Key takeaways
- A flat hem needs a flange of at least 4t before folding: 6 mm on 1.2 mm CRCA, and a 5 mm flange will tear at the fold.
- A 180° fold at an inside radius of 0.5t asks the outer surface to travel 1.6 times the neutral-axis length, which is why total elongation decides the hem, not yield strength.
- Work the hem allowance as 2.95t at R0.5t and K0.44, so a 10 mm flange on 1.2 mm sheet finishes at about 6.5 mm of hem width.
- Keep the hem gap under 0.15 mm, coat after hemming, and deburr the laser-cut edge before the fold or the burr becomes the crack starter.
- Grades below about 20% elongation, including 5052-H32 aluminium, need an open or teardrop hem rather than a flat one.
Hemming sheet metal is a 180° fold that puts the cut edge back inside the part. It removes the sharp edge a hand will find on a door or a car panel, it doubles the material at the edge so the edge stops buckling, and it hides the inner panel's raw edge on an assembly that is seen from one side only. The fold is also the most common place a 1.2 mm CRCA panel splits in production, because a hem asks the outer surface to stretch further than any other bend on the part.
The rule is short. A flat hem needs a flange of at least 4t, an inside radius of at least 0.5t, and a material with enough total elongation to survive a 180° fold at that radius. Break any of the three and the crack appears at the outside of the fold, usually on the second or third day of a production run, after the tool has been accepted.
This page covers the hem types, the numbers that decide whether a fold will close, the hem allowance arithmetic, and the drawing note that gets the part made right the first time.
What hemming sheet metal buys that a plain bend does not
A hem does three jobs at once, and only one of them is cosmetic. It buries the cut edge, it stiffens the edge band, and it gives the part a controlled thickness at the boundary so a mating panel or a gasket has something to sit against.
The edge band stiffness gain is real and measurable. A 1.2 mm edge folded flat becomes a 2.4 mm edge over the hem width, and local bending stiffness goes with the cube of thickness: 1.2 mm to 2.4 mm is a factor of eight on the same width, less the small radius at the fold. A hem band about 6.5 mm wide therefore behaves like an 8× stiffer edge, which is why a hemmed door stops the oil-canning that a plain 1.2 mm panel shows when it is handled.
The edge also becomes safe. A laser-cut 1.2 mm edge has a burr of 0.05–0.15 mm on the underside, and a burr that fine will still cut a glove and a hand. A closed hem replaces it with a fold radius of about 1.2 mm and no exposed burr.
Hem types and the limits we hold
| Parameter | Typical value | Ganesh limit or note |
|---|---|---|
| Hem type | Flat, teardrop, rope | Flat to 1.5 mm CRCA; teardrop above 2 mm |
| Flange before hemming, flat hem | at least 4t | 6 mm minimum at 1.5 mm; 5 mm tears |
| Flange before hemming, open hem | at least 3t | 4.5 mm at 1.5 mm |
| Inside radius, flat hem | 0.5–1.0t | 0.6 mm at 1.2 mm CRCA |
| Inside radius, teardrop hem | 1.5–2.0t | 2–3 mm at 2 mm; keeps the fold open |
| Hem width after closing | flange minus about 3t | 10 mm flange on 1.2 mm gives 6.5 mm |
| Hem gap, closed | 0.15 mm maximum | Above 0.2 mm the gap shows as a shadow line |
| Material elongation for a flat hem | 25% minimum, A80 | IS 513 grade D gives 28% minimum |
| Material elongation for an open hem | 18% minimum | 5052-H32 at 12% needs annealed stock |
| Thinning at the fold | 15–25% | Measure on the first article, not on the coupon |
| Hem to bend distance | 3t minimum | 3.6 mm at 1.2 mm; closer and the bend pulls the hem |
| Hem width tolerance | ±0.5 mm | ±0.3 mm on a CNC-folded edge |
The last column is a working limit, not a guarantee for every grade. A hem in 304 stainless at 1.5 mm behaves differently from a hem in CRCA at the same thickness, and the table's radius values move up for stainless because it work hardens through the fold.
Why a hem cracks, and what the pre-hem does about it
A 180° fold at an inside radius of 0.5t is geometrically severe. The outer surface has to travel a path about 1.6 times the neutral-axis length, and the only way the material accommodates that is by thinning at the fold. Mild steel with 28% elongation thins by 15–25% at the crown and survives. A grade with 12% elongation cannot, and it splits along the outside of the bend within the first few millimetres of the fold.
The pre-hem angle carries the forming
A hem is not made in one hit. The first stage folds the flange to about 100°, with a punch nose that sets the inside radius. The second stage closes the fold with a flat tool. The pre-hem does the stretching and the closing stage only flattens what is already formed, which is why a two-stage tool produces a sound hem on a grade that a single-hit crush would split.
Set the pre-hem too shallow, at 80°, and the closing tool has to drag the material through the last 100° with no control over the radius. The fold then lands wherever the material wants to go, the hem width varies by 0.5 mm along the panel, and the crack risk goes up because the outer surface is stretched at a radius the tool did not set.
Material grade and the elongation test
Total elongation, not yield strength, decides whether a hem closes. IS 513 grade D cold reduced sheet is specified at 28% minimum elongation over an 80 mm gauge, which is comfortable for a flat hem at 0.5t. Drawing quality grades with 34–38% are easier still.
Stainless 304 shows 40% elongation on the certificate and still cracks at a tight hem, because it work hardens as it folds and the local ductility at the crown is far below the nominal figure. Treat 304 as a 1.5t minimum radius material and use a teardrop hem at 1.5 mm and above. Aluminium 5052-H32 at 12% elongation will crack in a flat hem at any radius; either use O temper, which is soft and scratches easily, or design an open hem.
Galvanised sheet adds a coating variable. The zinc layer is ductile enough to fold, but a heavy coating (Z275 and above) can flake at the crown on a tight radius, and the exposed steel under the flake rusts first. IS 277 sets the coating mass classes; if the part is going outdoors, specify the coating and keep the hem radius at 1t rather than 0.5t.
The cut edge is part of the specification
A hem made from a laser-cut blank has a cut edge with a heat-affected zone of 0.1–0.3 mm and a slight taper. The taper means the two faces of the fold are not parallel before closing, so the hem gap closes unevenly along the edge. Deburring removes the burr but not the taper.
Two things help. Cut the blank with the hem edge on the inside of the nest so the edge is a clean straight cut, and specify a hem gap of 0.15 mm rather than 0.05 mm when the blank is laser cut, so the taper has somewhere to go. A 0.15 mm gap is still tight enough to look closed after coating.
Working out the hem allowance
The developed length of a 180° fold follows the same bend allowance arithmetic as any other bend, with the angle at 180°:
BA = π × (R + K × t)
At R equal to 0.5t and K equal to 0.44, the allowance is π × 0.94t, or 2.95t. For 1.2 mm sheet that is 3.54 mm. A flange drawn at 10 mm therefore finishes as a hem 6.46 mm wide, and the blank has to be cut 10 mm long at that edge, not 13.5 mm.
Most of the errors in hem design come from forgetting that subtraction. A designer who draws a 6 mm hem and a 6 mm flange gets a hem that is 2.5 mm wide after folding and a fold that sits inside the bend radius of the adjacent feature. Draw the finished hem width, then add the allowance to get the flange, or draw the flange and state the finished width. Do not leave both to the shop.
The K-factor moves with the radius. At R equal to 1t, K sits nearer 0.42 and the allowance becomes π × 1.42t, or 4.46t, so a 10 mm flange on 1.2 mm sheet finishes at 4.6 mm. That is the difference between a hem that covers the inner panel edge and one that leaves 2 mm of it showing.
Running a hem on a long panel
Long hems are a press-brake job with a two-stage tool, and the brake bed length sets the maximum panel. On the 200 T, 5-axis brake with the 4,450 mm bed, a 2,400 mm door panel is hemmed in one pass with the pre-hem tool and one pass with the closing tool. On a 3,000 mm bed the same panel needs two bites, and the overlap shows as a 0.3 mm step in the hem line that no amount of adjustment removes.
Two process details decide the result on a long edge:
- Crowning and shimming. A brake bed that is 0.1 mm out over 2,400 mm produces a hem that is tight at the ends and open in the middle, or the reverse. The pre-hem is the stage where this shows, because the closing tool hides it.
- Back gauge support. A 1.2 mm panel 2,400 mm long sags under its own weight when it is only held at the bend line. Support the outboard end, or the hem picks up a twist that reads as a wavy edge after coating.
The 4,450 mm bed also allows a hem and a bend on the same panel in one setup, which matters on a door where the hem runs along one edge and the bend that forms the return runs along the other. Doing both in one setup keeps the relationship between them inside 0.5 mm.
What to put on the drawing
- Hem type — flat, teardrop or rope — with the finished hem width, not the flange length.
- Inside radius at the fold, and the side the fold closes towards.
- Hem gap, if the part is coated after folding, because the coating thickness has to fit inside it.
- Flange length before folding, or a note that the hem allowance is 2.95t at R0.5t.
- Edge condition: deburred, and which face of the blank the burr may remain on.
- Straightness of the hem line, typically 0.5 mm per metre, with the measurement method.
- The distance from the hem to the nearest bend, so the two operations do not fight each other.
The minimum flange length guide covers the adjacent problem: a flange that is too short for the die opening. A hem is a flange with an extra fold, and a hem on a flange that is already at the minimum for the V-die will mark the outside face.
When a hem is the wrong answer
A hem is wrong when the material cannot take the fold. That covers most 5000-series aluminium in a work-hardened temper, high-strength low-alloy grades above 550 MPa yield, and any sheet where the elongation on the mill certificate is under 20%. It is wrong when the part is already coated, because the fold will crack the film at the crown whatever the base metal does.
It is also wrong when the edge is the datum. A hem has a ±0.5 mm width tolerance and a rounded crown, so it cannot be used as a locating edge for a fixture or as a sealing face for a gasket. If the edge has to seal, use a folded return with a flat land behind it, or a separate extrusion.
Above 3 mm the fold needs so much force and produces so much springback that the flat hem stops being a sheet metal operation and becomes a press-forming job. At that point a tee return, a welded lip or a rolled edge is the cheaper answer, and the bending capability page sets out the tonnage and bed limits for the alternative.
Hemming at Ganesh
Hems on enclosure doors, elevator car panels and canopy skins run on the 200 T, 5-axis brake with the 4,450 mm bed, in two stages: pre-hem at about 100° with a radius nose, then close with a flat tool. The blanks are cut on the 12 kW fibre laser with the 40 ft × 16 ft bed, deburred, and the first article is checked for hem width, hem gap and straightness before the run continues.
The bending tolerances guide covers the inspection method for the fold, and the elevator parts range shows where a hemmed car panel sits in an assembly. Where the material cannot take a flat fold, the drawing comes back with a teardrop hem at 2 mm radius and the reason written on it, which is cheaper than finding the crack after coating.
Send the panel drawing with the grade, the hem type and the flange length through the RFQ route, and the design and CAD hub has the rest of the drawing rules that decide whether a folded part can be made as drawn.
Frequently asked questions
What is the minimum flange length for a hem?
Four times the material thickness for a flat hem, so 6 mm on 1.2 mm CRCA. Three times the thickness is enough for an open or teardrop hem. Below that the fold has no material to grip and the edge curls instead of closing.
Why does a hem crack on the outside of the fold?
The outer surface has to stretch around 180° at a very small radius. At an inside radius of 0.5t the outer path is about 1.6 times the neutral-axis length, so a grade with less than about 20% elongation splits before the fold closes.
Can you hem 2 mm mild steel?
Yes, but not flat. At 2 mm the fold wants an inside radius of 1 mm or more, and a flat hem at that thickness needs a press with enough tonnage to coin the fold. A teardrop hem at 2–3 mm inside radius is the practical route and it keeps the edge round rather than sharp.
Should the part be painted before or after hemming?
After. Powder at 70 µm per face will not survive a 180° fold, and a hem closed over a coated surface traps solvent and blisters in the oven. Hem, then pretreat and coat, and mask the hem gap if the customer wants a visible shadow line.
How wide is the hem after folding?
The finished hem width is the flange length minus the bend allowance. At 1.2 mm with an inside radius of 0.6 mm the allowance is about 3.5 mm, so a 10 mm flange produces a 6.5 mm hem.
Sources and further reading
- IS 513 — Cold reduced carbon steel sheets and strips: grade D elongation and mechanical property limits · reviewed 3 Nov 2026
- ISO 2768-1 — General tolerances for linear and angular dimensions without individual tolerance indications · reviewed 3 Nov 2026
- IS 277 — Galvanised steel sheets: coating mass and surface condition for sheets that will be folded after coating · reviewed 3 Nov 2026
Have a panel that needs a folded edge?
Send the panel drawing with the material grade, the hem type and the flange length. We will check the fold against the material's elongation before the blank is cut.
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