Free Sheet Metal Hem & Tear Relief Tool
Calculate minimum press brake hem flange length ($L_{min} \ge 4T$), flattening tonnage, tear-drop radius, and corner tear relief cutout geometry per DIN 6935.
📏 Sheet Metal Thickness & Material
📊 Sized Hem Flange & Press Force
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Sheet Metal Hemming and Tear Relief Engineering
Hems are created by bending a sheet 30° to 45° in an acute tool and flattening it in a secondary hemming press brake stroke. Hems eliminate razor-sharp burrs on exposed cabinet edges while quadrupling section stiffness.
Minimum Flange Rules
If the return flange is too short, the sheet slips out of the V-die during the initial 30° acute bend:
$$L_{min} ge 4 cdot T$$
For a 1.5mm steel sheet, never specify a hem shorter than 6.0 mm.
Corner Tear Relief Cutouts
When bending two adjacent flanges that meet at a 90° corner, plastic strain will rip the material at the intersection without a relief cutout:
- Relief Width: (W_{relief} ge T) (recommended (R + T))
- Relief Depth: Must extend beyond the bend tangent line by at least (0.5 cdot T).
Frequently Asked Questions
Why does aluminum crack when creating a closed hem?
Tempered alloys like 6061-T6 or 5052-H34 have limited ductility. When crushed into a closed hem (zero inside radius), the outer grain fibers stretch past their ultimate strain and fracture. For aluminum, always specify an open hem (air gap = T) or tear-drop hem.
How much tonnage does a flattening hemming die require?
Flattening a closed hem requires 3x to 5x higher tonnage than an air bend—typically 40 to 70 tons per meter for mild steel. Ensure your press brake has adequate capacity.