Quick Lookup: Choose V-Die and Radius by Thickness
Use this table before detailed tonnage calculation. It gives a practical starting V-die opening and expected inside radius for common air-bending jobs. After selecting the die, verify force with the press brake bending calculator.
| Job condition | Starting V-die | Expected inside radius | Use when |
|---|---|---|---|
| Tighter radius | 6 × thickness | About 0.75t to 1.0t | Small flange or compact enclosure parts; check tonnage and cracking risk first. |
| Standard work | 8 × thickness | About 1.0t to 1.3t | Most mild steel and aluminum sheet metal jobs with balanced force, radius and accuracy. |
| Stainless / HSLA | 8 × to 10 × thickness | About 1.3t to 1.7t | Higher tensile strength, larger springback, or parts where surface cracking must be avoided. |
| Thick plate | 10 × to 12 × thickness | About 1.7t to 2.0t | 8 mm and thicker plate, long bend lengths, or limited press brake tonnage. |
What Is Minimum Bend Radius and Why It Matters
The minimum bend radius is the smallest inside radius a sheet metal part can be bent to without cracking, fracturing, or experiencing excessive thinning on the outer surface. It is one of the most critical parameters in press brake forming — get it wrong, and you risk scrapped parts, cracked flanges, or premature fatigue failure in service.
Bend radius depends on four primary factors: material type (ductility and tensile strength), material thickness, grain direction relative to the bend line, and V-die opening width. Understanding these relationships allows operators to select the correct tooling and program accurate bend parameters on the CNC press brake controller. The tooling selection guide covers how V-die opening affects achievable bend radii in detail.
A bend radius that is too tight causes cracking on the outside surface of the bend. A radius that is too large wastes material and may not meet part geometry requirements. The charts and formulas in this guide give you the data to find the optimal balance for every common sheet metal material.
Minimum Bend Radius Chart by Material and Thickness
The following table shows the minimum recommended inside bend radius for common sheet metals when air bending with a standard V-die (opening = 8× material thickness). Values are in millimeters. These are minimums — using a slightly larger radius improves part quality and reduces cracking risk.
| Material | 1 mm | 1.5 mm | 2 mm | 3 mm | 4 mm | 6 mm | 8 mm |
|---|---|---|---|---|---|---|---|
| Mild Steel (A36/Q235) | 1.0 | 1.5 | 2.0 | 3.0 | 4.0 | 6.0 | 8.0 |
| Stainless Steel 304 | 1.5 | 2.3 | 3.0 | 4.5 | 6.0 | 9.0 | 12.0 |
| Stainless Steel 316 | 2.0 | 3.0 | 4.0 | 6.0 | 8.0 | 12.0 | 16.0 |
| Aluminum 5052-H32 | 1.0 | 1.5 | 2.0 | 3.0 | 4.0 | 6.0 | 8.0 |
| Aluminum 6061-T6 | 2.0 | 3.0 | 4.0 | 6.0 | 8.0 | 12.0 | 16.0 |
| Copper (C110, soft) | 0.5 | 0.8 | 1.0 | 1.5 | 2.0 | 3.0 | 4.0 |
| Brass (C260, half-hard) | 1.0 | 1.5 | 2.0 | 3.0 | 4.0 | 6.0 | 8.0 |
| HSLA Steel (S355/Q345) | 1.5 | 2.3 | 3.0 | 4.5 | 6.0 | 9.0 | 12.0 |
Grain Direction Matters
The values above assume bending perpendicular to the rolling direction (transverse). When bending parallel to the grain, increase the minimum bend radius by 50–100%. For critical parts, always specify grain direction on the engineering drawing.
How to Calculate Minimum Bend Radius
The industry-standard formula for minimum bend radius uses a material-specific factor multiplied by the sheet thickness:
Where Rmin is the minimum inside bend radius (mm), k is the material factor, and t is the material thickness (mm).
Material Factor (k) Reference Table
| Material | k Factor | Condition |
|---|---|---|
| Soft Copper (C110 annealed) | 0.5 | Annealed, very ductile |
| Aluminum 5052-O / 3003 | 0.5–1.0 | Soft temper |
| Mild Steel (A36/Q235) | 1.0 | Hot rolled or cold rolled |
| Brass (C260) | 1.0 | Half-hard temper |
| HSLA Steel (S355/Q345) | 1.5 | Higher strength, lower ductility |
| Stainless Steel 304 | 1.5–2.0 | Work-hardens rapidly |
| Aluminum 6061-T6 | 2.0–3.0 | Heat-treated, low ductility |
| Stainless Steel 316 | 2.0 | Higher molybdenum content |
| Spring Steel | 3.0–4.0 | Hardened, very low ductility |
Calculation Example
Suppose you need to bend 3 mm 304 stainless steel. Using k = 1.5 (conservative):
This means the smallest inside bend radius you should program is 4.5 mm. A 5 mm or 6 mm radius would provide a safety margin and produce a cleaner bend surface.
V-Die Selection and Its Effect on Bend Radius
The V-die opening width is the single most controllable factor affecting inside bend radius during air bending. For quick workshop estimates, the relationship is approximately:
Where Ri is the resulting inside bend radius and V is the die opening width. A 24 mm V-die opening normally produces an inside radius around 3–4 mm in air bending, depending on material strength, punch radius and tooling condition. For a more practical estimate, the Rucheng bending calculator uses material-specific radius factors instead of treating every material exactly the same.
V-Die Opening Selection Rules
| Material Thickness | V-Die Opening (V) | Approx. Inside Radius (V/6) | Example |
|---|---|---|---|
| 0.5–3.0 mm | 6× to 8× thickness | 1× to 1.3× thickness | 2 mm → V16, Ri ≈ 2.7 mm |
| 3.0–8.0 mm | 8× to 10× thickness | 1.3× to 1.7× thickness | 6 mm → V50, Ri ≈ 8.3 mm |
| 8.0–16.0 mm | 10× to 12× thickness | 1.7× to 2.0× thickness | 12 mm → V120, Ri ≈ 20 mm |
| > 16 mm | 12× to 16× thickness | 2.0× to 2.7× thickness | 20 mm → V250, Ri ≈ 42 mm |
How Bend Radius Affects Machine and Tooling Selection
Tonnage and press brake reserve
A smaller V-die creates a tighter radius but sharply increases bending force. If the job requires 6× thickness or less, calculate force first and leave at least 20–30% machine reserve for production work.
Minimum flange length
A wider V-die lowers tonnage but increases the minimum flange. If the drawing has a short return flange, confirm that the flange can sit on the die shoulders before choosing a wide opening.
Cracking and surface quality
Stainless steel, 6061-T6 aluminum and high-strength steel need larger radii than mild steel. For visible panels, use polished tooling, protective film or a larger die to reduce marking and cracking.
Accuracy and springback
Larger radii usually produce more springback and require more controller compensation. Precision parts may need angle measurement, crowning and test bends before full production.
Warning: Narrow Die Risks
Using a die opening less than 6× the material thickness significantly increases tonnage requirements and risk of die damage. A V-die opening of 4× thickness requires approximately 3× the normal tonnage. Always verify tonnage capacity before selecting a narrower die. Use our free bending calculator to instantly check the required force for any die and material combination.
Material-Specific Bending Considerations
Mild Steel (A36 / Q235 / S235)
Mild steel is the most forgiving material for press brake bending. With a minimum bend radius of 1× thickness and low springback (1–3°), it is ideal for operators learning CNC press brake operation. Mild steel can be bent to very tight radii without cracking, though surface scratching on the outer radius may occur at R < 0.5t. Use a standard V-die (8× thickness) for best results.
Stainless Steel (304 / 316)
Stainless steel work-hardens during bending, which means the material becomes harder and more brittle at the bend zone. Key considerations:
- 304 stainless requires 1.5× thickness minimum radius; 316 requires 2.0×
- Springback is 3–5° — overbend accordingly on the CNC controller
- Use polished or chrome-plated tooling to prevent surface marking
- Tonnage is approximately 1.5× that of mild steel for the same thickness
- Avoid re-bending — work hardening at the bend zone makes corrections risky
Aluminum Alloys
Aluminum bending behavior varies dramatically by alloy and temper. Soft alloys like 5052-O bend as easily as mild steel (k = 0.5–1.0), while heat-treated 6061-T6 requires 2–3× thickness radius and is prone to cracking. For 6061-T6, consider annealing the bend zone before forming, or specify 5052-H32 in the design phase instead.
| Aluminum Alloy | k Factor | Springback | Notes |
|---|---|---|---|
| 3003-O | 0.5 | 1–2° | Very soft, excellent formability |
| 5052-O | 0.5 | 1–2° | Most common sheet aluminum |
| 5052-H32 | 1.0 | 2–3° | Strain-hardened, good balance |
| 6061-T4 | 1.5 | 2–4° | Partially heat-treated |
| 6061-T6 | 2.0–3.0 | 3–5° | Cracks easily — use caution |
| 7075-T6 | 4.0–5.0 | 4–6° | Not recommended for tight bends |
Copper and Brass
Annealed copper (C110) is the most ductile common sheet metal, with a k factor of just 0.5. It can be bent to extremely tight radii with virtually no cracking risk. Half-hard brass (C260) behaves similarly to mild steel with k = 1.0. Both materials produce minimal springback (0.5–2°), making them easy to program on CNC controllers.
Springback Compensation by Material
Springback is the elastic recovery of the material after the press brake ram retracts. The bent angle "opens up" slightly, and the inside radius increases. Understanding springback is essential for achieving the target angle on the first hit.
| Material | Air Bending Springback | Bottom Bending Springback | Compensation Method |
|---|---|---|---|
| Mild Steel | 1–3° | 0.5–1° | Overbend 2° on controller |
| Stainless 304 | 3–5° | 1–2° | Overbend 4°; use angle sensor |
| Aluminum 5052 | 2–4° | 0.5–1.5° | Overbend 3°; test first piece |
| Aluminum 6061-T6 | 3–5° | 1–2° | Overbend 4–5°; expect variation |
| Copper (soft) | 0.5–2° | ~0° | Minimal compensation needed |
| Brass (half-hard) | 1–3° | 0.5–1° | Overbend 2° |
Pro Tip: Reducing Springback
Three methods reduce springback: (1) Bottom bending or coining instead of air bending — forces material past yield point. (2) Using a smaller V-die opening — concentrates force on a smaller area. (3) Using a punch with a radius smaller than the target inside radius — the material wraps more tightly. Modern Delem and ESA controllers include automatic springback tables that adjust the ram depth based on material and thickness selection.
Common Bend Radius Mistakes and How to Avoid Them
Even experienced operators encounter bend radius issues. Here are the most frequent mistakes and their solutions:
1. Ignoring Grain Direction
Bending parallel to the rolling direction increases cracking risk significantly. Always orient blanks so the bend line is perpendicular to the grain. If bending parallel is unavoidable, increase the bend radius by at least 50% above the chart minimum.
2. Using the Wrong V-Die Width
A die opening that is too narrow (less than 6× thickness) dramatically increases tonnage and risks cracking the die. A die that is too wide (more than 12×) produces an unpredictable radius and poor angle accuracy. Stick to the 8× rule for standard work, and always check the tonnage calculation before running.
3. Not Accounting for Material Variation
Real-world sheet metal varies in thickness (±10% is common) and hardness. A batch of "2 mm mild steel" may actually be 1.85–2.15 mm, and yield strength can vary by 20%. Always run a test bend on the first piece and adjust the program accordingly. For production runs, use a press brake with angle measurement sensors for real-time correction.
4. Bending Too Close to Edges or Holes
Bend lines placed too close to punched holes or sheet edges cause distortion and tearing. As a rule, keep the bend line at least 2× material thickness away from any hole edge, and at least 4× thickness from the sheet edge.
5. Neglecting Springback for Stainless and Aluminum
Operators accustomed to mild steel often underestimate springback when switching to stainless or aluminum. Stainless 304 springs back 3–5° in air bending — failing to compensate means every part is out of spec. Program the springback offset before running the batch.
Engineering Checklist for Tooling Recommendation
When you ask Rucheng to recommend a press brake, V-die or punch set, send the following details. This lets the engineer check tonnage, die opening, minimum flange, punch interference, back gauge clearance and springback before quoting.
- Material grade and tensile strength if available.
- Thickness range and maximum bend length.
- Bend angle and required inside radius.
- Minimum flange length and return flange shape.
- Grain direction and whether the bend line is parallel or transverse.
- Surface finish requirement, protective film or no-mark tooling requirement.
- Production volume, accuracy tolerance and first-piece inspection requirement.
- Existing press brake model, tonnage, working length and tool clamping type.
Frequently Asked Questions
What is the minimum bend radius for mild steel?
The minimum bend radius for mild steel (A36/Q235) is typically 1.0× the material thickness when using air bending with a properly sized V-die. For example, 2 mm mild steel has a minimum inside bend radius of 2 mm. Using a narrower V-die opening can achieve tighter radii, but increases tonnage requirements and risk of cracking.
How do you calculate minimum bend radius?
Minimum bend radius is calculated using the formula: Rmin = k × t, where Rmin is the minimum inside bend radius, k is the material factor (ranging from 0.5 for soft copper to 3.0 for hardened stainless steel), and t is the material thickness. For air bending, the actual inside bend radius also depends on the V-die opening width, typically following the rule: Ri ≈ V/6 to V/8, where V is the die opening.
What V-die opening should I use for bending?
The standard rule is V-die opening = 8× material thickness for material up to 3 mm, 10× for 3–8 mm, and 12× for material over 8 mm. For example, 2 mm steel should use a 16 mm V-die, while 6 mm steel needs a 50–60 mm die. Narrower dies produce tighter bend radii but require higher tonnage. Wider dies reduce tonnage but produce larger radii. View our complete press brake die catalog for available sizes.
Does grain direction affect bend radius?
Yes. Bending perpendicular to the grain direction (transverse bending) allows tighter radii and produces cleaner bends. Bending parallel to the grain (longitudinal bending) requires a larger minimum bend radius — typically 1.5× to 2× greater — and increases the risk of surface cracking, especially in aluminum and high-strength steel. Always orient blanks so the bend line crosses the rolling direction when possible.
How does springback affect the final bend radius?
Springback causes the final bend radius to be larger than the punch radius. Mild steel springs back 1–3°, stainless steel 3–5°, and aluminum 2–4°. To compensate, operators overbend by the expected springback angle. Larger bend radii produce more springback. Bottom bending and coining minimize springback to near zero but require 3–5× more tonnage than air bending.
What information is needed to recommend press brake tooling?
Provide material grade, tensile strength if available, thickness range, bend angle, required inside radius, minimum flange length, grain direction, surface finish requirement, maximum bend length, production volume, and the press brake model or tonnage. Drawings or STEP files allow the engineer to check tool interference and back gauge clearance.
Conclusion
Selecting the correct bend radius is fundamental to producing quality press brake parts. Use the charts and formulas in this guide as a starting point: calculate Rmin = k × t for your material, select a V-die opening of 8× thickness, and always account for springback compensation in your CNC program. When in doubt, run a test bend on scrap material before committing to a production run.
For materials with low ductility like 6061-T6 aluminum or hardened stainless, consider specifying a more formable alloy in the design stage, or use a larger bend radius to maintain structural integrity. The right combination of tooling, material, and machine settings will give you consistent, crack-free bends every time.
Need Help Selecting the Right Press Brake or Tooling?
Rucheng Technology manufactures CNC press brakes, precision V-dies and bending molds, and complete bending tooling systems. Our engineers can recommend the optimal machine and die configuration for your specific materials and bend radius requirements. Also use our tonnage calculation guide to verify your machine can handle the required force.
Get a Free Quote →Press brake selection and production resources
Compare the full Rucheng press brake machine range before choosing tonnage, working length, controller, tooling and automation options.
- Electro-hydraulic CNC press brake for high-mix precision bending.
- Tandem press brake for long parts, poles, panels and structural work.
- Press brake tonnage calculation, tooling selection and controller selection help confirm the final specification.
- For blank preparation before bending, compare fiber laser cutting machines and plan the cutting-to-bending workflow together.
For a practical machine recommendation, send material grade, thickness, bending length, daily output and target budget through the press brake quote form.