Pick material, thickness, angle, and K‑factor to model bends with precision fast. Get minimum radius guidance plus bend allowance, deduction, and export files instantly.
| Material | Thickness (T) | Factor (R/T) | Rmin | Angle | K | BA |
|---|---|---|---|---|---|---|
| Mild steel (low carbon) | 2.0 | 1.0 | 2.0 | 90° | 0.33 | 3.68 |
| Stainless steel (304/316) | 1.5 | 1.5 | 2.25 | 90° | 0.40 | 4.56 |
| Aluminum (5052/6061) | 3.0 | 1.0 | 3.0 | 120° | 0.35 | 7.32 |
In construction fabrication, bend radius controls cracking risk, fit-up quality, and repeatability. Tight radii concentrate strain at the outer fibers. A practical rule is to start with Rmin = (R/T) × T and refine using tooling and finish requirements.
Low‑carbon steel often works near R/T ≈ 1.0 for standard tempers and tooling. Stainless commonly needs larger radii, frequently R/T ≈ 1.5 or more, while high‑strength steels can require R/T ≥ 2.0. Softer metals like copper may tolerate smaller ratios when bend direction and surface quality allow.
As thickness increases, the same punch radius produces a sharper effective bend. Grain direction also matters: bending across the grain usually reduces cracking risk compared with bending parallel. Burrs, laser‑cut heat zones, and notches can act as starters, so deburring and edge prep improve reliability.
The calculator uses the neutral axis radius R + K·T. K‑factor commonly falls between 0.30 and 0.45. Air bending with wider V‑dies often increases K slightly, while bottoming/coining can shift the neutral axis and reduce K, changing flat patterns.
Bend allowance is computed as BA = θ(rad) × (R + K·T). For a 90° bend (θ≈1.5708 rad), small changes in R or K produce measurable differences. That’s why shops keep bend tables: a 2 mm sheet with K=0.33 and R=2 mm yields BA near 3.68 mm.
When you enter Leg A and Leg B, the flat length uses Flat = A + B − BD, where BD = 2·OSSB − BA. This is useful for brackets, flashings, and folded trims, where finished outside dimensions must match drawings.
Air bending typically increases springback, especially in stainless and high‑strength steels. Using a larger die opening, changing punch radius, or applying over‑bend helps hit angles. If you must hold tight angles on site‑critical parts, validate with a test coupon and update K and radius in your records.
Compare your target radius against the recommended minimum to reduce field failures. Record thickness, material, radius used, and angle for each job. Exporting results to CSV/PDF supports inspection packages, RFIs, and repeat work where identical bends are needed across multiple phases.
1) What is the “inside bend radius”?
It is the radius measured on the inside surface of the bend. Tooling and material behavior determine how closely the formed part matches the punch radius.
2) What if my target radius is below the minimum?
A smaller radius increases cracking risk and may reduce fatigue life. Use a larger punch radius, softer temper, different bend direction, or redesign the feature to increase the radius.
3) How do I choose a K‑factor?
Start with 0.33 for many air bends, then calibrate using a test bend and measured flat pattern. Different dies, punch radii, and materials shift K.
4) Does bend angle affect bend allowance?
Yes. Bend allowance scales with the angle in radians. A 120° bend uses a larger θ than 90°, increasing BA and changing the final flat length.
5) Are Leg A and Leg B inside or outside lengths?
Use outside leg lengths when applying the standard BD method shown. Keep the same measurement convention in drawings and shop practice for consistent flats.
6) Why do stainless parts spring back more?
Stainless typically has higher yield strength and elastic recovery. Expect more springback, requiring over‑bend or tooling adjustments to achieve the final angle.
7) Should I use different radii for painted or galvanized parts?
Coatings can crack on tight bends. Increasing radius, improving edge condition, and avoiding severe forming helps protect finishes and reduce corrosion issues.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.