Understand the fundamental principles of CNC press brake bending, from force calculation to springback control. Learn how modern bending technology achieves precision results.
Introduction
Press brake bending transforms flat laser-cut blanks into three-dimensional parts. At Fulei Metal, our four CNC press brake machines handle everything from simple brackets to complex multi-bend enclosures for clients across the globe. Understanding the principles of CNC press brake bending helps engineers design better parts.
The Physics of Bending
How Bending Works
Press brake bending applies force to sheet metal between an upper punch and lower die. The punch descends into the die, forcing the metal to bend. The metal undergoes plastic deformation (permanent) and elastic deformation (springback).
The Bending Zone
When the punch presses the sheet, the outer surface stretches under tension, the inner surface compresses, and the neutral axis is where neither occurs. The neutral axis position determines bend allowance.
Bending Methods
Air bending: the punch descends but does not bottom out. The bend angle depends on punch depth. It requires less force, accommodates different angles with one tool set, but has more springback.
Bottom bending: the punch bottoms out in the die. More accurate with less springback but requires more force and dedicated tooling.
Coining: extreme force imprints the die profile. Most accurate with no springback but requires very high tonnage.
CNC Press Brake Technology
Machine Configuration
Modern CNC press brakes feature ram guidance systems, CNC crowning for bed deflection compensation, CNC-controlled back gauges, real-time angle measurement, and quick-change tool clamping.
CNC Control Systems
The controller manages ram position, back gauge position, crowning, sequence programming, and parameter databases. At Fulei Metal, our four CNC press brakes feature advanced control systems.
Bend Allowance and K-Factor
Bend allowance is the material length consumed in the bend. The K-factor is the ratio of distance from inside surface to neutral axis, divided by thickness. It ranges from 0.3 to 0.5. Soft materials have lower K-factors, hard materials higher.
Springback
Springback is the elastic recovery after bending, typically 1-5 degrees. Factors include material type, thickness, bend angle, inside radius, and bending method. Modern CNC press brakes compensate by overbending, using real-time angle measurement, and parameter databases.
Bend Radius
Minimum inside bend radius: mild steel 0.8-1.0 times thickness, stainless steel 1.0-1.5 times, aluminum 1.0-2.0 times. Preferred radius is at least 1 times thickness.
Bending Force
Force depends on material tensile strength, thickness, bend length, die opening width, and bending method. Our press brakes range up to 200 tons.
Conclusion
CNC press brake bending requires understanding of material behavior, force calculation, and machine control. At Fulei Metal, our four CNC press brakes and experienced operators produce complex bent parts with consistent precision for international clients.
Air Bending Reference Numbers
The theory above becomes useful when it is tied to real die choices. The figures below assume mild steel with a tensile strength of roughly 400 MPa formed by air bending, where the inside radius is set by the die opening rather than by the punch nose.
| Sheet thickness | V-die opening used | Resulting inside radius | Shortest practical flange | Forming force per metre |
|---|---|---|---|---|
| 0.8 mm | 6 mm | about 0.9 mm | about 5 mm | about 61 kN (6 t) |
| 1.5 mm | 12 mm | about 1.9 mm | about 9 mm | about 107 kN (11 t) |
| 2.0 mm | 16 mm | about 2.5 mm | about 12 mm | about 142 kN (14 t) |
| 3.0 mm | 24 mm | about 3.7 mm | about 17 mm | about 213 kN (22 t) |
| 4.0 mm | 32 mm | about 5.0 mm | about 23 mm | about 284 kN (29 t) |
Three ratios do most of the work in daily programming: the die opening is usually eight times the sheet thickness, the inside radius ends up close to 0.156 times that opening, and the required force falls roughly in proportion to the opening chosen. The last column is why a drawing asking for a very small radius on thick material can quietly push a job beyond the capacity of the machine intended for it — narrowing the die raises force, it does not reduce it.
Frequently Asked Questions
Why does my drawing specify an inside radius that the part cannot reach with air bending?
In air bending the sheet touches only the die shoulders and the punch tip, so the forming machine does not impose the radius — the die opening does, at roughly 0.156 times that opening. A radius tighter than this needs a narrower die and therefore more tonnage, or a switch to bottoming or coining.
Can you keep an existing part identical if we change material grade?
Only partly. Moving from mild steel to SUS304 changes both the springback and the achievable minimum radius, so we re-cut a first article and re-check the flat development before releasing a production program. Tell us the grade change before quoting rather than after the first batch.
What information speeds up a bending quote?
Material and thickness, the bend angles with inside radii, the dimensions you actually inspect, and any surface that must stay unmarked. A dimensional drawing with the datums marked removes more questions than anything else in the pack.
Questions about a specific part are usually faster to answer against the drawing — send it through the review route below.
Next step. Send the drawing for a CNC bending capability and machine list. We confirm the die set and the achievable radius before quoting, so the numbers you receive describe this part rather than an average. Useful background: DFM review before quoting, minimum bend radius tables and bend allowance and bend deduction.