakes
- Trying to close a hem on material that should use an open or teardrop hem.
- Forgetting to update the flat pattern for hem allowance before cutting blanks.
- Using rough tooling on stainless, painted or visible surfaces.
- Flattening too aggressively and crushing the part or leaving heavy tool marks.
- Skipping the first-piece check for cracks, edge thickness and final part size.
Press Brake Hemming FAQ
What is press brake hemming?
Press brake hemming is a sheet metal forming process where the edge is bent over and pressed close to the sheet. It creates a safer edge, improves stiffness, hides burrs and can improve the appearance of doors, covers, panels and enclosures.
What is the difference between an open hem and a closed hem?
An open hem leaves a small gap between the folded edge and the main sheet, while a closed hem is flattened nearly tight against the sheet. Open hems are safer for crack-sensitive materials, while closed hems are common on mild steel parts that need a clean finished edge.
Can a standard press brake make hems?
Yes. A standard press brake can make hems if it has suitable acute-angle tooling and a flattening or hemming tool. Dedicated hemming dies improve repeatability and speed for production runs.
How much hem allowance is needed?
Hem allowance depends on material thickness, radius, hem type and tooling. A practical starting point for thin sheet is at least 4 times material thickness, but the final allowance should be validated on the actual machine and tooling.
Which materials are difficult to hem?
Hard aluminum tempers, thicker stainless steel and pre-painted materials can be more difficult to hem because they may crack or mark. Use larger radii, open hems, teardrop hems or protective tooling when the material or finish is sensitive.
What press brake features help with hemming?
CNC control, accurate back gauges, tool libraries, sufficient open height, segmented tooling and mark-free hemming tools all help. For repeated cabinet, cover and panel work, a CNC hydraulic press brake with dedicated hemming tooling is usually the most efficient setup.
Conclusion: Hemming Is a Small Edge Detail With Big Production Impact
Press brake hemming improves safety, stiffness and appearance, but it must be designed and produced as a controlled process. The key decisions are hem type, material limit, hem allowance, pre-bend angle, tooling choice, flattening pressure and first-piece inspection.
If your factory makes electrical cabinets, stainless covers, HVAC parts, machine guards, elevator panels or metal furniture, send Rucheng your drawings, material thickness and production volume. Our engineers can recommend a CNC press brake, hemming tooling and setup workflow for your parts.
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