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Durmapress specializes in designing, manufacturing, and selling various metal processing equipment, including bending machines, shears, punches, and laser cutting machines. The company was founded in 2014, with years of experience and technology accumulation. DurmaPress has become one of the well-known brands in China's metal processing machinery industry.
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1. Introduction
In sheet metal bending, the single most important decision an operator makes is choosing the correct V die opening. It quietly controls three outcomes at once: the inside bend radius of your part, the tonnage your press brake must exert, and the minimum flange length you can safely form. Get it right and you produce accurate angles, a predictable radius, and long tool life. Get it wrong and you invite cracked parts, chipped dies, overloaded machines, and scrap.
This guide gives you a practical V die opening chart, the math behind it (the famous "Rule of 8"), and the material-specific adjustments that separate a passable bend from a precise one. Whether you run mild steel, stainless, aluminum, or high-strength plate, you'll be able to select the ideal die width with confidence.
2. Press Brake V Die Opening Chart
The chart below is your quick-decision command center for the most common case — mild steel (tensile strength ≈ 450 N/mm² / 60,000 PSI), 90° air bending. It links five key parameters: material thickness (t), V opening (V), inside bend radius (ir), minimum flange length (b), and required tonnage per meter (F). All V openings follow the Rule of 8 (V ≈ 8 × t).
| Material Thickness (t, mm) | V Die Opening (V, mm) | Inside Bend Radius (ir, mm) | Minimum Flange Length (b, mm) | Required Tonnage per Meter (F, t/m) |
|---|---|---|---|---|
| 1.0 | 8 | ≈ 1.3 | > 5.5 | ≈ 8 |
| 1.5 | 12 | ≈ 2.0 | > 8.5 | ≈ 10 |
| 2.0 | 16 | ≈ 2.6 | > 11.0 | ≈ 12 |
| 2.5 | 20 | ≈ 3.3 | > 14.0 | ≈ 14 |
| 3.0 | 25 | ≈ 4.0 | > 17.5 | ≈ 16 |
| 4.0 | 32 | ≈ 5.0 | > 22.0 | ≈ 22 |
| 5.0 | 40 | ≈ 6.5 | > 28.0 | ≈ 27 |
| 6.0 | 50 | ≈ 8.0 | > 35.0 | ≈ 32 |
| 8.0 | 63 | ≈ 10.0 | > 45.0 | ≈ 45 |
| 10.0 | 80 | ≈ 13.0 | > 55.0 | ≈ 56 |
Note: These are widely accepted industry reference values. Actual results vary slightly with material batch, tooling condition, and machine calibration. V openings shown are the ideal recommendations based on the "Rule of 8."
How to read the chart in three steps:
- Find your thickness (t) in column one.
- Read the recommended V opening (V) — your safest, most efficient starting point.
- Check the results — the inside radius, minimum flange, and tonnage are now predictable before you ever load a sheet.
Once you've fixed your V opening, our press brake die radius chart makes it quick to confirm the exact radius that opening will produce.
3. What Is V Die Opening?
Сайт V die opening (also called die width or "V") is the distance across the top of the V-shaped groove in the lower press brake die. It is the single most critical dimension in air bending, because it is not merely a support platform — it directly defines the floating inside bend radius of your part.
In air bending, the sheet contacts only three points: the punch tip and the two shoulders of the V-die. The metal "floats" in the opening, and the inside radius forms naturally across those two shoulders. This is why, in modern air bending, the punch tip radius is largely irrelevant — the V opening is the real programmer of your bend.
Three features of the die work together:
- H3: V Opening (die width) — governs the inside radius and required tonnage.
- H3: Die Angle — usually slightly sharper than the target (e.g., 88°, 85°) to allow over-bending and control springback.
- H3: Shoulder Radius — the smooth transition into the groove; a larger, polished shoulder radius reduces die marks on stainless and aluminum but requires a longer minimum flange. How long that shoulder holds its shape comes down to press brake tooling hardness, which is worth understanding before you buy.
4. How to Calculate V Die Opening: The Rule of 8 Formula
The industry's golden standard is the "Rule of 8": the ideal V opening for 90° air bending of mild steel is approximately eight times the material thickness.
4.1 The Formula
V Die Opening = 8 × Material Thickness
4.2 Worked Example
For a 2 mm mild steel sheet:
2 mm × 8 = 16 mm → Use a V16 die
This ratio strikes an almost magical balance between bending force, angle accuracy, and springback control. It naturally produces an inside radius roughly equal to the material thickness (about 1/6 of the V opening), which keeps bend allowance calculations simple and repeatable.
4.3 When to Vary the Multiplier
The 8× rule is tuned for mid-range mild steel. Adjust the multiplier by thickness and material:
- Thin gauge (< 3 mm): 6× – 8× t
- Mid-range mild steel: 8× t
- Heavy plate (> 10 mm): 10× – 12× t
The science behind it: when V = 8t for mild steel, the neutral-axis position (the K-factor) delivers an inside radius ≈ material thickness. It is backed by both empirical validation and solid engineering — but it is a starting point, not an unbreakable law.
5. Recommended V Die Opening for Different Materials
The Rule of 8 is not one-size-fits-all. Different materials have different tensile strengths and ductility, so the multiplier must change. Use the chart below as your conversion reference, then read the material-specific notes.
| Материал | Tensile Strength (UTS) | V-Opening Rule | Tonnage Factor vs. Mild Steel | Inside Radius (% of V) |
|---|---|---|---|---|
| Mild Steel (baseline) | ≈ 450 N/mm² | 8 × t | 1.0× | 16–17% |
| Stainless Steel (304) | ≈ 600 N/mm² | 10–12 × t | 1.4–1.5× | 20–22% |
| Aluminum (5052, soft) | ≈ 230 N/mm² | 5–6 × t | 0.5–0.6× | 13–15% |
| Aluminum (6061-T6, hard) | Higher, low ductility | 10–12 × t | ≈ 1.0× | 13–15% |
| High-Strength Steel | > 800 N/mm² | 12–20 × t | 2.0–3.0× | varies |
5.1 Mild Steel
Apply the Rule of 8 (V = 8t). It's the safest, most efficient default and yields an inside radius ≈ material thickness. Example: 3 mm mild steel → V24 (chart shows V25 as the practical die size).
5.2 Stainless Steel
Stainless (e.g., 304) has ~50% higher forming force, greater toughness, and pronounced springback and work hardening. Give it more room: V = 10t to 12t. A wider opening disperses stress, lowers tonnage, and makes springback larger but far more predictable and controllable. Example: 2 mm stainless → V20.
5.3 Aluminum
Soft grades (5052) are ductile but delicate — prone to outer-fiber cracking and surface denting. Use a narrower V = 5t to 6t. Example: 2 mm aluminum → V12. Hard/T6 grades (6061-T6) crack easily, so switch to 10t–12t to keep the radius larger and safer. Protect the finish with polyurethane film or non-marking (urethane-insert / roller-shoulder) dies.
5.4 High-Strength Steel
Alloys such as Hardox® and Weldox® combine high yield strength with low ductility. Disregard the 8× rule — start at 12t, 16t, or even 20t. A wider V forces a larger inside radius, spreading intense stress and preventing fracture along the bend line. Because the loads are extreme, these jobs also call for heavy-duty
оснастка для листогибочных прессов and sharper die angles (75°, 70°, 60°) to accommodate 15–20° of springback.
6. Relationship Between V Die Opening and Bend Radius
Here is the truth that upgrades a beginner into a process engineer: in air bending, the V opening — not the punch tip — determines the inside bend radius. The sheet is stretched across the two die shoulders, and the radius self-forms as a percentage of the opening.
6.1 The Percentage (16%) Rule
Here is the truth that upgrades a beginner into a process engineer: in air bending, the V opening — not the punch tip — determines the inside bend radius. The sheet is stretched across the two die shoulders, and the radius self-forms as a percentage of the opening.
ir ≈ V × Material Factor
- Mild steel: ir ≈ 16% of V (0.16 × V)
- Stainless steel: ir ≈ 20–22% of V (higher springback and toughness)
- Aluminum: ir ≈ 13–15% of V
Example: Bending mild steel in a 32 mm V opening — whether the punch tip is R0.8 or R2 — gives an inside radius of about 32 × 0.16 ≈ 5.1 mm.
6.2 Punch Radius Limits
- Too large: the punch radius must not exceed the natural radius the V-die forms, or it will dominate and destabilize the bend.
- Too small: the punch radius should not drop below 63% of the material thickness, or it stops bending and starts "cutting" — leaving marks, stress concentrations, and crack risk.
So, if you need a larger radius, choose a wider V-die; for a tighter radius, choose a narrower one — and always verify against the drawing.
7. How V Die Opening Affects Press Brake Tonnage
Bending force is inversely related to V opening width: the narrower the V, the higher the tonnage — and it rises non-linearly.
7.1 Air Bending Tonnage Formula (Metric)
Tonnage = (Length(mm) × Thickness²(mm) × UTS(N/mm²) × 1.42) ÷ (V-Opening(mm) × 1000)
Required tonnage scales with:
- Ultimate Tensile Strength (UTS) — higher-strength material needs more force.
- Thickness squared (T²) — doubling thickness quadruples the force.
- Bend length (L) — doubling length doubles the force.
- Inversely with V — doubling the V opening cuts required tonnage by roughly 50%.
7.2 Two Safety Checks — Always Both
- Total force must stay well below the press brake's rated tonnage.
- Force must stay below the tooling's linear load rating (tons per meter/foot). Concentrating even a "safe" total tonnage into a narrow V can overload the die locally, causing chips, cracks, or catastrophic failure.
⚠️ Using too narrow a V-die on thick plate is a gamble: the die edges chip, and at worst the ram or bed permanently deforms, destroying machine accuracy.
8. Minimum Flange Length Based on V Die Opening
To prevent the sheet from slipping into the die during bending, the flange must rest firmly on both die shoulders. The industry rule of thumb:
Minimum Flange Length (b) ≈ 0.77 × V-Opening (a simpler shop rule uses 0.7 × V)
Example: With a V20 die → 20 × 0.7 = 14 mm. Your flange must be at least 14 mm. If the drawing calls for a 10 mm flange, a V20 will not work — you must select a smaller die (and re-check tonnage) or change the design.
| V-Opening Width (V, mm) | Minimum Required Flange Length (mm) |
|---|---|
| 8 | 6.2 |
| 12 | 9.2 |
| 16 | 12.3 |
| 24 | 18.5 |
| 32 | 24.6 |
| 40 | 30.8 |
| 50 | 38.5 |
| 63 | 48.5 |
| 80 | 61.6 |
| 100 | 77.0 |
This is a classic design-versus-manufacturing constraint: a part with a very short flange forces a narrower V-die, which increases tonnage and tightens the radius — raising crack risk. Catch it at design review, not on the shop floor.
9. How to Choose the Right V Die Opening Step by Step
Follow this disciplined workflow to turn any drawing into a safe, accurate setup — every time. If you want a deeper walkthrough of the whole decision, our guide to choosing press brake dies covers material, thickness, and bend requirements together.
Step 1 — Lock Down Your Input Parameters Confirm material grade and actual UTS (not just "stainless"), measured thickness (use a micrometer — a 0.1 mm error can shift the angle by 0.5°), required inside radius, bend angle, and minimum flange from the drawing. If UTS is missing, check the Material Test Certificate.
Step 2 — Select the Baseline V Opening Start with the Rule of 8 (V = 8t), then apply the material multiplier from Section 4 (×1.0 mild, ×1.2–1.5 stainless, ×0.8–1.0 aluminum, ×1.5–2.0+ HSS).
Step 3 — Verify Radius and Flange Cross-check the resulting inside radius (ir ≈ V × material factor) against the drawing, then confirm the part's flange ≥ 0.77 × V. If it falls short, drop to a smaller V and recalculate.
Step 4 — Calculate Tonnage and Confirm Safety Run the tonnage formula. Ensure it's below both the machine's total rating and the tooling's linear load rating.
Step 5 — Test Bend and Document Bend identical scrap, measure the actual angle and radius, dial in springback compensation in the CNC, and log every parameter. This builds your proprietary bending database — turning personal skill into a company asset.
10. Common Mistakes When Selecting V Die Opening
Mistake 1 — The "One V-Die Fits All" Trap Using one mid-sized V for every thickness and material. On thick plate a narrow V spikes tonnage and chips the die; on thin stock a wide V gives poor support, uncontrolled radius, and unstable springback. Fix: one material, one solution — 8× for mild steel, 10–12× for stainless, 5–6× for aluminum.
Mistake 2 — "Harder Tooling Is Always Better" Chasing HRC 60+ blindly. Over-hard dies are brittle and can fracture explosively under overload. Fix: match toughness to the job — 42CrMo (HRC 47–52) as the all-rounder for stainless, HSS, and thick plate; T8/T10A (HRC 58–62) for high-volume mild steel where wear resistance matters. If you're unsure which grade fits your work, our tool and die steel selection guide breaks it down.
Mistake 3 — Estimating the Inside Radius "By Feel" Ignoring that the V opening programs the radius, or grabbing a razor-sharp punch (< 63% of thickness) that pierces instead of bends. Fix: design and verify the radius with ir ≈ V × material factor.
Mistake 4 — Skipping the Flange and Tonnage Checks Forgetting the 0.77×V flange rule or the tooling's linear load limit — the two fastest routes to dropped parts and broken dies.
11.FAQ
It states the ideal V opening for 90° air bending of mild steel equals about 8 times the material thickness (V = 8 × t). It balances bending force, radius, and springback, and yields an inside radius roughly equal to the sheet thickness.
In air bending, the V opening does. The sheet floats across the die shoulders, forming an inside radius ≈ 16% of V for mild steel (20–22% stainless, 13–15% aluminum). The punch radius is secondary as long as it's smaller than the naturally formed radius and larger than 63% of the thickness.
Apply 10–12× t: 2 mm × 10 = V20 (up to V24). Stainless needs ~50% more force and a wider opening to control springback and prevent cracking.
Tonnage is inversely proportional to V. Doubling the V opening cuts required force by about 50%; halving it roughly doubles the force. Always verify against both machine and tooling load ratings.
Use b ≈ 0.77 × V (shop rule: 0.7 × V). For a V16 die, the flange must be about 12.3 mm or the part will drop into the die.
For quality work, no. Their tensile strengths and springback differ dramatically, and steel residue on a die galls aluminum. Match tooling and V opening to each material.
12. Conclusion
The right V die opening is never guesswork — it's an engineering decision that governs radius, tonnage, and flange feasibility simultaneously. Start from the Rule of 8, adjust for your material's tensile strength, verify the radius and minimum flange, confirm tonnage against both machine and tooling limits, then prove it with a test bend.
Need a V-opening that isn't standard? We manufacture custom press brake dies and punches to fit any bending requirement — contact our engineering team for a quote.
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