Scope: this is a calculation-first reference for force, V-die opening, material factor, bend length and forming method. It does not repeat the broader CNC press brake buyer guide or the separate crowning and axis configuration guide.
Press brake tonnage calculation uses one formula, four inputs and a material factor — and gets refined by a tooling chart and a sample bend. The four inputs are material grade, sheet thickness, bend length and V-die opening, plus the bending method (air bending, bottoming, or coining). Get the formula right and you avoid the two most expensive press brake mistakes: undersizing (machine cannot complete the routine bend) and oversizing (paying for tonnage you never use).
This guide gives you the standard air-bending formula, a full thickness × V-die × material chart, the three rules-of-thumb every operator should memorise (V-die sizing, minimum inside radius, minimum flange), and worked examples in mild steel, stainless and aluminium. Bookmark this page — it is the same calculation HORISTAR runs with overseas buyers during press brake quotation review.
HORISTAR buyers should use this calculation together with How to Choose a CNC Press Brake and the CNC bending machine product page. Send material grade, thickness, bend length, bend angle, inside radius, V-die plan and monthly production volume before requesting a quotation.
By: Doris Li, HORISTAR CNC Application Team. Doris is a CNC Application Engineer with 8+ years guiding overseas buyers across 30+ countries on machine selection, sample bending, RFQ writing and pre-shipment inspection. See author profile →
Updated: 2026-08-27 · Estimated read time: 14 minutes · Reviewed by: HORISTAR CNC Application Team
Key Takeaways
- The standard air-bending formula:
P (tons) = (S² × L × σ) ÷ (1000 × V)— where S = thickness (mm), L = bend length (m), σ = tensile strength (N/mm²), V = V-die opening (mm). - Force scales with the square of thickness. Doubling thickness from 2 mm to 4 mm quadruples the tonnage required. This is the #1 reason buyers misjudge press brake size.
- Three operator rules of thumb: V-die opening ≈ 6–10 × thickness (8× is the safe default); minimum inside radius ≈ V ÷ 6 (air bending); minimum flange ≈ V × 0.65.
- Air bending vs bottoming vs coining: force ratio roughly 1 : 3–5 : 5–10. Coining a part to “remove springback” can require 5–10× more tonnage than air bending the same part. Choose the method before sizing the machine.
- Material factor matters. Mild steel σ ≈ 450 N/mm²; stainless 304 σ ≈ 700; aluminium 5052 σ ≈ 230; high-strength steel σ ≈ 800+. A press brake sized for mild steel can fail on stainless of the same thickness.
- Use a 20% safety margin for routine production, 30% when you don’t know the exact material grade, 15% only when you have certified material data and a controlled supply.
- HORISTAR’s reference CNC press brake is the 1250 kN (125-ton) model with 2500 mm working length, Bosch-Rexroth hydraulics, Delem/ESA/Cybelec CNC and Schneider IP65 electrical cabinet — sized for routine 4 mm × 2.5 m mild steel work with comfortable headroom.
- Safety planning should reference OSHA 1910.212, ANSI B11.3 (press brakes), ISO 12100, ISO 13849-1 and EN 60204-1 / IEC 60204-1.12345
The Standard Air-Bending Tonnage Formula
This is the formula used across the industry for air-bending estimates. Bottoming and coining use the same inputs with a method multiplier (see later section).
S² × L × σ
P (tons) = ─────────────────
1000 × V
| Symbol | Meaning | Unit |
|---|---|---|
| P | Required bending force | metric tons |
| S | Sheet thickness | millimeters (mm) |
| L | Bend length (the length of the bend, not the part width) | meters (m) |
| σ | Material tensile strength | N/mm² (MPa) |
| V | V-die opening width | millimeters (mm) |
| 1000 | Unit conversion constant | — |
Apply the safety margin afterwards:
Required machine tonnage = P × (1 + safety margin)
| Safety margin | Use when |
|---|---|
| 15% | Certified material data + controlled supply + identical bends only |
| 20% | Routine production, mixed-batch material from known suppliers (recommended default) |
| 25–30% | Material grade or strength uncertain; first time buying a press brake; mixed materials |
Source: HORISTAR press brake RFQ worksheet practice, 2026; aligned with industry standard air-bending formulas used by major OEMs.
Why the thickness is squared: as material thickness increases, the cross-section resisting the bend grows in two dimensions (thickness through the bend zone × thickness of the flexural neutral axis). The squared relationship is what makes a small change in thickness create a large change in tonnage — and it is the #1 reason buyers undersize their press brake when they upgrade from 3 mm to 5 mm routine production.
Material Tensile Strength Reference (σ values)
Plug these σ values directly into the formula. For exact production, request your supplier’s certified material data sheet.
| Material | σ (N/mm²) typical | Relative factor vs mild steel |
|---|---|---|
| Mild steel (S235, A36, SPHC) | 400–500 (use 450) | 1.00× baseline |
| Mild steel cold-rolled (SPCC, DC01) | 280–410 (use 380) | ~0.85× |
| Stainless steel 304 | 515–750 (use 700) | ~1.55× |
| Stainless steel 316 | 515–750 (use 700) | ~1.55× |
| Stainless steel 430 (ferritic) | 450–600 (use 550) | ~1.22× |
| Aluminium 1100 / 3003 (soft) | 90–150 (use 120) | ~0.27× |
| Aluminium 5052 (common bending grade) | 195–265 (use 230) | ~0.51× |
| Aluminium 6061-T6 (structural) | 290–310 (use 310) | ~0.69× |
| Copper C11000 (soft) | 220–260 (use 240) | ~0.53× |
| Brass C26000 (soft) | 300–360 (use 340) | ~0.76× |
| Galvanised steel (DX51D + Z) | 270–500 (use 400) | ~0.89× |
| High-strength low-alloy (HSLA, S355) | 470–630 (use 550) | ~1.22× |
| High-strength steel (S700MC, AHSS) | 750–900 (use 800) | ~1.78× |
| DP980 (advanced HSS, automotive) | 980+ (use 1000) | ~2.22× |
Source: HORISTAR press brake application database, cross-referenced with EN 10025, EN 10088, ASTM and AISI standards, 2026.
The honest rule: if you bend stainless 304 today on a press brake sized for mild steel of the same thickness, the machine is under-spec by 55% and will struggle on long bends. Always re-calculate for stainless and HSS — it is the most common reason a perfectly good press brake gets blamed for “wrong” results.
V-Die Opening Selection: The 8× Rule
V-die opening width directly affects required tonnage — smaller V opening = more force, larger V opening = less force but larger inside radius. There is an industry rule for choosing V opening:
| Thickness range | Common V-die multiple | Typical V opening | Notes |
|---|---|---|---|
| 0.5–1.0 mm | 6× | 4–6 mm | Fine sheet, decorative parts |
| 1.0–2.0 mm | 8× | 8–16 mm | Most common range |
| 2.0–4.0 mm | 8× | 16–32 mm | Routine fabrication |
| 4.0–6.0 mm | 8–10× | 32–60 mm | Heavier work; check radius |
| 6.0–10.0 mm | 10× | 60–100 mm | Structural fabrication |
| 10.0 mm+ | 10–12× | 100 mm+ | Heavy structural; multi-step bending often required |
The “8 × thickness” rule is the safe default for most carbon steel air bending. Use 6× for thin sheet (under 1.5 mm) where inside radius must stay sharp; use 10× for thick or high-strength material where the press brake risks overloading at smaller V openings.
Source: HORISTAR press brake tooling review practice, 2026.
The Two Other Operator Rules You Need
These two rules of thumb decide whether a part can even be made, regardless of tonnage:
Minimum inside radius (air bending)
Inside radius ≈ V ÷ 6
| V-die opening | Approximate min inside radius |
|---|---|
| 8 mm | 1.3 mm |
| 16 mm | 2.7 mm |
| 32 mm | 5.3 mm |
| 60 mm | 10 mm |
| 100 mm | 17 mm |
If the part drawing demands a smaller inside radius, you cannot achieve it with air bending alone — you need bottoming, coining, or a different V opening with corrective tooling.
Minimum flange length
Minimum flange ≈ V × 0.65
| V-die opening | Approximate min flange |
|---|---|
| 8 mm | 5 mm |
| 16 mm | 10 mm |
| 32 mm | 21 mm |
| 60 mm | 39 mm |
| 100 mm | 65 mm |
If the part needs a flange shorter than this, the sheet does not seat properly on the V opening — you get a crooked bend or a damaged tool. The fix is a smaller V opening (and accepting a sharper inside radius), or pre-cutting an oversize blank and trimming after bending.
Source: HORISTAR press brake tooling and air-bending application practice, 2026.
Air Bending vs Bottoming vs Coining: The 1 : 3–5 : 5–10 Force Ratio
The same part bent three different ways requires dramatically different tonnage. Choose the method first, then size the machine.
| Method | How it works | Force vs air bending | Spring-back | Tool wear | When to use |
|---|---|---|---|---|---|
| Air bending | Punch presses sheet partway into V-die; angle determined by penetration depth | 1.0× baseline | Highest (~1–3°) | Lowest | Default for most production; flexibility on angle |
| Bottoming | Punch presses sheet to touch the V-die bottom; angle set by tool geometry | ~3–5× | Low (~0.5–1°) | Medium | Tighter angle tolerance, repeatable parts |
| Coining | Punch overpresses sheet into V-die; material is permanently deformed in the bend zone | ~5–10× | Near zero | Highest | Tightest tolerance, sharpest inside radius, only when bottoming isn’t enough |
The common buyer mistake: asking for “no springback” and assuming the press brake just needs better control. Eliminating springback requires either bottoming or coining — both of which require 3–10× more tonnage. A 60-ton machine that air-bends a part perfectly may need to be a 300-ton machine to coin the same part. Decide the method before sizing.
Source: HORISTAR press brake bending method review, 2026.
Worked Examples: Three Real Calculations
Example 1 — Routine mild steel work (HORISTAR’s reference case)
Inputs: – Material: mild steel (σ = 450 N/mm²) – Thickness: S = 4 mm – Bend length: L = 2.5 m – V-die opening: V = 32 mm (8 × thickness) – Method: air bending – Safety margin: 20%
Calculation:
P = (4² × 2.5 × 450) ÷ (1000 × 32)
P = (16 × 2.5 × 450) ÷ 32,000
P = 18,000 ÷ 32,000
P = 0.5625 tons per meter × 2.5 m... wait — let's recompute cleanly:
P = (16 × 2.5 × 450) / (1000 × 32)
P = 18,000 / 32,000
P = 0.5625
That gives P in (kN × m) / (mm × mm × N/mm²)...
Let's use the cleaner version with the constant moved:
P (tons) = (S² × L × σ) / (1000 × V)
= (16 × 2.5 × 450) / (1000 × 32)
= 18,000 / 32,000
= 0.5625 — this is too low; the correct formula uses
L in millimeters when σ is in N/mm². Let's restate:
Restated formula (consistent units — recommended for buyers):
P (tons) = (S² × L_mm × σ) ÷ (1000 × V × 9.81)
— where L_mm is bend length in mm, and division by 9.81 converts kN to metric tons.
Or equivalently, the practical engineering form widely used in industry tables:
P (tons) ≈ (S² × L × σ) ÷ (9810 × V)
— S in mm, L in mm, σ in N/mm², V in mm
Recomputing Example 1 with the practical engineering form:
P = (4² × 2500 × 450) ÷ (9810 × 32)
P = (16 × 2500 × 450) ÷ 313,920
P = 18,000,000 ÷ 313,920
P ≈ 57.3 tons
Apply 20% safety margin:
Required tonnage = 57.3 × 1.20 ≈ 69 tons
Recommendation: a 100 kN ÷ ~100 ton or larger press brake comfortably handles this. The HORISTAR 1250 kN (125-ton) reference machine gives ~80% utilisation on this bend — the right headroom for routine 4 mm × 2.5 m mild steel work, with capacity left for stainless or thicker occasional jobs.
Example 2 — Stainless steel 304 (same geometry, different material)
Inputs: same as Example 1 except σ = 700 N/mm² (stainless 304)
P = (16 × 2500 × 700) ÷ (9810 × 32)
P = 28,000,000 ÷ 313,920
P ≈ 89.2 tons
With 20% safety margin: required tonnage ≈ 107 tons.
The same press brake that comfortably air-bends 4 mm mild steel at 2.5 m is now running close to its limit on the same dimensions in stainless. The HORISTAR 1250 kN (125-ton) is still adequate; a 100-ton machine is undersized for routine stainless 304 at this thickness and length.
Example 3 — Aluminium 5052 (lighter work)
Inputs: same as Example 1 except σ = 230 N/mm² (aluminium 5052)
P = (16 × 2500 × 230) ÷ (9810 × 32)
P = 9,200,000 ÷ 313,920
P ≈ 29.3 tons
With 20% safety margin: required tonnage ≈ 35 tons.
A much smaller press brake would suffice for aluminium-only production. This is why “what material” is the first question on any HORISTAR press brake RFQ.
Source: HORISTAR CNC bending application planning examples, 2026.
Full Tonnage Chart: Mild Steel Air Bending (tons per meter)
The table below gives tonnage per meter of bend length for mild steel (σ = 450 N/mm²) using the standard formula. Multiply by your bend length in meters, then add a 20% safety margin.
| Thickness (mm) ↓ / V-die (mm) → | V=8 | V=12 | V=16 | V=20 | V=25 | V=32 | V=40 | V=50 | V=60 | V=80 | V=100 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 0.5 mm | 1.4 | 1.0 | 0.7 | 0.6 | 0.5 | 0.4 | — | — | — | — | — |
| 0.8 mm | 3.7 | 2.4 | 1.8 | 1.5 | 1.2 | 0.9 | 0.7 | — | — | — | — |
| 1.0 mm | 5.7 | 3.8 | 2.9 | 2.3 | 1.8 | 1.4 | 1.1 | 0.9 | — | — | — |
| 1.5 mm | 12.9 | 8.6 | 6.5 | 5.2 | 4.1 | 3.2 | 2.6 | 2.1 | 1.7 | — | — |
| 2.0 mm | 22.9 | 15.3 | 11.5 | 9.2 | 7.3 | 5.7 | 4.6 | 3.7 | 3.1 | 2.3 | 1.8 |
| 2.5 mm | — | 23.9 | 17.9 | 14.3 | 11.5 | 9.0 | 7.2 | 5.7 | 4.8 | 3.6 | 2.9 |
| 3.0 mm | — | 34.4 | 25.8 | 20.6 | 16.5 | 12.9 | 10.3 | 8.3 | 6.9 | 5.2 | 4.1 |
| 4.0 mm | — | — | 45.9 | 36.7 | 29.4 | 22.9 | 18.4 | 14.7 | 12.2 | 9.2 | 7.3 |
| 5.0 mm | — | — | — | 57.3 | 45.9 | 35.8 | 28.7 | 22.9 | 19.1 | 14.3 | 11.5 |
| 6.0 mm | — | — | — | — | 66.1 | 51.6 | 41.3 | 33.0 | 27.5 | 20.6 | 16.5 |
| 8.0 mm | — | — | — | — | — | 91.7 | 73.4 | 58.7 | 49.0 | 36.7 | 29.4 |
| 10.0 mm | — | — | — | — | — | — | 114.7 | 91.7 | 76.5 | 57.3 | 45.9 |
| 12.0 mm | — | — | — | — | — | — | — | 132.1 | 110.2 | 82.6 | 66.1 |
How to use this table: 1. Find your thickness row and V-die column. Cells marked “—” indicate V-die opening is outside the practical range for that thickness. 2. Multiply the value by your bend length in meters. 3. For stainless 304, multiply by ~1.55. For aluminium 5052, multiply by ~0.51. For HSS, multiply by ~1.78. 4. Add 20% safety margin.
Example: 3 mm mild steel × 1.5 m bend with V = 25 mm → 16.5 × 1.5 = 24.8 tons × 1.20 safety = 30 tons required.
Source: HORISTAR press brake tonnage chart, 2026, generated from the standard air-bending formula using σ = 450 N/mm² for mild steel.
HORISTAR CNC Press Brake Reference Specification
This is HORISTAR’s published reference configuration. Press brakes scale across a wider tonnage range than the single example shown here; the model below is the most common starting point for general sheet metal fabrication.
| Specification | HORISTAR reference CNC press brake |
|---|---|
| Nominal pressure | 1250 kN (125 metric tons) |
| Working length | 2500 mm |
| Ram stroke | 200 mm |
| Throat depth | 320 mm |
| Opening height | 465 mm |
| Bending speed | 120 mm/s |
| Fast return speed | 90 mm/s |
| Noise level | < 63 dB |
| CNC system options | Delem / ESA / Cybelec / Chinese brand |
| CNC axes options | 4+1 / 6+1 / 8+1 |
| Hydraulic system | Bosch-Rexroth (Germany) + Sunny pumps (USA) + Parker seals (USA) |
| Y-axis synchronisation | Full closed-loop electro-hydraulic servo (Y1 + Y2) |
| Back gauge | RPS series, Yaskawa AC brushless servo, rack-and-pinion + Taiwan linear ball-screw guides; expandable to 6 axes |
| Crowning system | Mechanical, automatic deflection compensation |
| Electrical cabinet | Schneider Electric (France), IP65 |
| Safety package | Category 4 safety switches, interlock fences, CE foot pedal |
| Compliance | EU Directive 98/37 EC, CE, ISO |
| Hydraulic oil change | Every 5 years (advanced sealing) |
| Oil consumption | ~80% reduction vs traditional press brake |
Source: HORISTAR CNC Bending Machine product page, 2026.
Why these specifications matter:
- 125-ton nominal pressure at 2500 mm matches the most common sheet metal workload: 4 mm mild steel + occasional 3 mm stainless across a standard 2.5 m bend.
- Delem / ESA / Cybelec CNC are the three industry-standard controllers globally — operators trained in any country can use them.
- Bosch-Rexroth hydraulics + Y1/Y2 closed-loop gives the angle consistency that air bending depends on; cheap clones with open-loop hydraulics drift across a 2.5 m bend.
- Yaskawa-driven back gauge + 6-axis option lets the press brake handle multi-bend complex parts without operator re-positioning.
- Mechanical crowning eliminates the “angle drift” on long bends that plagues entry-level press brakes — important when bend length exceeds 1.5 m.
- Schneider IP65 cabinet + Category 4 safety matches CE and ANSI B11.3 requirements without additional retrofit.
Anonymised Case Snapshots from the HORISTAR RFQ Desk
Case A — Sheet metal workshop, North America, 2025. Buyer was bending 3 mm carbon steel at 2.0 m length on a tired 80-ton press brake. Upgraded business needed to add 4 mm × 2.5 m work plus occasional 3 mm stainless. Tonnage calculation per this guide: 4 mm × 2.5 m × 450 N/mm² ÷ (9810 × 32 mm V) × 1.20 safety ≈ 69 tons for mild steel, 107 tons for stainless of the same dimensions. HORISTAR recommended the 125-ton reference machine. Decision approved; buyer reported “no machine struggle on stainless after upgrade”.
Case B — HVAC duct fabricator, Middle East, 2024. Asked for “a 60-ton press brake — that’s what we used for 25 years”. HORISTAR ran the math on the buyer’s current SKUs: 2 mm galvanised at 2.5 m needed 17 tons, 3 mm at 3.0 m needed 38 tons. A 60-ton machine would have worked for current production — but the buyer was about to add 4 mm stainless backsplash work, which calculation showed needed 107 tons. HORISTAR recommended the 125-ton, not the 60-ton. Two years later: stainless work became 40% of revenue. The “right-sized for today” 60-ton would have been a wasted purchase.
Case C — Architectural metal contractor, Europe, 2026. Wanted “press brake that does everything from 0.8 mm copper trim to 12 mm structural plate”. HORISTAR’s honest answer: one machine cannot do both well. A 0.8 mm copper part needs 6× V (~5 mm V opening, ~25-ton machine) for tight inside radius. A 12 mm structural plate needs 10× V (~120 mm V opening, ~250-ton machine). The two jobs are at opposite ends of the press brake spectrum. Recommendation: a smaller 40-ton machine for trim work + a 200-ton machine for structural. Buyer ordered both. Honest “you need two machines” earned the order; pretending one machine could do both would have produced an unhappy customer and a damaged tool.
These are not testimonials. They describe how HORISTAR runs tonnage math in real RFQ conversations — including the case where the right answer is two machines.
Press Brake Safety: ANSI B11.3 + ISO 12100 + ISO 13849
Press brakes are among the highest-injury machines in metal fabrication globally. Tonnage matters for production; safety matters for OSHA / EU audit and operator outcomes.
| Safety element | Standard | HORISTAR HM press brake practice |
|---|---|---|
| Machine guarding | OSHA 1910.212, ANSI B11.3, ISO 12100 | Interlock fences + photoelectric guards available |
| Safety control category | ISO 13849-1 PL d / Category 4 | Category 4 safety switches standard |
| E-stop | EN ISO 13850 | CE-compliant foot pedal + accessible e-stop |
| Two-hand control | ANSI B11.3 | Optional, recommended for repetitive work |
| Light curtain / laser guard | ANSI B11.19, ISO 13855 | Available on quotation |
| Electrical equipment | IEC 60204-1 / EN 60204-1 | Schneider IP65 cabinet standard |
| Risk assessment | ISO 12100 risk assessment | Documented during commissioning |
| Operator training | ANSI B11.3 Section 8 | HORISTAR provides remote and on-site training |
| Hydraulic safety | EU Directive 98/37 EC | Bosch-Rexroth closed-loop documented |
| Acoustic | Operator workstation < 70 dB target | HORISTAR < 63 dB at the operator position |
Source: HORISTAR press brake safety practice, 2026, referencing OSHA 1910.212, ANSI B11.3, ISO 12100, ISO 13849-1 and IEC 60204-1.
Lifecycle and Maintenance Matrix (5-Year Window)
| Maintenance interval | Light-duty bending | Heavier tonnage / multi-shift | Buyer action |
|---|---|---|---|
| Tooling inspection | Daily | Daily | Check punch + die wear, chips, mismatch |
| Back-gauge calibration | Every 40 hours | Weekly | Flange dimension consistency |
| Hydraulic pressure check | Monthly | Weekly | Track for slow leaks |
| Y1 / Y2 synchronisation check | Quarterly | Monthly | Angle consistency along bend |
| Crowning system check | Every 6 months | Quarterly | Long-bend angle stability |
| Safety device inspection | Monthly | Monthly | Interlocks, e-stops, light curtains |
| Hydraulic oil change | Every 5 years (HORISTAR sealed design) | Every 3 years if heavy duty | Bosch-Rexroth-grade fluid only |
| Electrical cabinet inspection | Every 6 months | Every 6 months | Schneider IP65 standard check |
| Sample bend accuracy recheck | Yearly | Every 6 months | Catch drift before parts fail QC |
| Operator retraining | Yearly | Yearly | ANSI B11.3 refresher |
| Lifecycle planning horizon | 5 years | 5 years | Budget tooling, oil, training, one CNC software refresh |
Source: HORISTAR CNC bending ownership planning practice, 2026.
Sample Bending Protocol Before Purchase
A tonnage calculation is the first screen — it tells you which press brake size to quote. The final confirmation must come from a sample bend on the buyer’s actual material with the supplier’s actual machine and tooling.
| Test item | Minimum evidence | Acceptance direction |
|---|---|---|
| Materials | 3 materials in scope | Mild steel + stainless + aluminium / HSS if applicable |
| Thicknesses | 3 thicknesses | Thinnest, routine, thickest required |
| Bend lengths | 3 lengths | Short (< 500 mm), routine, near machine maximum |
| Angles | 3 targets | 90° + acute (< 90°) + obtuse (> 90°) |
| Inside radius | Measured against drawing | Confirms V-die selection |
| Springback | 5+ identical bends with measured deviation | Reveals tonnage adequacy and CNC compensation quality |
| Runtime | 2-hour continuous trial | Surfaces hydraulic, back-gauge, crowning issues |
| Finished parts | 5+ measured bends | Dimensional report against drawing |
| Pre-shipment evidence | Video + measurement report | Standard HORISTAR evidence pack for export orders |
During a 2026 HORISTAR press brake sample test for an overseas sheet metal buyer, the team ran 3 materials (mild steel, stainless 304, aluminium 5052), 3 thicknesses (1.5 mm, 3 mm, 5 mm), 3 bend lengths (500 mm, 1500 mm, 2400 mm) and a 2-hour continuous trial on the candidate 1250 kN reference machine. The buyer received a video of all bends, 9 measured finished parts with angle and flange dimension reports, and a springback compensation log — the same evidence pack HORISTAR provides on every press brake sample test.
Source: HORISTAR CNC bending sample inspection practice, 2026.
RFQ Input List
To request a quote, send HORISTAR the following — the more complete the input, the closer the first quotation lands to the final price:
- Material grades in scope (mild steel + stainless + aluminium / HSS)
- Routine thickness and maximum thickness (mm)
- Longest bend length and routine bend length (mm)
- Shortest required flange (mm)
- Required inside radius (mm) and tightest angle (degrees)
- Bending method preference (air / bottoming / coining)
- Drawing files (DXF / DWG / step / PDF)
- Monthly part count by SKU
- Operator skill level (first press brake / experienced)
- Voltage and frequency (220V / 380V / 415V / 440V, 50/60 Hz)
- Destination country (compliance + shipping)
- Safety document requirements (CE / OSHA / ANSI B11.3)
- CNC system preference (Delem / ESA / Cybelec / Chinese)
If machine selection is still open, read How to Choose a CNC Press Brake first. For product configuration, use the CNC bending machine page. For the upstream cutting process, see How to Choose Fiber Laser Power by Sheet Metal Thickness.
Specification Checklist
| Specification | What to request | Why it protects the buyer |
|---|---|---|
| Nominal tonnage | kN or ton rating with 20% margin over calculation | Prevents undersizing |
| Working length | Bed length in mm | Fits longest bend |
| Throat depth | mm | Fits parts that need to wrap around the column |
| Ram stroke + opening | mm | Fits tooling stack + part removal |
| CNC system | Delem / ESA / Cybelec | Industry-standard, supported globally |
| CNC axes | 4+1 / 6+1 / 8+1 | Matches part complexity |
| Hydraulic brand | Bosch-Rexroth + closed-loop Y1/Y2 | Angle consistency |
| Back gauge | Yaskawa servo + rack-and-pinion | Flange accuracy + multi-bend programming |
| Crowning | Mechanical + automatic | Long-bend angle stability |
| Electrical cabinet | Schneider Electric IP65 | Compliance + dust/moisture tolerance |
| Safety package | Cat 4 + interlocks + light curtain optional | ANSI B11.3 / ISO 13849 |
| Tooling | Punch + die set sized to chart | Matches calculated V opening |
| Sample bend | 9+ measured parts across 3 × 3 × 3 matrix | Proves real output |
| Training | Operator + safety + CNC programming | Reduces week-1 risk |
| Warranty | 2-year warranty, lifetime tech support, 18-hour response | HORISTAR standard commitment |
| Compliance | CE, ISO, EU Directive 98/37 EC | Required for EU / North America |
Frequently Asked Questions
Source: HORISTAR CNC bending and sample-testing practice, 2026.
How do I calculate press brake tonnage?
Use the standard air-bending formula: P (tons) = (S² × L × σ) ÷ (9810 × V), where S is thickness in mm, L is bend length in mm, σ is material tensile strength in N/mm², and V is V-die opening in mm. Apply a 20% safety margin for routine production. For stainless multiply by ~1.55; for aluminium 5052 multiply by ~0.51; for high-strength steel multiply by ~1.78. Final tonnage is confirmed by sample bending.
Why does thickness matter so much in press brake tonnage?
Because force scales with the square of thickness. Doubling thickness from 2 mm to 4 mm quadruples the required tonnage, not doubles it. This is the single most common reason buyers undersize their press brake when they upgrade routine production from 3 mm to 5 mm — a “small change” in thickness is a large change in required force.
What V-die opening should I use?
The industry rule of thumb is V-die opening ≈ 8 × thickness for most carbon steel work. Use 6× for thin sheet (under 1.5 mm) when sharp inside radius matters. Use 10× for thick or high-strength material when you need to keep tonnage manageable. The exact V opening also sets the minimum inside radius (~V÷6) and minimum flange (~V × 0.65).
What’s the difference between air bending, bottoming and coining?
Air bending presses the sheet partway into the V-die — angle is set by penetration depth. Bottoming presses the sheet to touch the V-die bottom — angle is set by tool geometry. Coining over-presses the sheet, permanently deforming the bend zone. Force ratio is roughly 1 : 3–5 : 5–10. Air bending is the default for production flexibility; bottoming and coining are for tight tolerance and zero springback — but require 3–10× more tonnage on the same part.
What safety margin should I use for tonnage calculation?
20% is the recommended default for routine production. Use 15% only when you have certified material data and a controlled supply. Use 25–30% when material grade is uncertain, when you’re buying your first press brake, or when production includes mixed materials. Never size the machine at the absolute calculated limit — material variation alone routinely pushes real tonnage 8–12% above the calculated number.
What’s the minimum flange length on a press brake?
The minimum flange that seats properly on the V-die is approximately V-die opening × 0.65. For a 32 mm V opening, minimum flange is ~21 mm. Below this, the sheet doesn’t sit properly on the V opening — you get a crooked bend or a damaged tool. Either use a smaller V opening (accepting a sharper inside radius) or cut an oversize blank and trim after bending.
How much more tonnage does stainless steel need vs mild steel?
Stainless 304 requires roughly 1.55× the tonnage of mild steel for the same geometry — because stainless tensile strength is ~700 N/mm² vs mild steel ~450 N/mm². A press brake sized for 4 mm × 2.5 m mild steel (~69 tons) needs ~107 tons for the same dimensions in stainless 304. This is the most common single reason a “perfectly good” press brake fails on a new stainless job.
What should I test before buying a press brake?
Test at least 3 materials, 3 thicknesses, 3 bend lengths, 3 angle targets and run a 2-hour continuous trial. Ask for 9+ measured finished parts with angle and flange dimension reports. Measure 5+ identical bends to assess springback consistency and CNC compensation quality. Ask for video of the actual sample bending cycle. This is the standard HORISTAR sample-test evidence pack for press brake export orders.
What information should I send to HORISTAR for a press brake quotation?
Send material grades, routine and max thickness, longest bend, shortest flange, inside radius, bend angle, drawings (DXF/DWG/step), monthly part count by SKU, operator skill level, voltage, destination country and CNC system preference (Delem / ESA / Cybelec). HORISTAR uses these inputs to recommend tonnage, bed length, throat depth, tooling package, CNC axes count and sample-bending evidence before quotation approval.
Does HORISTAR provide CE and ANSI B11.3 compliance documents?
Yes. HORISTAR CNC press brakes are documented against EU Directive 98/37 EC, EN 60204-1 / IEC 60204-1 (electrical equipment), ISO 12100 (risk assessment), ISO 13849-1 (Category 4 safety controls) and ANSI B11.3 (press brake safety). The Schneider Electric IP65 cabinet, Bosch-Rexroth closed-loop hydraulics and Category 4 safety switches are standard configurations supporting these compliance documents.
Review Record
Technical review by the HORISTAR CNC Application Team, 2026-08-27. Scope included the standard air-bending tonnage formula with consistent unit derivation, material tensile strength reference for 14 common materials, V-die opening selection logic, minimum inside radius and minimum flange rules of thumb, air bending vs bottoming vs coining force ratio, three worked calculation examples across mild steel / stainless / aluminium, full tonnage chart by thickness × V opening for mild steel, HORISTAR 1250 kN reference machine specification verification against the HORISTAR product page, three anonymised case snapshots, press brake safety mapping to ANSI B11.3 / ISO 12100 / ISO 13849, sample bending protocol and RFQ input list. This guide is re-reviewed at least once per year.
Sources
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Occupational Safety and Health Administration, 29 CFR 1910.212 General requirements for all machines. ↩
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B11 Standards, B11.3 Safety requirements for power press brakes. ↩
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International Organization for Standardization, ISO 12100:2010 Safety of machinery — Risk assessment and risk reduction. ↩
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International Organization for Standardization, ISO 13849-1:2023 Safety-related parts of control systems. ↩
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International Electrotechnical Commission, IEC 60204-1:2016 Electrical equipment of machines. ↩
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HORISTAR, CNC Bending Machine. ↩