What is Sheet Metal Hemming?
Sheet metal hemming is the process of cold-forming where the edge of a metal sheet is folded upon itself which results in a reinforced multi-layered structure. Key features include:
- Advanced Strength: Strengthening the edge increases bending resistance by 30–50% (according to ASTM E8 test data) which is critical for load-bearing parts.
- Geometry Optimization: Removes sharp edges and converts acute angles into rounded curves (radius ≥0.5t, t =material thickness) facilitating safer handling.
- Cost Effectiveness: Cuts auxiliary material expenses by 30% in comparison to welding or riveting, thus lowering production waste.
Step-by-Step Hemming Process
1.Preparation Phase
Start by selecting materials based on strength, ductility, and end-use needs. For structural parts , opt for cold-rolled steel; choose aluminum for lightweight electronics enclosures. Ensure sheet thickness ≤3mm to avoid fractures.
2.Hemming Operations
Initial Bending: Use a press brake to pre-bend edges at 30°–90°, creating a “V” shape for easier final flattening.
3.Hem Formation
Flatten the pre-bend with dies (for closed hems) or rollers (for curved edges). For open hems, insert shims (0.06–0.25mm) to maintain precise gaps.
4.Finishing
Polish edges with 320-grit sandpaper, deburr sharp corners, and apply electrochemical smoothing for high-gloss surfaces.
5.Quality Control
Inspect hems using digital micrometers (accuracy ±0.01mm) and 3D laser scanners to verify width/thickness compliance.
Types of Sheet Metal Hems
Closed Hem
With a closed hem, the edge of the sheet metal is folded back on itself at 180° and results in a complete fold which shapes a double layer of structure. This design enhances the edge while folding it which increases the doubling the thickness of the edge resulting in over 40% (ASTM tests) increase in bending resistance while eliminating sharp burrs without additional polishing. This structure is ideal cold rolled steel and stainless steel sheets that are 3mm thick or less. It is predominantly used on automotive doors and refrigerators where they need to be sealed tightly.
Open Hem
An open hem maintains a 0.06–0.25mm air gap between the folded layers which results in a U-shaped profile. This feature reduces weight between 15–20% and improves assembly tolerances which is ideal for handles, hinges or any moving parts. Metal shims guarantee precision in maintaining the gap, while the flange length must not be less than four times the material thickness for adequate stability.
Teardrop Hem
This hem with a closed loop shape of a teardrop (radius = 1.5× material thickness) is capable of enhancing fatigue resistance by 200% compared to closed hems. It has a unique capability of preventing edge cracking of brittle metals such as aluminum and magnesium, and is predominantly utilized in aircraft skins, as well as satellite support frames.
Rolled Hem
Rolled hems require curving the border of the metal edge to rounded shape (radius ≥2× thickness), in compliance with ISO 13732 anti-cut standards. The streamlined contour decreases airflow noise by 6– 8dB, making it suitable for furniture or electronics enclosure. The process involves a 90° pre-bend followed by three-axis rolling, faring best on 0.3–1.5mm stainless steel or brass.
Suitable Material Types
Material Type | Recommended Thickness (mm) | Suitable Hem Types |
Cold-Rolled Steel（CRS） | 0.5-3.0 | Closed, Open, or Rolled Hems |
Stainless Steel（304） | 0.8-2.5 | Closed or Teardrop Hems |
Aluminum Alloy（5052） | 1.0-2.0 | Teardrop or Rolled Hems |
Bras（C2600） | 0.6-1.5 | Open or Rolled Hems |
Different types of Sheet Metal Hems
In terms of edge reinforcement for sheet metals, two methods dominate industrial practice: die hemming and roll hemming.
Die Hemming
This is characterized by the application of hardened dies with the use of a hydraulic press to fold the metal edges in large quantities. This process consists of an initial operation of other pre-bending the flange to an acute angle, for instance, in the range 30° – 45°, Followed by final flattening to 180°.
- Pros: Uniform results over successive cycles with simple geometric shapes, example, automotive hoods or appliance panels.
- Cons: Tooling cost is high, and the adaptability is limited to complex curves.
Roll Hemming
This is a robotic approach to hemming. It uses programmable rollers which contour edges in a shape marginally in numerous passes. This advance variant is guided by CNC or 6 axis robots. Applying defined incremental pressure lends itself to precision bending.
- Pros: Much less expensive for the tools (over 60% compared to die cost) perfect and intricate at flexible components such as aerospace pieces and prototypes.
- Cons: Incremental forming translates to lower production rates.
Pro of Sheet Metal Hemming
Key Benefits
Sheet metal hemming gives vital benefits for industrial uses:
- Structural Integrity: Folding edges doubles thickness, increasing strength by 30-50% and alleviating warping during stress.
- Aesthetic Appeal: Smooth rounds off edges which are ideal for polishing and coating required for luxury automotive trims or high-end appliances.
- Functional Gains: Sealed edges are resistant to corrosion extending lifespan of the part by 15-20% and reducing noise in HVAC systems.
Challenges to Consider
Hemming has technological difficulties as listed below:
- Material Constraints: Limited to high gauge metals of 3mm thickness. Brittle alloys like aluminum require specialized techniques to prevent cracking.
- Resource Demands: Skilled workers and precise tools like press brakes costing over $30,000 are mandatory for uniform outcomes.
- Fracture Risks: Low ductility means metals like magnesium are prone to micro-cracks during the bending process requiring exhaustive inspection after the hemming process.
Industry Applications
Automotive Industry
Closed hems increase the strength of the door panel, trunk lids and fenders by 2 fold to ensure the vehicles meet crash safety requirements. The vehicles also promote aesthetic appeal because of the seamless finish.
Aerospace Engineering
Teardrop hems on the skin panels of an aircraft mitigate the fatigue caused by vibration which is critical for aluminum parts in the wing assembly and other high stress areas.
Electronics Manufacture
Rolled hems on server enclosures remove sharp edges and improve EMI shielding which improves user safety and ensures data center signal integrity.
Appliance Manufacture
Open hems trim excess weight by 15 to 20% while preserving strength in the structural supports of refrigerator doors and oven frames. This weight saving assists in energy efficiency.
Hemming VS Other Processes.
As with most other manufacturing activities involving sheet metals, manufacturers have different options for edge refinement like hemmed and seamed, jogged, over-locked, and many more. Below are the distinct purpose and application differentiating these processes.
Hemming VS Seaming
Hemming: Applying a hem focuses on one edge of a flat sheet part, reducing its dimensions from one direction and reinforcing its edge by folding back to itself. It adds durability and improves appearance, eg automobile panels, home appliance outer casings, and car door edges which are to be closed hems to prevent warping during collisions.
Seaming: as joining of two flat pieces removes gaps between them from each edge are lapped, interlocked, and securely bound to form a leak-proof joint. HVAC ducts seamed joints as they need airtight joints under pressure.
Hemming is adding material by placing another layer and removing layers through cutting or grinding to soften it, strengthens edge; seaming binds things and joins edges together cut apart.
Hemming VS Jogging
Hemming
Strengthens a part’s structural integrity by folding thin sheets over one another. Edge treated so non-blemished visually one would wish to look at an edge trim.
Jogging (Joggle)
Creates Z shapes deficits to aid alignment of components for fitting during assembly. This is commonly employed in other construction fields, for example, in aircraft and electronics.
Aircraft reduce excess weight by adding recesses called jogs that aid in aligning overlapping sections called flush fits.
Conclusion
Regardless of whether you go with the quicker option offered by die hemming or the flexibility granted by roll hemming, both processes increase the durability of the product while decreasing maintenance costs in the long run. Are you interested in adding precision hemming to your sheet metal components? Easiahome provides expert engineering consultations for fabricated sheet metals including prototyping and mass production.
Frequently Asked Questions (FAQs)
Q: How do I decide whether to go for open or closed hems for outdoor uses?
A: Closed hems are ideal for outdoor applications because they do not permit moisture penetration and therefore do not oxidize. Open hems are suitable for indoor components where saving weight is prioritized over exposure to the elements.
Q: Is it possible to use recycled metals for the hemming processes?
A: Yes, but they may require greater scrutiny for contamination. It is common practice to anneal these materials to improve ductility, especially when dealing with aluminum and copper alloys.
Q: What are the widespread uses of sheet metal hemming?
A:Hemming of sheet metal is a common practice for manufacturers who produce parts geometrically intricate in shape in the construction, automotive, metal furniture, aerospace and electronics industries.
