Sheet Metal Hemming & Flange Unfolding Prepress Guide

1. The Two-Stage Hemming Process

Unlike a standard 90° air bend, a sheet metal hem cannot be formed in a single stroke. Hemming requires a sequential two-stage tooling setup:

• Stage 1 (Acute Pre-Bend): An acute punch (typically 28° to 30°) presses the sheet into a matching acute 30° V-die to bend the return flange past 145°–150°.
• Stage 2 (Flattening / Closing): The pre-bent part is moved to a flat flattening table or spring-loaded hemming die where vertical pressure flattens the flange against the sheet body.

2. Minimum Flange Length (Lmin) Rule

To safely form the acute pre-bend, the flange must span across both shoulders of the 30° V-die without falling into the die opening:

If the return flange is shorter than 4T, the edge will slip off the die shoulder during punch descent, causing severe part distortion, inconsistent angles, and tool damage.

3. Three Styles of Sheet Metal Hems

• Closed / Flat Hem (Rope Hem): Completely flattened against the sheet (0 mm gap). Delivers maximum edge stiffness but exerts extreme tensile strain on the outer bend radius. Ideal for ductile cold rolled mild steel and stainless steel.
• Open Hem: Features a calibrated air gap (g ≈ 0.5T - 1.5T) maintained by a shim or stop block. Used for sliding interlocks and weather seals.
• Teardrop Hem: Forms a smooth bulbous loop (R ≈ 1.0T). Recommended for heavier aluminum alloys (such as 5052-H32 or 6061) that would crack if flattened completely.

4. Flat DXF Blank Elongation Deduction

Because the metal undergoes plastic stretching during the 180° folding and coining process, the flat DXF pattern must account for hem growth. The unfolded flat blank addition for a hem is calculated as: