How To Hem Sheet Metal
Content Menu
● Understanding Hemming Types and Their Uses
● Material Behavior During Hemming
● Advanced Methods and Research Insights
Introduction
Hemming sheet metal is one of those operations that separates rough work from finished work. A clean folded edge keeps fingers safe, stiffens the panel, hides the raw cut line, and gives the part a factory appearance. In automotive body panels, appliance housings, electrical enclosures, and even custom ductwork, the hem is usually the last forming step before paint or assembly. Done right, it looks effortless. Done wrong, it cracks, waves, or springs open and the whole job has to be redone.
The process itself is straightforward: bend the edge up 90°, then fold it over until it lies flat against the sheet, or sometimes only partway for an open hem. What makes it tricky is that the metal fights back every step of the way. Springback, work hardening, and material thickness all affect the final shape. Over the years I’ve hemmed everything from 0.6 mm galvanised steel to 2 mm 5052 aluminum, and the same basic rules always apply — but the details change with every alloy and gauge.
This article walks through the entire operation from layout to final close, with practical examples taken from daily shop work and backed by real journal papers. Whether you run a manual brake, a press brake with hemming dies, or a robotic roller hemmer, the goal stays the same: a tight, flat, defect-free edge every time.
Understanding Hemming Types and Their Uses
A hem is nothing more than a folded edge, but the geometry varies depending on the job.
Flat hem (180° closed): the flange is folded completely flat. Most common on visible edges of doors, lids, and covers. Open hem (typically 135–150°): leaves a small gap for drainage or to accept a gasket. Used on roof panels and some enclosures. Teardrop or rope hem: a rounded closed hem with a small radius on the outside. Adds stiffness and looks premium on appliance doors. Roll hem: formed in several gradual passes with rollers. Essential for high-strength steels and sharp contours.
In practice, a typical car door skin uses a combination: flat hems on the bottom and sides, teardrop along the window line, and roll hems around tight corners.
Material Behavior During Hemming
Sheet metal does not like being folded twice in the same place. The first 90° flange is easy. The second fold, when the flange is crushed flat, puts the outer fibers in tension and the inner fibers in compression. Aluminum can handle about 6–8 % outer-fiber strain before it necks or cracks; DP780 steel is closer to 4 %. Exceed that limit and you get orange peel or outright splits.
Springback is the other enemy. After the final close, the hem wants to open a few degrees. The thicker the material and the higher the yield strength, the worse it gets. A 1.5 mm AHSS part can spring open 8–10° if no compensation is built into the tooling.
Real-world example: a batch of 1.2 mm 5182-O aluminum battery tray covers kept cracking along the hem during final assembly. The fix was to increase the pre-hem angle from 135° to 142° and use a slightly larger bend radius on the flanging die. Cracks disappeared immediately.
Tools and Equipment
Manual and Bench-Top Methods
For one-offs or repair work, a good vise, a 3-pound dead-blow hammer, and a hardwood or delrin block are enough. Clamp the sheet over a steel angle, bend the flange with a mallet, then fold it flat with the block. Works fine up to about 1 mm mild steel or 0.8 mm aluminum.
Small sheet metal shops often use hand-operated folding machines (Eastwood, Tennsmith, Baileigh 48-inch folders). These give repeatable 90° flanges and can close simple flat hems.
Press Brake Hemming
Most production shops use a two- or three-stage press brake sequence:
- Flanging die – 30° to 45° acute punch to create a clean 90° flange.
- Pre-hemming die – flattens the flange to roughly 135°.
- Final flattening die – spring-loaded or hydraulic pad that closes the hem completely flat.
The flattening die is the expensive part. Good ones have adjustable pads that compensate for springback and material thickness variation.
Roller Hemming
For contoured parts and high-strength steels, roller hemming wins. A typical tabletop roller hemmer (Malco, Mittler Bros) has three or four wheel sets: 45°, 90°, 135°, and final close. Each wheel takes a light pass. The gradual bending keeps strains low and virtually eliminates cracks.
In automotive plants, robotic roller heads (usually 6-axis ABB or Fanuc) follow the contour at 200–400 mm/s. Path compensation is programmed from CAD data.
Step-by-Step Process
Layout and Allowance
Rule of thumb for a closed flat hem: allowance = 4 × material thickness + bend radius. Example: 1.0 mm steel, 1 mm inside radius → allowance = 5 mm total. Mark the trim line accordingly.
Edge Preparation
Remove burrs with a fine file or 180-grit flap wheel. A sharp burr acts like a stress riser and starts cracks.
Stage 1 – Flanging
Bend the edge exactly 90° ± 1°. Any deviation here carries through to the finished hem. On a press brake, use a sharp punch and a die opening 8 × thickness.
Stage 2 – Pre-Hemming
Fold to 130–145°, depending on material. Aluminum usually needs more overbend than mild steel.
Stage 3 – Final Closing
Close completely flat. If the hem springs open more than 2–3°, either increase pre-hem angle or add a slight coined groove on the panel side to lock the edge.
Finishing
Light sanding or scotch-brite on the folded edge removes tool marks before painting.
Advanced Methods and Research Insights
Livatyali et al. (2000) showed that the single biggest factor in hem quality is the flanging radius and angle. By increasing the radius from 0 to 1 × thickness and bending to 92–95° instead of 90°, surface strains dropped 25 % and cracks vanished on 1 mm DX56 steel.
Hu et al. (2002) developed an analytical model that predicts final hem angle from material properties alone. The model is still used today to set initial overbend values in new programs.
Wang et al. (2015) used commercial FEA (AutoForm) to optimize roller paths for an aluminum hood. Simulation reduced physical tryouts from 12 die sets to 3 and cut cracking issues to zero.
Common Problems and Fixes
Problem: Edge cracking on the outside of the hem Fix: Increase flanging radius, add 3–5° overbend, slow down the final close.
Problem: Wavy or buckled hem Fix: Too much material – reduce allowance or stretch the flange slightly before hemming.
Problem: Hem opens after a few days Fix: Material relaxation. Use a flattening die with a coining rib or stake the hem lightly.
Problem: Visible line or step on the show surface Fix: Make sure the pre-hem die pad contacts the panel evenly; any gap leaves a witness line.
Conclusion
Hemming looks simple until you have to do it a thousand times a day without a single reject. The operation combines basic bending principles with very specific compensation for springback and strain limits. Start with generous bend radii and proper allowance, pay attention to the 90° flange quality, and adjust the pre-hem angle until the final close lies flat. Once those fundamentals are dialed in, everything else — roller paths, robotic programs, adhesive bonding — becomes straightforward.
Every shop has its own favorite tools and tricks, but the physics never changes. Respect the material limits, measure every stage, and the hems will come out clean every time.