Hemming bend reduction calculator

What is hemming bend reduction?

A hem folds the edge of the sheet back on itself, so the bend region consumes material that the finished part no longer shows. The bend reduction is the amount to subtract from the hem length to get the flat blank. Standard bend allowance and bend deduction formulas stop being reliable once the bend angle passes about 174 degrees, which is why hemming uses its own allowance.

How much material does a hem consume?

The published rule for material under 2 mm is a hem allowance of 43 percent of the material thickness: multiply the thickness by 0.43 and subtract the result from the hem length. A 1.5 mm flange hemmed flat therefore loses about 0.65 mm against the finished length.

Does the rule hold for thicker material?

The 0.43 factor is published for stock under 2 mm. Above that, the reduction grows with the thickness and the pre-bend radius, and published values diverge, so the safe practice is to test-bend one piece and measure before cutting the batch. Tight-tolerance work should be test-bent whatever the thickness.

How much force does hemming take?

Hemming needs several times the force of an ordinary air bend of the same thickness, because the final stage flattens the fold almost like coining. The table below gives the commonly published hemming tonnage per metre of bend; the calculator interpolates between the rows.

What is the hemming sequence on a press brake?

Hemming is done in stages rather than one hit, which keeps the strains low and avoids cracking. First bend the flange to 90 degrees, then fold it part-way back, then close it flat.

1. Flange: bend the edge to 90° ± 1°; any error here carries through to the finished hem.
2. Pre-hem: fold back to about 130–145°; aluminium usually needs more overbend than mild steel.
3. Close: flatten fully. If the hem springs open, increase the pre-hem angle or add a slight coined groove on the panel side.

How do I stop a hem from cracking?

Cracks start at the outside of the fold where the elongation is highest. A generous pre-bend radius and a slower final close fix most cases.

• Increase the pre-bend radius: the single biggest factor in hem quality; raising it from sharp toward one times the thickness cuts surface strain sharply.
• Add 3–5° of overbend in the pre-hem stage to compensate for springback instead of forcing the close.
• Slow the final close so the material flows instead of tearing.
• Check the material: high-strength steel, hard aluminium alloys and some stainless grades crack at radii mild steel accepts.
• Deburr the edge first: a sharp burr acts as a stress riser and starts the crack.

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