Why does the flat pattern calculated by CAD differ from the real result on the shop floor?

CAD software often uses a fixed default inside radius (e.g. 1 mm) and a static K-factor. In air bending, the actual inside radius depends mainly on the V-die opening (about V/6 for carbon steel) and the material's behaviour. If the CAD radius does not match the radius produced by the selected V-die, the bend deduction will be wrong and the error will accumulate across the part.

How do rotating WingBend or Rolla-V dies differ from conventional dies?

With rotating-wing dies (WingBend / Rolla-V), the sheet rests on moving inserts that rotate with the punch. This reduces sliding friction and surface marking. Minimum flange and inside radius depend on the specific die design and must be checked against its technical data.

How do you choose the V-die opening based on sheet thickness?

For carbon steel (Fe360/S235), a common starting range is V = 6–8 times thickness for sheets up to 3 mm and V = 8–10 times thickness for thicker sheets. Stainless steel often requires a wider opening, while aluminium may allow a narrower one. Always check the material, tooling manufacturer's guidance and press-brake load limits.

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Calculate your sheet-metal flat pattern.

Set the material, tooling and bend geometry. Get working parameters with the model assumptions and limits clearly stated.

Calculator Notes & cases

Flat pattern · K-factor · S/V/F table Method and formulas ↘

Calculate the K-factor from a sample

Enter dimensions measured on an already-bent part. The K-factor is derived from the measured geometry, not looked up in a table.

How to measure the flanges

Dimension A/B runs parallel to its respective leg. Extension lines are perpendicular to the leg and reach the projection of the bend outer envelope. Each dimension includes the straight portion plus the outside setback (OSS); at 90°, OSS = Ri + s.

Dimension convention: The A/B dimension line runs parallel to the leg; extension lines are perpendicular to it and end at the projected outer bend envelope. The formula uses OSS = (Ri + s) × tan(bend angle / 2), which equals Ri + s for a 90° bend.

Thickness s (mm)

Inside angle after bending (°)

Measured inside radius Ri (mm)

Initial flat length (mm)

Outside dimension A — bend projection (mm)

Outside dimension B — bend projection (mm)

Bending setup (optional)

These details document the setup used to produce the sample and can be included in the shared link. They do not directly contribute to the geometric K-factor calculation.

Material (optional)

Nominal Rm (N/mm², optional)

Bending method

V opening / equivalent V (mm)

Nominal punch radius Rp (mm)

K-factor derived from the sample

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Calculated from the entered dimensions.

Bend Allowance

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Bend Deduction

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Sample setup

Enter the sample measurements.

Flat-pattern parameters

The V-die opening can be selected from the available sizes. Ri and minimum flange are calculated by the model and cannot be edited.

Material

Material Rm (N/mm²)

Thickness s (mm)

Bending method

Cava V suggerita (selezionabile)

The suggested size follows a practical guideline and can be changed among the available sizes.

Punch-tip radius Rp (mm)

Inside angle (°)

Flangia esterna A (mm)

Flangia esterna B (mm)

Bend geometrySchema dinamico

Illustrative diagram, not to scale. The angle represents the selected inside angle.

V / V equivalente selezionata

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Inside radius Ri

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Non modificabile

Minimum flange

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Non modificabile

K-factor predittivo

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Model estimate; not a value certified by the tooling manufacturer.

Bend Allowance

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Bend Deduction

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Flat pattern piano teorico

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Parametri derivati dal modello; verificare le tabelle dell’utensile installato.

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Shop-floor experience

Over 25 years of experience in bending, tooling and CAD/CAM.

Radius first, then K-factor

Start with the geometry produced by the bend, not an isolated value.

Transparent limits

Misure, dati importati e stime restano distinti.

Diario di officina · Caso 01

A fit problem. A better question.

A component does not fit as expected. The drawing specifies R0.8; the punch is R0.5. The photo documents the issue, but the actual formed radius still needs to be measured.

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Osservazione reale di produzione

When the actual radius changes the fit

Air bending · 2 mm pickled mild steel · 30° V10 die · R0.5 punch · 90° and 120° inside angles.

Drawing radius R0.8Actual radius not yet measuredProblema documentato in foto

Read the case

Technical Guide: Bending Mechanics and Flat-Pattern Calculation

1. Air Bending vs Bottoming/Coining vs Rotating Dies (WingBend)

In traditional air bending, the sheet contacts the tooling at only three points: the punch tip and the two shoulders of the V-die. As a result, the inside radius (r_i) is not determined by the punch radius (unless the punch radius is larger than the natural radius); instead, it forms freely in proportion to the V-die opening:

Carbon steel: r_i ≈ 0.16 × V (about V/6)
Stainless steel: r_i ≈ 0.20 × V (about V/5, due to greater springback)
Soft/medium aluminium: r_i ≈ 0.14 × V (about V/7)

In tangential rotating-wing dies (such as WingBend or Rolla-V), the die shoulders are replaced by semi-round inserts that rotate in sync with the descending punch. This reduces the leverage needed for the minimum flange (down to 0.45 × V), helps prevent cosmetic marks (ideal for brushed stainless steel or pre-painted sheet), and makes the sheet closely follow the punch radius (r_i ≈ R_p).

2. Why CAD Flat Patterns Can Be Wrong (Especially for Multi-Bend Parts)

Many designers create 3D models using their software's default parameters (e.g. r_i = 1.0 mm and K = 0.33 or 0.44). On the shop floor, the operator selects the die based on material thickness; for example, a 3 mm sheet may be bent using a V = 24 mm opening.

With a 24 mm V-die opening, the actual inside radius may be about 3.8–4.0 mm, not 1.0 mm. A larger radius changes the bend deduction significantly. If the error is 0.5 mm per bend, a box or omega profile with four bends can accumulate an error exceeding 2.0 mm, potentially making the part unusable.

Air-bending force: F [kN] = (8 × R_m × L × s²) / V
Bend Allowance (BA): π × (β / 180) × (r_i + K × s)
Outside Setback (OSS): tan(β / 2) × (r_i + s)
Bend Deduction (BD): 2 × OSS − BA (where β is the bend angle)

Frequently Asked Questions (FAQ)

When can a punch that is too sharp cause a scoring effect?

When the punch radius (R_p) is less than about 63% of the sheet thickness (R_p < 0.63 × s), the tip may indent or score the inner surface instead of allowing it to bend smoothly. This can affect the K-factor and increase the risk of cracking on the outside of the bend.

How does the rolling direction affect bending?

Bending perpendicular to the rolling direction may require slightly more force but generally improves resistance to cracking and can allow tighter radii. Bending parallel to the rolling direction may reduce springback, but can increase the risk of surface cracking in high-strength steels or 6000-series aluminium.

What is the difference between Bend Allowance (BA) and Bend Deduction (BD)?

The Bend Allowance is the arc length along the neutral axis within the bend region; it is used with the straight portions to calculate the flat length. The Bend Deduction is the amount subtracted from the sum of outside dimensions measured to theoretical sharp corners (L = A + B − N × BD).