An engineering-based, practical guide to air-bending force, V-die selection, material strength, working length, tooling limits and real KRRASS press brake configurations.
Quick answer: For a preliminary press brake tonnage chart, start with the material thickness, actual bend-line length, V-die opening and ultimate tensile strength. Under KRRASS’s published metric air-bending convention, the estimated force is P (kN) = 650 × S² × L / V × (Rm / 450), where thickness S and die opening V are millimeters, L is meters, and Rm is tensile strength in MPa. Divide the result by 9.80665 for metric ton-force. Use the KRRASS press brake tonnage calculator to check a particular job, then verify machine capacity, tool load ratings, forming method, flange clearance and the manufacturer’s operating instructions before production.
This article expands the existing KRRASS explanation rather than discarding it. Protecting the machine, preserving punch-and-die life, controlling the V-opening and avoiding inaccurate bending remain the reasons to calculate force. The new emphasis is on what a tonnage chart assumes, where an estimate can fail, and how to connect a calculated load to an actual machine specification. That distinction matters to operators, production engineers and buyers who want more than an attractive tonnage number.
Engineering and safety note: All calculations and recommendations here are planning estimates, not a certified safe-working-load determination. The published calculator uses a simplified air-bending model. Actual loads vary with material batch, yield-to-tensile behavior, punch and die geometry, contact friction, forming angle, tool position and machine settings. A nominal press brake rating never overrides a lower rating for a tool segment, holder, clamp, machine position or specific job. Follow the machine and tooling manufacturer’s instructions and local safety rules.
Press Brake Tonnage Chart at a Glance
A press brake tonnage chart gives an approximate forming force for a defined set of conditions. It is not a universal thickness-to-capacity lookup. The most useful chart tells you whether its numbers are in kilonewtons, metric tons, or US short tons; whether force is shown per meter or for a complete bend; what material and tensile strength it assumes; and what lower V-die opening is used. Without these details, two respectable charts can produce different answers that appear contradictory but are simply based on different inputs.
For a typical 90-degree air-bending estimate, KRRASS uses 450 MPa mild steel as the reference material and an 8 × thickness V-opening as a convenient default. The online tool also offers 6×S, 8×S and 10×S presets, a custom opening, alternative materials and process choices. The preset is a beginning, not a promise of compatibility: the exact die on the machine must still be checked. As the KRRASS live calculator makes clear, initial capacity planning should normally include approximately 20–25% headroom, followed by a separate tool and machine review.
Fast reference: 450 MPa mild steel, air bending, V = 8 × thickness
| Thickness S (mm) | V opening (mm) | Air-bend force (kN/m) | Metric ton-force (t/m) | Estimated inside radius (mm) | 90° minimum flange estimate (mm) |
|---|---|---|---|---|---|
| 1.0 | 8 | 81.3 | 8.3 | 1.3 | 6.2 |
| 1.5 | 12 | 121.9 | 12.4 | 1.9 | 9.4 |
| 2.0 | 16 | 162.5 | 16.6 | 2.6 | 12.5 |
| 3.0 | 24 | 243.8 | 24.9 | 3.8 | 18.7 |
| 4.0 | 32 | 325.0 | 33.1 | 5.1 | 25.0 |
| 5.0 | 40 | 406.3 | 41.4 | 6.4 | 31.2 |
| 6.0 | 48 | 487.5 | 49.7 | 7.7 | 37.4 |
| 8.0 | 64 | 650.0 | 66.3 | 10.2 | 49.9 |
| 10.0 | 80 | 812.5 | 82.9 | 12.8 | 62.4 |
| 12.0 | 96 | 975.0 | 99.4 | 15.4 | 74.9 |
| 16.0 | 128 | 1,300.0 | 132.6 | 20.5 | 99.8 |
| 20.0 | 160 | 1,625.0 | 165.7 | 25.6 | 124.8 |
Calculation assumptions: 450 MPa representative ultimate tensile strength; one-meter bend; air bending; V = 8S throughout; P = 650S²L/V. Radius = approximately 0.16V; preliminary 90-degree flange = approximately 0.78V. Rounding is to the nearest displayed decimal. Wider openings may be required for thick plate, high-strength materials or specific die geometries. These are calculated examples, not independent load-test results.
A 3 mm sheet on a 24 mm die therefore needs about 24.9 metric tons per meter for this reference case, before a sizing allowance. If the active bend length is 3.2 m, estimated total force rises to approximately 79.5 metric tons (780 kN); at 25% added design headroom, the preliminary capacity target becomes 975 kN. This one example shows why the unit t/m must never be confused with total press capacity in tonnes.
What a Press Brake Tonnage Chart Really Measures
“Press brake tonnage” is the shop-floor name for the forming force exerted by the ram and punch through the workpiece into the lower die. It is not a measure of finished part quality, an automatic guarantee of bending thickness or a prediction of electricity consumption. A 200-ton nominal press brake can be inappropriate for a bend requiring much less than 200 tons if the lower die is too small, the top tool is weak, the part does not fit between the columns, or the force is concentrated over a short length.
There are three related quantities that purchasing teams and operators often mix up. Bending force is the modeled demand for one specific bend. Nominal machine force is the press brake’s advertised overall force capacity under the manufacturer’s conditions. Allowable tooling load is the permitted loading of a specified tool, holder and clamping system, commonly expressed as kN/m. A safe setup has to satisfy all three constraints along with applicable machine geometry and point-loading restrictions.
In general, thickness affects force through a square term; bend length affects force linearly; tensile strength is treated as a linear multiplier in KRRASS’s simplified model; and increasing the V-opening reduces force inversely. The model is therefore sensitive to errors in thickness and die selection. A nominal thickness stamped on a purchase order might differ from actual stock thickness. Likewise, a die marked as an 8V may refer to a specific nominal opening, not a dimension that can be guessed from photographs or body width.
The chart can also support estimating a production envelope. A fabricator can calculate representative components from its order book, group them by material, bend length and tooling requirements, and identify whether a 1100 kN, 1750 kN, 2100 kN or 2500 kN machine is a reasonable shortlist. But that is a selection exercise, not permission to bend every combination of thickness and length that appears in a chart.
The KRRASS Bending Force Formula — With Units That Actually Match
The original article used a convenient approximate coefficient of 65 to express force in metric tons. This revision uses the same kN-based reference formula displayed on the current KRRASS press brake tonnage calculator and in the KRRASS press-brake product selector:
P (kN) = [650 × S² × L / V] × [Rm / 450]
Here, P is the estimated total forming force in kilonewtons; S is sheet thickness in millimeters; L is the total active length of the bend in meters; V is the lower die’s actual V-opening in millimeters; and Rm is the material’s representative ultimate tensile strength in MPa (numerically the same as N/mm²). The baseline is 450 MPa mild steel. Writing every unit beside its symbol makes the expression usable outside a single calculator interface.
If a drawing gives the bend length in millimeters, first calculate L (m) = L (mm) / 1,000. Do not enter 3,200 when the formula expects 3.2, or the answer becomes a thousand times too large. For total force in metric ton-force, use t = P (kN) / 9.80665. For US short tons-force, use US tons = P (kN) / 8.89644. Metric ton-force and US short tons-force are not interchangeable. A nameplate may identify force in kN even when the machine model code contains a familiar “T” number.
Why the old 65 coefficient is not the same as the new 650 coefficient
With L measured in meters, 65 × S² × L / V is an older rounded method for expressing force in metric tons-force, whereas 650 × S² × L / V gives kN under KRRASS’s currently stated baseline. Because 650 / 9.80665 = 66.28, the approximate 65-ton-force expression is within about two percent of the converted 650-kN expression for the same inputs. The difference is rounding and unit convention, not a mysterious change in sheet-metal physics. Keeping the published kN calculator as the single reference avoids accidental double conversion.
Another common source of confusion is a different manufacturer’s formula with a different reference tensile strength or empirical bending coefficient. A published chart may assume 420 MPa carbon steel, a different contact model, a particular thickness range, or a specific air-bend angle. Independent tooling and steel producers emphasize that force is influenced by actual die geometry and material behavior. For example, SSAB’s engineering guide to press-brake force discusses thickness, bend length, tooling, tensile strength, friction and changing moment arm. Compare assumptions before comparing numbers.
Calculation conditions: when the published formula is appropriate
The model is a practical air-bending screening estimate, usually associated with a conventional V-die and a normal bend setup. In air bending, the sheet is supported on the die shoulders while the punch advances to create the required angle without fully pressing the material into the entire die cavity. The inside radius is strongly influenced by the V-opening, material and tool geometry. It is important to distinguish this from bottoming or coining, where contact and force behavior are substantially different.
The estimate should be reviewed whenever the tooling is unusually narrow, the material is hardened, the bend is near an edge, the punch has a large or sharp nose, the part is offset from the machine center, or the operation has special friction or restrike requirements. These are not edge cases for a technical team; they are the ordinary reasons a shop-floor tonnage estimate needs a documented verification step.
Reading the KRRASS Calculator Interface Correctly
The KRRASS bending force calculator is the practical counterpart to this guide. It accepts metric and inch dimensions, material selection, sheet thickness, bending length and a die-opening choice. It lets the user review an air-bend estimate and choose different forming methods. It also provides force per meter and estimates for air-bend radius and minimum flange length. Its user interface exposes 6S, 8S, 10S and custom die choices, which are useful for comparing reasonable tooling scenarios.
For accurate use, start with the actual production inputs, not the nearest attractive chart row. Select a material whose tensile strength is representative of the batch. Enter measured or approved stock thickness and the full bend-line length active under the punch. Then enter the opening stamped on the selected lower die, rather than simply accepting the default. If the required drawing radius, flange dimension or part geometry cannot be achieved on that die, selecting another opening is a genuine engineering change, not merely a calculator setting.
After calculating, read the kN value first. Convert to ton-force only if a procurement form or legacy operating instruction specifically uses those units. Read kN/m or t/m as a distributed load, then compare it with tooling and holder ratings. Record the estimated inside radius and short-flange result as preliminary geometry checks, not guaranteed finished dimensions. Finally, decide whether the selected machine’s rated working length, clearances and allowable load distribution can support the operation.
The tool’s automatic result cannot detect every physical interference, nonstandard segment, worn clamping surface or material defect. This is why an effective calculator workflow concludes with a setup review. Use the calculator for the arithmetic and the engineering process for the approval. When a job changes from air bending to bottoming or coining, do not silently reuse the air-bending tonnage chart.
What each input changes
| Input | Where it is obtained | Effect in the KRRASS air-bend model | Verification before release |
|---|---|---|---|
| Sheet thickness S | Drawing plus stock measurement | Force scales with S² at a fixed V | Material tolerance, coating, batch thickness |
| Bend length L | Developed part drawing / bend program | Force increases linearly with active length | Actual punch contact length and machine width |
| V-die opening V | Marking, tool catalog or measurement | Force scales inversely with V | Tool capacity, radius, flange clearance |
| Material strength Rm | Material specification / mill certificate | Force multiplied by Rm/450 | Grade, temper, heat and lot identity |
| Forming method | Process plan | Air-bend model or higher-force process estimate | Bottoming/coining verification and die design |
| Final angle / geometry | Engineering drawing and tryout | Not fully captured by single coefficient | Springback, crowning and finished radius |
| Machine and tooling limits | Model documentation and tooling markings | Not part of calculated force equation | Rated force, tool holder, clamp, local load |
How V-Die Opening Changes Tonnage, Radius and Flange Length
The V-opening is the most useful adjustable parameter in many bending setups, but it is also the easiest place to create a misleading “saving.” Because force is inversely related to V in the simplified model, choosing a 60 mm die instead of a 40 mm die for the same plate and bend length lowers estimated force by one-third. That does not mean the larger die is automatically a better die. It may produce a larger inside radius, lose support for a short leg, create a different visible finish or fail a drawing requirement.
A simple starting range for common air bending is 6–10 times material thickness, with wider openings often useful for heavier or harder plate. KRRASS’s live calculator provides 6×, 8× and 10× presets, while the appropriate production choice depends on actual material and tool requirements. For heavy plate, a toolmaker or steel producer may recommend 10–12× or a different opening. Follow the material and tooling recommendations rather than enforcing one universal ratio.
For the calculator’s preliminary 90-degree geometry estimates, use inside radius ≈ 0.16 × V for its mild-steel example and minimum flange ≈ 0.78 × V. Those quantities are linked to the die. A larger opening reduces force but simultaneously increases the approximate inside radius and the minimum supported flange. For stainless and particular aluminum alloys, the calculator provides material-specific radius assumptions that differ from the mild-steel 0.16V starting point; do not transfer one radius factor blindly across alloys.
Die-opening sensitivity: 6 mm, 450 MPa mild steel, one-meter bend
| Actual opening | V/S ratio | Force (kN/m) | Metric t/m | Approx. Ri using 0.16V | Approx. flange using 0.78V |
|---|---|---|---|---|---|
| 36 mm | 6× | 650.0 | 66.3 | 5.8 mm | 28.1 mm |
| 48 mm | 8× | 487.5 | 49.7 | 7.7 mm | 37.4 mm |
| 60 mm | 10× | 390.0 | 39.8 | 9.6 mm | 46.8 mm |
| 72 mm | 12× | 325.0 | 33.1 | 11.5 mm | 56.2 mm |
Compared with a 36 mm opening, a 72 mm opening appears to halve the required load. But its approximate supported flange changes from 28.1 to 56.2 mm, while the natural radius roughly doubles. A part with a 35 mm flange therefore cannot be approved merely because the wider die makes the tonnage number comfortable. Check minimum edge support, tooling geometry, the drawing’s actual dimensioning convention and the machine’s collision envelope.
Material Strength: Why Stainless Steel, Aluminum and High-Strength Steel Differ
Tonnage charts typically begin with a reference low-carbon steel. That is convenient but incomplete because the metal’s actual strength matters. The KRRASS calculator applies a simple Rm/450 correction to the baseline load. A 650 MPa material, under otherwise identical conditions, produces 650/450 = 1.444 times the calculated load of 450 MPa reference steel. A 240 MPa aluminum sheet gives 240/450 = 0.533 times the reference load. Both values are model factors, not a guarantee that every commercial coil or plate of that named family has exactly that strength.
The calculator’s material guide includes example tensile strengths for common alloys and structural grades. Actual values change with standard, temper, rolling condition, thickness and lot. An annealed sheet and a heat-treated sheet can react very differently; some high-strength materials impose bend-radius and cracking limits long before the press is short of nominal force. For procurement, ask for grade, thickness, temper and a mill test certificate when the application is critical. For highly strengthened steels, SSAB’s bendability recommendations emphasize bending direction, punch radius and grade-specific limits rather than a single universal V-rule.
Model factors used for example force comparisons
| Representative material | Assumed tensile strength Rm | Model multiplier Rm/450 | 4 mm × 1 m, V32: estimated force |
|---|---|---|---|
| Aluminum 5052-H32 example | 240 MPa | 0.53× | 173.3 kN / 17.7 t |
| General aluminum sheet example | 280 MPa | 0.62× | 202.2 kN / 20.6 t |
| Lower-strength carbon steel example | 400 MPa | 0.89× | 288.9 kN / 29.5 t |
| Reference mild steel | 450 MPa | 1.00× | 325.0 kN / 33.1 t |
| Structural steel example | 550 MPa | 1.22× | 397.2 kN / 40.5 t |
| Austenitic stainless example | 650 MPa | 1.44× | 469.4 kN / 47.9 t |
| High-strength steel example | 900 MPa | 2.00× | 650.0 kN / 66.3 t |
These illustrative tensile strengths follow the representative inputs published in the KRRASS calculator. The values do not certify a specific ASTM, EN, JIS or GB grade and are not material test data. The example holds thickness, bend length and V-opening constant to isolate the tensile-strength term.
For stainless steel enclosures, the correct selection may also be constrained by surface appearance and marking. Narrower openings may help achieve tighter geometric targets, but they raise the force. For aluminum parts, the tonnage might be low even when the allowable inside bend radius is relatively large due to cracking sensitivity. For wear-resistant plate, do not accept a simplistic “two times steel tonnage” multiplier as sufficient evidence of feasibility. Request the producer’s bending data, observe grain direction and confirm a suitable punch-nose radius.
Worked Example: Reproduce the Calculator’s 6 mm × 4,000 mm Result
The live KRRASS calculator publishes a useful reference case: 6 mm thick mild steel, 4,000 mm active bend length, V48, 450 MPa tensile strength. Use the same values here to make the editorial guidance traceable to the website’s interactive tool. This is an especially helpful quality-control check because the formula, the table and the example should all agree to ordinary rounding.
First, convert the drawing length: 4,000 mm = 4.0 m. Then use S² = 6² = 36. The base force is 650 × 36 × 4.0 / 48 = 1,950 kN. Because Rm = 450 MPa, the material factor is exactly one. Divide by 9.80665 and the result is 198.8 metric ton-force. Divide by 8.89644 and the result is approximately 219.2 US short tons-force. The distributed load is 1,950 / 4 = 487.5 kN/m or 49.7 t/m.
For the calculator’s mild-steel geometry estimates, the inside radius is about 0.16 × 48 = 7.7 mm, and the preliminary minimum flange is 0.78 × 48 = 37.4 mm. A 25% initial capacity allowance takes 1,950 kN to 2,437.5 kN, which is approximately 248.6 metric ton-force. On paper, that creates a strong case for examining a 2,500 kN machine with at least a 4,000 mm working length, but it is not the final machine or tooling approval.
Check the punch and die’s rated distributed load, the holder and clamping system, the machine’s load curve and the ability to support a 4,000 mm part. The press may have crowning limitations, backgauge clearance or an opening-height requirement that the tonnage formula does not see. If the drawing requires a 25 mm flange or an inside radius smaller than the predicted value, the tooling choice must be reconsidered. This is the precise difference between an estimated bending force and a verified production setup.
To verify the arithmetic on the live page, open the KRRASS tonnage calculator, choose 450 MPa carbon steel, enter thickness 6 mm, bend length 4,000 mm, V-opening 48 mm and air bending. A printed or shared result can become part of a setup sheet, but it should be paired with tool identification and approval evidence.
Press Brake Tonnage Chart by Material and Bend Length
A chart is most useful when shop-floor staff can connect force per meter to the actual workpiece length. The following comparison uses the published KRRASS coefficient, a V-opening of 8S, and representative tensile strengths of 450 MPa for mild steel, 650 MPa for stainless and 240 MPa for 5052-H32 aluminum. All values are metric ton-force for the complete bend, not machine model numbers. They are rounded screening estimates; verify your actual material and die before making the first piece.
| Thickness | V opening | Mild steel t/m | Mild steel total 3.2 m | Stainless t/m | Stainless total 3.2 m | 5052 aluminum t/m |
|---|---|---|---|---|---|---|
| 1 mm | 8 mm | 8.3 | 26.5 | 12.0 | 38.3 | 4.4 |
| 2 mm | 16 mm | 16.6 | 53.0 | 23.9 | 76.6 | 8.8 |
| 3 mm | 24 mm | 24.9 | 79.5 | 35.9 | 114.9 | 13.3 |
| 4 mm | 32 mm | 33.1 | 106.0 | 47.9 | 153.2 | 17.7 |
| 5 mm | 40 mm | 41.4 | 132.6 | 59.8 | 191.5 | 22.1 |
| 6 mm | 48 mm | 49.7 | 159.1 | 71.8 | 229.8 | 26.5 |
| 8 mm | 64 mm | 66.3 | 212.1 | 95.7 | 306.4 | 35.4 |
| 10 mm | 80 mm | 82.9 | 265.1 | 119.7 | 383.0 | 44.2 |
| 12 mm | 96 mm | 99.4 | 318.1 | 143.6 | 459.6 | 53.0 |
Data basis: KRRASS 650 coefficient and Rm/450 material multiplier; figures rounded to one decimal. The 3.2 m column is calculated by multiplying the underlying unrounded force per meter by 3.2. The chart is intentionally focused on formula consistency, not on predicting the final angle or approving short flanges.
The linear scaling principle is easy to apply: if the chosen sheet and die need 33.1 t/m, a 2.0 m bend needs about 66.3 t before margin, while a 3.2 m bend needs about 106.0 t. This assumes uniform contact and an appropriate centered setup. Do not scale a short tool segment’s rating as if it were the same as the whole press brake’s nominal tonnage. A small bend can create a severe point load even when the total force looks comfortable compared with the machine nameplate.
How to interpolate a chart row without guessing
Use the actual numerical formula whenever thickness or V-opening sits between listed rows. For instance, a 3.5 mm sheet on a 28 mm die is not necessarily represented by averaging 3 and 4 mm rows, because thickness is squared. A direct calculation gives P = 650 × 3.5² × L / 28 × (Rm/450). For 450 MPa steel and a one-meter bend, that is approximately 284.4 kN/m. If the chosen real die is 30 mm rather than 28 mm, the result drops to about 265.4 kN/m. Recording the exact V-opening is more reliable than inventing a result by visual interpolation.
For gauge-based drawings, convert the specified gauge to the correct actual thickness before computing. Gauge numbering is not a unique global thickness standard, and stainless, aluminum and uncoated steel schedules may differ. Many international procurement teams now prefer dimensions in millimeters for precisely this reason. If stock tolerance could materially change the calculation, evaluate the high side of the thickness range and document the assumption.
Air Bending, Bottoming and Coining: Never Mix the Force Charts
Three familiar bending methods can share a press brake but demand different forces. Air bending uses the punch penetration and the workpiece’s contact with the lower die shoulders to create an angle. In practical CNC work, the bend angle can be adjusted through ram position, subject to material springback and process limitations. The nominal tonnage chart in this article is based on this method. It should not be applied unchanged when the punch is driven into full die contact.
Bottoming drives the material substantially into a die geometry, which can reduce some of the variability associated with air bending but typically increases force and makes punch/die geometry more restrictive. Coining uses much higher localized pressure to plastically form material against the tooling profile. A machine that can air-bend a 4 mm sheet on a given V-die is not automatically capable of coining the same part. Tooling rated for air bending may also be inappropriate at the higher process force.
The KRRASS calculator currently lists 3–5 times the air-bend force as an initial bottoming range and 5–10 times as a preliminary coining range. These multipliers are planning flags, not certified process calculations. Punch radius, die angle, material properties, surface condition and the amount of overtravel can produce different behavior. For heavy-pressure operations, determine the load with the tooling manufacturer and trial evidence rather than using the lowest multiplier to justify an underpowered setup.
| Process | What generally controls the result | Appropriate chart use | Typical engineering concern |
|---|---|---|---|
| Air bending | Punch penetration, V-opening, material behavior | Main KRRASS 650-coefficient chart | Springback, radius, short flanges, material variation |
| Bottoming | Close tool contact and angle geometry | Estimate only after separate process factor review | High tonnage, precise die match, excessive tool loading |
| Coining | Localized plastic deformation under pressure | No direct substitution from air-bending values | Very high contact stress, tool failure and machine suitability |
It is better to write “bottoming feasibility subject to engineering review” than to publish a false claim that a particular tonnage always works. In an RFQ, state the forming method, not just “bend to 90 degrees.” A 90-degree drawing describes the finished part, not the process used to make it. Process clarity prevents a manufacturer from quoting a machine on the wrong force basis.
The Tooling Load Is Often the Real Limitation
Press-brake tooling has a rated load, sometimes given as kN per meter or tons per foot. This matters because a machine’s total force is distributed through a specific active tool length. A 100 mm punch segment does not acquire the full safe capacity of a 3,200 mm machine merely because the operator has installed it in the center. The punch, lower die, intermediate holder and clamping mechanism can each impose a different permissible limit, and local machine-loading rules still apply.
WILA’s published explanation of tool load capacity gives a clear example: a 100 mm tool segment rated at 1,000 kN/m can carry 100 kN of evenly distributed load, not 1,000 kN. WILA also warns that the toolholder or press brake itself may be the limiting element. These principles are more useful to engineers than a blanket statement that any segment can bend a certain thickness.
Suppose the chart predicts 487.5 kN/m for 6 mm mild steel over V48. A matched punch, die and clamping system would each need an adequate rating at the intended active length and location. If the die is rated 450 kN/m, it is insufficient for the calculated 487.5 kN/m loading even if the press brake has 2,500 kN of nominal force. If a 100 mm segment is rated 1,000 kN/m, its uniform-load rating is 100 kN over that segment. Its real permissible use still depends on tooling shape and the manufacturer’s instructions.
The operator should also identify which part of a bending load is being checked. A 2,000 mm long workpiece might rest on multiple die segments, but the distribution depends on actual contact. Gaps, unequal heights, wear, offsets and unusual part geometry can cause loading to concentrate unexpectedly. Do not presume that simply summing catalog ratings of several segments proves a valid setup. The complete assembly needs consistent support and an approved load path.
A four-level capacity check
Check, in order: (1) the modeled bend demand; (2) the punch, die and toolholder capacity; (3) the press brake’s overall and local allowable loading; and (4) clearances and part geometry. The setup must satisfy every level. If one fails, choosing a larger motor or increasing hydraulic pressure is not a legitimate workaround. Select different tooling or a different machine, revise the bending sequence, or seek engineering approval for a changed process.
Actual KRRASS Machine Configurations: Force, Length and Clearances
A trustworthy buying guide should connect the calculation to published, identifiable machine specifications. The table below uses real model configurations displayed in KRRASS’s AdvancedBend, StandardBend and EssentialBend series information. Values are published nominal machine specifications, not measured bending trials or an assertion that the full listed force is available at every position and setup. Product configuration and selected options should be reconfirmed when requesting a quotation.
| Published KRRASS model example | Nominal bending force | Rated bending length | Column distance | Series / application note |
|---|---|---|---|---|
| PBS 110T/3200 | 1,100 kN | 3,200 mm | 2,700 mm | AdvancedBend/PBS servo-driven hydraulic configuration |
| PBS 175T/3200 | 1,750 kN | 3,200 mm | 2,700 mm | AdvancedBend/PBS medium-heavy CNC configuration |
| PBS 210T/3200 | 2,100 kN | 3,200 mm | 2,700 mm | AdvancedBend/PBS higher-force 3.2 m configuration |
| PBS 250T/4000 | 2,500 kN | 4,000 mm | 3,500 mm | AdvancedBend/PBS longer working-length option |
| PBS 320T/3200 | 3,200 kN | 3,200 mm | 2,700 mm | AdvancedBend/PBS high-force 3.2 m option |
| MB8 175T/3200 | 1,750 kN | 3,200 mm | 2,700 mm | StandardBend/MB8 conventional motor hydraulic option |
| MB8 250T/4000 | 2,500 kN | 4,000 mm | 3,500 mm | StandardBend/MB8 longer working-length option |
| WC67K 160T/3200 | 1,600 kN | 3,200 mm | 2,580 mm | EssentialBend/NC-oriented production alternative |
The difference between rated bending length and column distance deserves attention. A 3,200 mm press can have approximately 2,700 mm between frame uprights. This does not mean that every 3,200 mm part is impossible, but it does mean a long part, formed return flange or unusual rotation can collide with the machine or be limited by frame geometry. Operators need a drawing-based collision check, especially for boxes, large panels and multi-bend components.
Likewise, a 2,500 kN model is not necessarily rated to apply 2,500 kN through any extremely short tool. The load-distribution chart, tooling capacity and selected position must be consulted. The customer’s part range also needs adequate throat depth, daylight, stroke, backgauge travel and support capacity. These machine characteristics cannot be inferred from tonnage alone.
The real-product comparison is relevant to procurement because it makes the difference between a marketing label and a data sheet tangible. For example, KRRASS publishes PBS 250T/4000 with 2,500 kN nominal force, 4,000 mm bending length and 3,500 mm column distance, while PBS 210T/3200 publishes 2,100 kN and 3,200 mm length. A requirement for 4,000 mm continuous bending cannot be satisfied merely by choosing the shorter machine with a sufficiently high force rating. Force and geometry are two separate selection axes.
Which KRRASS series fits which type of fabrication work?
The KRRASS press brake range groups machines into families built for different operating and investment requirements. EssentialBend addresses straightforward forming work where a simpler control approach can be appropriate. StandardBend addresses a broad range of repeatable CNC bending tasks, while AdvancedBend offers servo-related options and CNC configurations for demanding production flows. E-Bend is a separate electric bending family that can be considered for appropriately sized parts; it is not a substitute for heavy-plate tonnage simply because electric actuation can be fast or efficient.
Control axes, automation options and drive technologies can improve workflow, setup efficiency or repeatability. They do not change the fundamental forming force required by the material and tool geometry. Comparing two machines by controller branding alone is a poor method of sizing their capacity. A buyer should request a quotation that specifies the exact force/length model, controller, backgauge, crowning, upper and lower tools, safety equipment and applicable rated-load documentation.
Four Application Cases: Turn the Chart into an Equipment Shortlist
The following cases are calculated decision examples, not factory test reports. Their purpose is to demonstrate a repeatable selection method with identifiable KRRASS nominal configurations. Each example assumes mild steel at 450 MPa, air bending, V = 8S and a 25% initial machine-sizing allowance. The model choice is a preliminary candidate only and requires tooling, load distribution and fit validation. No output-quality or production-capacity guarantee is implied.
| Case | Job details | Calculated force | With 25% planning allowance | Example series candidate | What must still be checked |
|---|---|---|---|---|---|
| A | 2 mm × 3.2 m, V16 | 520 kN | 650 kN | PBS 110T/3200 (1,100 kN) | 3.2 m tool fit, part support and short return flanges |
| B | 3 mm × 3.2 m, V24 | 780 kN | 975 kN | PBS 110T/3200 (1,100 kN) | tool rating, position and sufficient practical headroom |
| C | 4 mm × 3.2 m, V32 | 1,040 kN | 1,300 kN | PBS/MB8 175T/3200 (1,750 kN) | 1100 kN machine would not meet this chosen margin |
| D | 6 mm × 3.2 m, V48 | 1,560 kN | 1,950 kN | PBS 210T/3200 (2,100 kN) | 2,100 kN capacity is a planning shortlist, not tool approval |
| E | 6 mm × 4.0 m, V48 | 1,950 kN | 2,437.5 kN | PBS/MB8 250T/4000 (2,500 kN) | 4 m access, long-tool load and tight margin above target |
| F | 8 mm × 3.2 m, V64 | 2,080 kN | 2,600 kN | PBS 320T/3200 (3,200 kN) | die geometry and plate-handling requirements |
Case B is a useful warning against choosing a nominal machine with no reserve. A 3 mm × 3.2 m air bend on V24 needs approximately 780 kN in the simplified model. The 1,100 kN candidate provides an adequate nominal comparison against the stated 975 kN preliminary sizing target. But if actual material is much stronger than 450 MPa, or the customer changes the die to V18, the calculated demand rises and the shortlist must be rerun.
Case C demonstrates an even more important distinction. A 4 mm × 3.2 m part requires approximately 1,040 kN, so a 1,100 kN nameplate might appear large enough when viewed without headroom. Adding the stated 25% allowance gives 1,300 kN, however, which exceeds the 1,100 kN nominal reference. Therefore, a 1,750 kN machine is the more credible initial candidate for this conservative sizing method. The specific required margin is an engineering judgment, not a universal legal threshold, and exact machine performance must be verified.
Case E is the same 6 mm × 4 m job used by the calculator. The 2,500 kN machine only exceeds the preliminary 2,437.5 kN target by 62.5 kN. This may satisfy that numerical screen, but the relatively narrow gap means every other limit deserves scrutiny. If the job requires high-strength steel, a smaller V-die or a non-air-bending method, that shortlist may change substantially. A quote that says “250 ton, 4 meters” without a die and load verification is not sufficient evidence for this application.
Machine Selection Is an Envelope, Not a Single Number
A credible press brake selection begins with a family of parts. Record the smallest and largest thicknesses, materials, bend lengths, leg dimensions, bend angles, desired radius, weight, annual quantity and expected future work. A shop that bends light enclosures every day and one very long heavy part each quarter may require a different investment approach from a manufacturer that produces long chassis sections continuously. The average job matters for productivity, while the worst approved job matters for capacity.
Choose the tooling strategy before comparing machine nameplates. A nominal 200-ton press with only a narrow V-die may be worse suited to heavy plate than a lower-tonnage machine equipped with the appropriate large-opening tooling, subject to geometry and tool rating. The die determines leverage and often controls the natural inside radius. In the same way, an exceptionally short flange can require a smaller die opening, which raises the force and can alter the machine shortlist.
The machine must physically accommodate the workpiece and its movement during bending. Verify working length, column distance, throat depth, daylight, ram stroke, tool height, backgauge approach, front supports and part rotation. If the workpiece has a deep box flange, height and clearance are often more decisive than force. Where repeatability over the entire bend is essential, confirm crowning and load distribution as part of the acceptance plan, rather than treating them as accessories unrelated to tonnage.
A sensible purchase review compares at least two viable configurations with the actual tooling quoted. Request the nominal force in kN, the working-length drawing, available tool interfaces, holder load ratings, control-axis configuration and operator safeguards. For complex production, include a sample part drawing and planned processing steps. KRRASS’s press-brake tooling selection guide provides complementary information on punches, dies and compatibility without duplicating the force chart here.
The 20–25% Margin: What It Means and What It Does Not
The KRRASS calculator suggests an initial 20–25% capacity margin before final machine sizing. This allows a buyer to screen alternatives without assuming that a theoretical force estimate equals the desired nameplate limit. A margin can absorb some normal uncertainty in model assumptions and selection, but it is not a substitute for a manufacturer-specified safety factor, a tool-specific proof load or an approved operation. In technical quotations, label it clearly as “planning allowance” or “sizing headroom.”
The correct sequence is to calculate the nominal air-bending demand, multiply by the chosen preliminary factor, and compare the result in the same units as the machine’s documented capacity. Then recheck the tool and machine’s allowable loading independent of the allowance. If any necessary condition is missing, flag the job for engineering review instead of treating the extra percentage as blanket protection against poor tooling or unusual forces.
An illustrative mistake is to add 25% to the machine nameplate rather than to the required job force. If the calculated load is 1,200 kN, a 25% planning allowance produces a target of 1,500 kN. It does not mean that a 1,200 kN press may legally or safely be run at 1,500 kN. Similarly, a 2,000 kN press does not make a 900 kN/m lower die safe at 1,200 kN/m. Machine capacity and local tool load remain distinct constraints.
The optimum margin can differ across applications and operating environments. A high-volume line with controlled certified material and a well-characterized tooling system might justify one selection rationale; a job shop regularly receiving unknown plate batches might adopt a different one. Make the reasoning explicit and have the responsible engineer approve the final setup. A percentage printed in a guide should never replace actual equipment documentation.
More Accurate Setup Records: A Practical Example
A tonnage calculation becomes useful operating evidence when its assumptions can be traced later. The following example describes a recommended setup sheet, not a claim that KRRASS conducted a documented physical test on this part. It can be adapted into an ERP route, traveler or CNC-program note. Actual forces, measured angles and acceptance records must be entered by the production team rather than invented by a marketing article.
| Setup field | Example entry | Purpose |
|---|---|---|
| Part / drawing | Bracket example — drawing revision to be assigned | Prevent wrong-revision production |
| Material batch | Mild steel; actual certificate to be attached | Confirm Rm, grade and thickness |
| Material thickness | 3.0 mm nominal; measure receiving stock | Validate S² assumption |
| Active bend length | 3,200 mm | Validate the L value |
| Bending method | Air bend | Avoid applying chart to bottoming |
| Lower die | V24; segment ID / rating to be recorded | Confirm V and local load |
| Upper punch | Radius, section and load rating to be recorded | Protect tool and finished radius |
| Calculated nominal force | 780 kN | KRRASS formula result |
| Initial sizing target | 975 kN at +25% | Machine shortlist reference |
| Proposed model | PBS 110T/3200, 1,100 kN nominal | Match actual rated force and length |
| Approval status | Pending tooling, geometry and operator-safety review | No assumption of production approval |
| First-part result | To be measured and signed | Capture actual angle/radius evidence |
A useful record should also identify the operator, machine serial number, program version, punch and lower-die IDs, actual ram position, crowning setting, bend sequence and any special support arrangements. If the job fails inspection, those fields help engineering determine whether the issue was springback, tooling misalignment, thickness variance, backgauge repeatability or force capacity. A well-documented first-article process is stronger E-E-A-T evidence than an unsupported claim of “perfect accuracy.”
For published case studies, use real dated setup photographs, verified drawing inputs, an anonymized material certificate where appropriate and actual before/after inspection data. Do not create illustrative “measured” results from a force formula. Readers researching a capital machine purchase need to know which numbers were calculated, which came from published specifications, and which have been physically tested.
Common Press Brake Tonnage Calculation Mistakes
The most consequential mistake is mixing units. Converting 3,200 mm to 3.2 m is mandatory when using the KRRASS 650 coefficient as published. Mixing US short tons with metric ton-force is another frequent problem, particularly when suppliers from different countries use the word “ton” without qualification. Keep kN as the master unit in calculations and show ton-force as a labelled conversion for communication.
A second error is using nominal material names as if they defined a single strength. “Stainless steel” includes multiple grades and tempers. Even for a familiar grade, actual tensile strength may differ from the reference example in the online tool. A third error is treating the V-opening as adjustable without consequence. A larger die lowers load but can compromise inside radius or minimum flange; a smaller die does the opposite and may exceed a tool’s safe rating.
A fourth error is assuming a short bend is always easy because total force is smaller. The force per meter can be high enough to damage a short tool segment or overload the machine locally. The fifth is assuming that the machine’s rated bending length means any large formed part will pass between the uprights or rotate without collision. Sixth, air-bending chart values are sometimes misapplied to bottoming, coining or unconventional tooling where contact stresses can be much higher.
Finally, operators may use a tonnage chart to troubleshoot an angle problem that is actually caused by crowning, deflection, springback, inaccurate backgauge position, an unsuitable punch radius or inconsistent material. Tonnage is an input to the process, not a universal adjustment knob. If the bend angle is wrong, investigate the complete process before increasing load. The equipment manual, qualified personnel and safe operating procedures take precedence over an editorial formula.
Operator Safety and Tooling Inspection
Press brake safety includes guarding, workpiece support, controlled setup and clear communication between people working around the machine. A calculation on a screen does not detect a person’s hand near the point of operation, a shifting plate or damaged clamping equipment. In the United States, OSHA’s machine-guarding rule, 29 CFR 1910.212, addresses point-of-operation hazards; other jurisdictions have their own applicable requirements and standards. Use the rules and safeguards applicable to the installed machine and workplace.
Before a bend, ensure the upper and lower tools are correctly seated, locked and supported according to the manufacturer’s instructions. Inspect for cracks, chipped shoulders, deformation and incorrect segment combinations. Check that the backgauge, guards, protective devices, foot pedal and emergency-stop functions are suitable for the intended operation. Heavy and long sheets need a support plan; a sudden change in balance during bending can create significant hazards even if the force calculation is accurate.
When work requires maintenance or exposing personnel to hazardous stored energy, follow the documented energy-isolation process. OSHA’s lockout/tagout standard, 29 CFR 1910.147, describes energy control for applicable servicing and maintenance activities in the US. Do not bypass guarding or improvise higher hydraulic settings in an attempt to overcome insufficient tonnage. The correct response to an unsafe or unverified setup is to stop, revise the method and obtain the required approval.
Frequently Asked Questions About Press Brake Tonnage
How do I calculate press brake tonnage for a 90-degree bend?
For an initial KRRASS air-bending estimate, use P (kN) = 650 × S² × L / V × (Rm/450) with thickness and die opening in millimeters, bend length in meters, and tensile strength in MPa. For 3 mm reference mild steel over V24 with a 1 m active bend, the result is 243.8 kN, or 24.9 metric ton-force. Check the real die and tool ratings before bending. Use the online calculator to adjust the inputs.
What does “tons per meter” mean in a tonnage chart?
It describes the estimated force along one meter of active bend length, not the capacity of the entire press brake. Multiply the chart’s force-per-meter figure by the planned bend length to obtain a total-force estimate. Compare the same distributed figure with any tool or toolholder rating stated in kN/m, while separately checking the overall machine rating and allowable local loading.
Is the recommended V-die opening always eight times material thickness?
No. Eight times thickness is a helpful starting point in conventional air bending, not a universal standard. The KRRASS calculator also shows 6S and 10S presets and permits custom die openings. Thicker plate, stronger steel, radius requirements, available tool ratings and minimum flange dimensions may demand another opening. The selected die must be physically available and approved for the expected load.
How many tons do I need to bend 3 mm mild steel?
There is no useful answer without length and die information. At V24, 450 MPa and air bending, the estimate is approximately 24.9 metric ton-force per meter. A 2 m active bend is about 49.7 t, and a 3.2 m bend is about 79.5 t, before any chosen initial machine-sizing margin. Different materials and V-openings produce different results.
Does stainless steel always need 50% more tonnage?
No. The familiar “about 50% more” rule is a rough shop-floor shortcut, not a universal measured ratio. Under the KRRASS tensile-strength method, a 650 MPa example compared with 450 MPa reference mild steel produces a multiplier of 1.44, or about 44% more. Another stainless grade, temper or lot may require a different factor. Use material certification whenever the difference affects capacity selection.
What is the difference between kN, metric tons and US tons?
Kilonewtons are units of force. One metric ton-force equals approximately 9.80665 kN; one US short ton-force equals approximately 8.89644 kN. A 1,950 kN air-bending estimate is approximately 198.8 metric ton-force or 219.2 US short tons-force. Machines are commonly marketed with tonnage-related model codes, so check the manufacturer’s actual force data in kN before comparison.
Can a 100-ton press brake bend a 6 mm plate?
It depends on the material, bend length, lower-die opening and specific machine/tool ratings. At 450 MPa with V48, a one-meter bend needs approximately 49.7 metric ton-force, while two meters need about 99.4 t before sizing headroom. A nominal “100-ton” label alone does not establish that the second job is suitable. Local loading, rated length, material variation and tool compatibility all need review.
Why does a wider V-die reduce bending force?
A larger opening changes the geometry and gives the punch more leverage over the supported sheet, reducing the force required in the simplified air-bending model. But it also increases natural inside radius and the leg length needed to remain supported on the die shoulders. If the part has a short flange, changing from V32 to V48 may invalidate the drawing even though the tonnage estimate falls.
Can I use the chart for bottoming or coining?
Not without a separate process assessment. The main chart and 650-coefficient formula are air-bending estimates. Bottoming and coining use different contact conditions and often need substantially higher force. The KRRASS calculator provides broad preliminary multipliers, but the specific die, punch, material and process loading must be confirmed by tooling and machine specialists.
What if the press brake has enough total tonnage but the die does not?
The setup is not adequately rated. The tooling limit can govern independently of machine capacity. Obtain a higher-rated compatible die or revise the approved process, and also check the punch, holder and clamp. WILA’s published loading guidance explains why a tool segment’s safe load must be calculated from its per-meter rating and loaded length.
How much reserve capacity should I allow when buying a press brake?
KRRASS’s calculator suggests approximately 20–25% initial sizing headroom as a practical comparison allowance. It is not a universal safety certification or a replacement for machine-specific capacity curves. Calculate the demand, add the chosen preliminary allowance to the demand, then evaluate machine geometry and all tooling and loading restrictions before ordering.
Does a CNC controller reduce bending force?
No. CNC control can improve program management, axis coordination, backgauge repeatability and setup processes, but does not remove the force required to plastically bend metal on a given tooling geometry. Machine drive technology and frame characteristics influence control and productivity; material thickness, length, strength and V-opening remain fundamental force variables. Compare rated force and usable geometry as well as controller features.
Where can I find KRRASS press brake models for a calculated requirement?
Begin with the press brake tonnage calculator and then review the KRRASS press brake range. The AdvancedBend, StandardBend and EssentialBend pages list published configurations. Share a drawing, material details, planned tooling and expected throughput with KRRASS to confirm a specific machine/tooling combination.
A Practical Engineering Checklist Before You Choose a Press Brake
Use this checklist as a bridge between an online force estimate and an engineering-approved production or purchasing decision:
- Define the part family: record material grade, thickness range, bend length, flange dimensions, inside radius, angle and annual volume.
- Check material evidence: obtain the relevant specification or material certificate and identify the tensile strength assumption used in the calculation.
- Select the actual tooling: record punch profile, lower-die V-opening, active segment length, load ratings and holder/clamp compatibility.
- Calculate in consistent units: use the 650-coefficient kN equation or KRRASS calculator and archive input values.
- Review geometry: check natural inside radius, minimum flange, allowable return-leg clearance, opening height and part rotation.
- Apply the initial sizing allowance: document why the planned headroom is appropriate, and do not mistake it for a tool’s allowable overload.
- Compare actual machines: verify rated force in kN, working length, column spacing, local load limits and machine-specific capacity information.
- Approve the complete setup: check guarding, supports, operator instructions, setup procedure and first-part inspection requirements.
In purchasing discussions, this sequence gives sales and engineering teams a shared basis for comparison. It also makes quotations more useful: instead of asking only for “a 200-ton bending machine,” buyers can provide the longest and heaviest representative part, its V-die, the selected method and the intended daily workload. The supplier can then explain whether a 2,100 kN, 2,500 kN or other configuration is appropriate and what tooling or geometry conditions must be satisfied.
Choosing the Right Next Step with KRRASS
Calculating tonnage is valuable precisely because it prevents unnecessary guessing. The result tells you how force demand changes with material strength, bend length, thickness and V-opening. A good press brake tonnage chart makes those relationships visible, while the KRRASS press brake tonnage calculator makes them interactive. Together, they help an operator verify a proposed setup and help a buyer prepare a more meaningful machine specification.
Start by entering your real job details into the free KRRASS tonnage calculator. Next, compare the nominal force and bending length in the KRRASS CNC press brake range. If the job involves complex geometry, request tooling and clearance guidance through the press-brake tooling guide and the machine’s technical documentation. For questions about capacity, safety margins, machine axes or automation, share your drawing and planned production conditions with the KRRASS technical sales team.
The practical principle is simple: calculate the bend, verify the tool, verify the machine, confirm the part can move safely, and document the first approved setup. That approach is more reliable than choosing a machine by model number alone and more useful than a tonnage chart without stated assumptions.
Methodology and transparency: This guide uses the public KRRASS online calculator’s air-bending equation, its stated 450 MPa mild-steel baseline and its example material multipliers. All displayed example force values are mathematically derived, not presented as physically measured test data. Machine specifications are attributed to current public KRRASS series pages. External guidance from SSAB, WILA and OSHA informs the limits of simplified predictions and setup safety. These sources do not certify the capacity of an individual KRRASS installation. An appropriately qualified KRRASS engineer should review the machine-specific and safety statements before this draft is published.
Share