What is a hemming die?
A hemming die folds the edge of a sheet back on itself to produce a safe, stiff, double-thickness edge. A closed hem is flattened completely; an open, or teardrop, hem leaves a controlled gap. Hemming is normally a two-step operation: a pre-bend to about 30 degrees, then closure.
What is the difference between an open hem and a closed hem?
A closed hem is flattened so the folded edge has no gap, giving the strongest and safest edge. An open hem is closed only part of the way, leaving a rounded gap that is easier to form and avoids the marking and cracking risk of full flattening.
| Hem type | Fold | Typical total thickness | Typical pre-bend flange |
|---|---|---|---|
| Closed (flattened) | 180°, no gap | ≈ 2t plus the hem radius | ≈ 4t minimum, material dependent |
| Open (teardrop) | 180° with a gap | ≈ 2t plus the gap | ≈ 4t minimum |
| Joggle hem | Offset, not closed | Depends on the offset | Set by the offset depth |
What tooling is needed to hem?
The usual route is a gooseneck or straight punch for the 30 degree pre-bend, followed by a hemming die with a spring-loaded or rocking bottom that rolls the edge over progressively. A dedicated hemming tool set does both steps without removing the part.
- Pre-bend: a punch that can reach over the upstand without marking it.
- Closure: a hemming die with a movable bottom, so the edge is rolled rather than simply crushed.
- Short flanges: a punch with a narrow tip, because a hem flange is short by definition.
- Cosmetic faces: an anti-mark film or urethane insert on the outer face.
How much does a hem shorten the part?
A hem consumes roughly the flange minus the material already accounted for in the fold, so the blank has to be longer than the finished part by the hem allowance. Because the flange is folded onto itself, the hem allowance is close to the flange length less the material thickness taken up in the radius.
Why does a hem crack along the outside?
The outer surface is stretched well beyond the material's usual elongation limit. Cracking at the hem is normally a material problem rather than a tooling problem, and is managed by using a softer temper, increasing the pre-bend radius, or annealing the fold line.
| Defect | Usual cause | Typical remedy |
|---|---|---|
| Cracking on the outside | Material elongation exceeded | Softer temper, larger pre-bend radius |
| Crease or mark line | Punch contact at the fold | Anti-mark film or urethane insert |
| Open hem, will not close | Insufficient force or wrong die | Heavier hemming die, check tonnage |
| Uneven hem height | Unbalanced segments or wear | Align and reseat tooling |
Related technical pages
Gooseneck punches
Profile that clears the upstand during the pre-bend.
Press brake dies
Single-V, 2V, multi-V and urethane die types.
Anti-mark tooling
Protecting a cosmetic face through a hem or a sharp bend.
Custom tooling
Hemming tool sets made for a specific part and material.
Next step
Send us the material, thickness and hem type and we will recommend a hemming tool set and the hem allowance to build into the blank.
Request a quotation