A sheet metal bracket that came out of the press brake at the wrong angle. The bracket was 2 mm mild steel, 50 mm leg, intended to bend at 90 degrees. The first batch came out at 87 degrees. The operator adjusted the die depth. The next batch came out at 93 degrees. The operator couldn’t dial in 90. The issue wasn’t the press brake — it was springback. The material bent past 90 during the stroke, then sprung back 3 degrees when the die opened. This is about press brake bending and how to compensate for springback.
What is springback
When sheet metal is bent, the outer fibers stretch and the inner fibers compress. Some of this deformation is plastic (permanent) and some is elastic (recoverable). When the die opens, the elastic strain recovers — the part springs back slightly. For mild steel (2 mm), the springback is about 2-3 degrees for a 90-degree bend. For aluminum, it’s 4-6 degrees. For high-strength steel, it can be 8-10 degrees.
The press brake operator sets the die depth to achieve 90 degrees. But the springback means the actual angle is 87 degrees. The operator over-bends to 93 degrees to compensate. The springback brings it to 90. But the springback varies with material batch, grain direction, and thickness. The over-bend that works for one batch fails for the next.
Air bend vs bottoming
Two bending methods control springback differently:
Air bending: the punch doesn’t bottom on the die. The sheet rests on the die shoulders and contacts the punch tip. The bend radius is determined by the die opening (about 1x material thickness for V-die). The operator sets the bend depth. Springback is present. The operator compensates by over-bending. Air bending is fast, flexible, and uses less tonnage. It’s the default for 90% of press brake work.
Bottoming (coining): the punch bottoms the sheet on the die. The material is forced into the V. The bend radius is controlled by the punch tip radius. Springback is nearly eliminated (the material is cold-worked at the bend). The angle is accurate to ±0.5 degrees. But bottoming uses 5x more tonnage and requires a dedicated die per bend angle. It’s used for precision parts where 90±0.5 is required.
What was changed
1. Set the over-bend angle. The operator bend 93.5 degrees (3.5 degrees past 90) for 2 mm mild steel. The springback brought it to 90±0.5. The over-bend was set in the CNC program. The first-article inspection verified the angle. Subsequent parts held the angle within ±0.5. The over-bend angle depends on material: 3 degrees for mild steel, 5 degrees for aluminum, 8 degrees for HSLA steel.
2. Used a 3-point die. For precision parts, a 3-point die measures the angle during bending. The CNC adjusts the ram depth automatically. The part comes out at exactly 90 degrees. No springback compensation needed. The die costs $2000 but eliminates first-article inspection. Used for production parts (thousands per month).
3. Changed the grain direction. The parts were bent across the grain (bend line perpendicular to the rolling direction). Across-grain bending causes more springback and can crack. The parts were re-nested in the blank so the bend line ran with the grain. The springback dropped from 3.5 to 2.5 degrees. The bend was cleaner. For holes near the bend, with-grain bending prevents distortion.
The bend tonnage calculation
The press brake tonnage depends on material thickness, tensile strength, and die opening:
P = (S × t²) / W × L
Where S is the material tensile strength (400 N/mm² for mild steel), t is thickness (mm), W is die opening (mm), and L is bend length (m). For 2 mm mild steel on a 12 mm V-die, 1 m bend: P = 400 × 4 / 12 × 1 = 133 kN. The press brake is rated for 1000 kN. Plenty of margin. The V-die opening should be 8x material thickness (16 mm for 2 mm). A smaller die opening needs more tonnage and can crack the bend. A larger die opening creates a radius that’s too wide.
| Thickness | Die opening (V) | Ton/meter (mild steel) | Min punch radius |
|---|---|---|---|
| 1 mm | 8 mm | 35 kN/m | 1t |
| 2 mm | 16 mm | 133 kN/m | 2t |
| 3 mm | 24 mm | 300 kN/m | 3t |
| 5 mm | 40 mm | 833 kN/m | 5t |
The inside bend radius rule
The punch radius (which becomes the inside bend radius) should be at least 1x material thickness for mild steel, 2x for aluminum. A tighter radius cracks the outside of the bend. A wider radius (3x thickness) reduces springback but creates a large radius that’s hard to dimension. The standard is 1t for mild steel, 2t for stainless. The outside bend radius = inside radius + thickness. This dimension goes on the drawing.
The bending rule: air bend with over-bend compensation, or bottom for precision. The 87-degree bracket wasn’t a press brake problem — it was 3 degrees of springback. Set the over-bend angle in the CNC program. Use a 3-point die for production. Bend with the grain to reduce springback and cracking. The die opening is 8x material thickness. The punch radius is 1t for mild steel.
