Press Brake Bend Radius & Springback Guide
A press brake bend radius is not selected from the punch tip alone. In air bending, the V-die opening, material strength, thickness and grain direction all influence the final inside radius. The part then opens slightly after the load is removed - a change called springback. This guide explains how to estimate the natural radius, plan springback compensation and confirm the setup with a controlled test bend.
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What Is the Inside Bend Radius?
The inside bend radius is the radius measured on the concave, inside surface of the formed sheet. It affects the finished dimensions, bend allowance, outside strain, cracking risk and the tooling required to make the part. The outside radius is approximately the inside radius plus material thickness.
Do not confuse three different values:
- Specified inside radius: the radius required on the drawing.
- Natural air-bend radius: the radius produced by the material, die opening and process.
- Minimum permitted bend radius: the smallest radius the particular material grade, thickness, temper and grain orientation can tolerate without unacceptable damage.
How V-Die Opening Changes the Natural Radius
WILA explains that the V opening largely determines the product radius during air bending. A larger opening generally creates a larger natural radius, reduces the required force and increases the minimum supported flange. A smaller opening creates a tighter radius but raises the force and can reduce precision or increase tooling stress if it is too narrow for the material.
Mate Precision publishes useful natural-radius starting factors for 90-degree air bending:
| Reference material | Natural-radius factor | Typical springback tendency | Important note |
|---|---|---|---|
| Cold-rolled / mild steel reference | About 0.156 × V | Baseline | Actual tensile and yield strength still matter. |
| Aluminum reference | About 0.14 × V | Alloy and temper dependent | 5052 and 6061-T6 do not bend alike; confirm alloy, temper and grain. |
| Stainless steel reference | About 0.21 × V | Generally higher than mild steel | Higher strength can require more force, a larger opening and more compensation. |
These factors estimate the radius created by a specific air-bending setup. They are not minimum-bend-radius limits for every grade.
Example Natural Radius Chart
| V opening | Cold-rolled steel estimate | Aluminum estimate | Stainless estimate |
|---|---|---|---|
| 12 mm | 1.87 mm | 1.68 mm | 2.52 mm |
| 16 mm | 2.50 mm | 2.24 mm | 3.36 mm |
| 24 mm | 3.74 mm | 3.36 mm | 5.04 mm |
| 32 mm | 4.99 mm | 4.48 mm | 6.72 mm |
| 40 mm | 6.24 mm | 5.60 mm | 8.40 mm |
Round the estimate only after comparing it with the approved manufacturer chart and a measured test bend. Changing the opening also changes tonnage, minimum flange and the flat pattern.
What Is Press Brake Springback?
Springback is the elastic recovery that occurs when bending force is removed. The angle normally opens and the inside radius can increase. The programmed or loaded angle therefore needs to be more acute than the desired released angle. That planned extra movement is commonly called overbending or springback compensation.
Mate notes that large-radius work is especially difficult to predict. It defines a large or profound radius as an inside radius at least eight times material thickness and reports that springback rises as the inside-radius-to-thickness ratio increases. For these applications, calculators are useful for planning but should not be treated as exact.
What Increases or Changes Springback?
| Variable | Practical effect | What to check |
|---|---|---|
| Higher yield or tensile strength | Usually increases springback and force. | Exact grade, temper, certificate and lot. |
| Larger radius-to-thickness ratio | Can make the final angle and radius harder to predict. | Whether special radius tooling or staged forming is required. |
| Wider V opening | Generally increases radius and can increase springback while reducing force. | Die angle must allow enough overbend without bottoming. |
| Material thickness variation | Changes penetration, force and released angle. | Measure the actual stock, not nominal gauge alone. |
| Rolling direction | Changes bendability and crack risk. | Orient the bend line using the material supplier's guidance. |
| Tool and machine condition | Wear, deflection and alignment can change the angle along the bend. | Tool radii, holders, crowning, ram alignment and maintenance. |
Air Bending vs Bottoming vs Coining
| Method | How the radius is formed | Springback control | Planning concern |
|---|---|---|---|
| Air bending | Material forms naturally over the die opening. | Controlled mainly with penetration, overbend and feedback. | Lowest force of the three, but sensitive to material variation. |
| Bottoming | Material is pressed closer to the punch and die geometry. | Reduced compared with air bending, but not eliminated. | Higher force and tooling geometry must match the job. |
| Coining | Very high pressure plastically sets the bend zone. | Can greatly reduce springback. | Very high force; unsuitable unless the machine and tooling are specifically rated. |
Do not use the air-bending radius factors or the air-bending tonnage calculator for bottoming or coining.
How to Compensate for Springback
- Confirm the material. Record grade, temper, measured thickness and rolling direction.
- Select a safe starting V opening. Check the desired radius, minimum flange and tonnage using the actual tooling chart.
- Confirm the tool angles. The punch and die must allow the required overbend without bottoming or interference.
- Run a representative test coupon. Use the same lot, thickness, bend direction, bend length and tooling planned for production.
- Measure after unloading. Check the released angle and inside radius with calibrated tools.
- Correct in small controlled steps. Follow the brake and controller manufacturer's procedure for depth, angle correction or adaptive feedback.
- Save the proven setup. Record the material lot, tooling IDs, opening, radius, program correction and measured result.
A universal springback-degree table is not a substitute for the coupon. Wilson Tool notes that material sold to the same specification by different vendors can produce different springback and final radii.
Preventing Cracks and Inconsistent Radii
- Confirm the material supplier's minimum bend radius for the exact grade and temper.
- When permitted by the drawing, orient the rolling direction perpendicular to the bend line. SSAB notes that plate can often bend tighter in this orientation than with the bend line parallel to rolling direction.
- Deburr thermal-cut or sheared edges and remove surface defects that could start a crack.
- Use a punch radius and V opening appropriate for the material strength.
- Inspect punch radii and die shoulders for wear or damage.
- Do not narrow the V opening solely to chase a smaller radius without recalculating force and checking tool capacity.
- For a large specified radius, use purpose-designed radius tooling and review the risk of springback and multi-breakage.
Worked Example: 3 mm Mild Steel
Assume a 3 mm mild-steel coupon, a 24 mm V opening and a 90-degree air bend. Using Mate's cold-rolled-steel starting factor:
The estimate is useful for the first setup and flat-pattern review, but it does not predict the final released angle. Next, calculate the required load for the actual bend length, confirm that the punch and die angles permit overbending, and run a coupon from the production material. Measure the released radius and angle, then save the proven correction.
Press Brake Tooling to Compare
Tool angle is only one selection point. Confirm the clamping style, height, radius, V opening, sectional lengths and load rating before ordering.
1011 Straight Punch, 88°
Amada/Promecam-style straight punch with a listed 0.8 mm tip radius and 100 T/m maximum.
From $202 CAD
View 88° Punch
1024 Straight Punch, 45°
Acute Amada/Promecam-style punch for compatible applications that require additional overbend clearance.
From $214 CAD
View 45° Punch
2081 Single-V Die, 45°
Amada-style die with a listed 40 mm opening and 100 T/m maximum for compatible acute-bending setups.
From $580 CAD
View 45° DieSend the machine and holder model, material grade, thickness, bend length, specified radius, angle, flange and part drawing.
Request a Tooling Review or Quote
Press Brake Bend Radius and Springback FAQ
Does the punch tip radius equal the inside bend radius?
Not normally during air bending. The material forms a natural radius influenced mainly by the V opening and material behavior. Punch radius becomes more critical when it is too sharp, when a large radius is specified, or when the process moves toward bottoming.
How much should I overbend for springback?
There is no universal value. Material grade, lot, strength, thickness, V opening, radius-to-thickness ratio and bend direction all matter. Use the manufacturer's starting data and confirm the correction with a representative test coupon.
Does stainless steel spring back more than mild steel?
Stainless commonly requires more compensation than a mild-steel baseline because of its material properties, but the exact result depends on the grade and setup. Do not apply one correction to every stainless alloy.
Does a wider V die reduce tonnage?
Generally yes for air bending. It also increases the natural radius and minimum flange and may increase springback, so all four results must be checked together.
What is considered a large-radius bend?
Mate describes an inside radius at least eight times material thickness as a large or profound radius. These bends can involve substantial springback and may require special tooling or staged forming.
Manufacturer References
- WILA: V-opening size - natural radius, opening ratios, flange and force considerations.
- Mate Precision: Large Radius Bending - springback and multi-breakage in large-radius work.
- Mate Precision tooling catalog - bending basics, natural-radius references and tooling guidance.
- Wilson Tool Q&A - radius-tool selection and material-vendor variation.
- SSAB: Bending of Strenx - rolling direction, tool condition, high-strength steel and springback.
Continue learning: Visit the Education & Product Guides hub, use the press brake tonnage calculator, or browse press brake tooling.
Flat-pattern planning: Continue with the press brake bend allowance, K-factor and bend deduction guide.
Compare Tonnage by Material and Thickness
Use the metric and imperial air-bending charts for mild steel, stainless and aluminum, then enter the exact bend length in the calculator.