K-Factor, Bend Allowance & Bend Deduction
The three numbers every press-brake operator fights with. Here's what each one means, how they relate, and the material-specific K-factor ranges that get you close enough to cut test coupons for the final calibration.
K-Factor Table by Material & Method
Empirical averages from Machinery's Handbook, Wilson Tool, Protocase, and Xometry design guides. Air-bend assumes 8×T V-die opening with 85–88° punch.
| Material | Thickness band | Air bend | Bottoming | Coining |
|---|---|---|---|---|
| Mild steel (1018, A36) | 0.025–0.040" | 0.33 | 0.38 | 0.42 |
| Mild steel (1018, A36) | 0.040–0.125" | 0.38 | 0.42 | 0.44 |
| Mild steel (1018, A36) | 0.125–0.250" | 0.42 | 0.44 | 0.46 |
| Stainless 304 / 316 | 0.025–0.062" | 0.38 | 0.40 | 0.43 |
| Stainless 304 / 316 | 0.062–0.187" | 0.40 | 0.43 | 0.45 |
| Stainless 304 / 316 | 0.187–0.250" | 0.42 | 0.45 | 0.47 |
| Aluminum 5052-H32 | 0.025–0.062" | 0.40 | 0.43 | 0.46 |
| Aluminum 5052-H32 | 0.062–0.250" | 0.43 | 0.46 | 0.48 |
| Aluminum 6061-T6 | 0.040–0.125" | 0.42 | 0.45 | 0.48 |
| Aluminum 6061-T6 | 0.125–0.250" | 0.45 | 0.47 | 0.50 |
| Copper C11000 (soft) | 0.025–0.125" | 0.38 | 0.41 | 0.44 |
| Brass C26000 (½-hard) | 0.025–0.125" | 0.38 | 0.41 | 0.44 |
| Galvanized / galvanneal | 0.040–0.125" | 0.40 | 0.43 | 0.45 |
No data? Start at K = 0.44 for mild-steel air bending and calibrate down from there.
Formulas
All angles in degrees. T = material thickness, R = inside bend radius, A = bend angle (complement of the included angle — a 90° bend means the material turns 90°).
BA = (π / 180) × (R + K × T) × A
BD = 2 × (R + T) × tan(A / 2) − BA
Lflat = Leg1 + Leg2 − BD
Multi-bend parts: subtract one BD per bend from the sum of all OML flange lengths.
Worked Example
0.090" mild steel · R = 0.125" · 90° air bend · K = 0.44 · Legs OML: 2.000" and 1.500".
BA = (π/180) × (0.125 + 0.44 × 0.090) × 90 = 0.2585"
BD = 2 × (0.125 + 0.090) × tan(45°) − 0.2585 = 0.1715"
Lflat = 2.000 + 1.500 − 0.1715 = 3.329"
Minimum Inside Bend Radius
Tighter than this and you get outer-fiber cracking, orange-peel, or fracture. Expressed as a multiple of thickness T, transverse to rolling direction.
| Material | Condition | Min inside radius |
|---|---|---|
| Mild steel 1018, A36 | As-rolled | 0.5 × T – 1.0 × T |
| HSLA (50 ksi+) | As-rolled | 1.0 × T – 1.5 × T |
| Stainless 304 / 316 | Annealed (2B) | 1.0 × T – 2.0 × T |
| Stainless 301 | ½-hard | 2.0 × T – 3.0 × T |
| Aluminum 5052 | H32 | 1.0 × T – 2.0 × T |
| Aluminum 6061 | T4 | 1.5 × T – 2.5 × T |
| Aluminum 6061 | T6 | 3.0 × T – 5.0 × T |
| Aluminum 2024 | T3 | 3.0 × T – 5.0 × T |
| Copper C11000 | Soft / annealed | 0.5 × T – 1.0 × T |
| Spring steel 1074/1095 | Hardened | 4.0 × T – 8.0 × T |
| Titanium Gr. 2 | Annealed | 2.5 × T – 3.5 × T |
For air bending, the achievable radius is also constrained by V-die opening: R ≈ 0.16 × V for mild steel (the "8× rule"). Parallel-to-grain bends require ≈1.5× the transverse values above.
Springback Compensation
Elastic recovery when the punch retracts. Overbend by this angle to land at target. Values for 90° air-formed bends in 8×T V-die.
| Material | Typical overbend |
|---|---|
| Mild steel (1018, A36) | 1° – 2° |
| HSLA / dual-phase | 3° – 5° |
| Stainless 304/316 annealed | 3° – 5° |
| Stainless 301 ½-hard | 6° – 10° |
| Aluminum 5052-H32 | 2° – 3° |
| Aluminum 6061-T6 | 3° – 5° |
| Aluminum 2024-T3 | 4° – 6° |
| Spring steel (hardened) | 10° – 20° |
| Titanium Gr. 2 | 4° – 7° |
Springback increases with higher yield, larger R, smaller T/R ratio, and air bending vs. bottoming/coining. Coining nearly eliminates springback at the cost of 5–10× the tonnage.
L. (2) Bend 90° on
production tooling. (3) Measure Leg1, Leg2, achieved angle, and
inside radius. (4) Back-calculate BA:
BA = L − (Leg1 + Leg2 − 2(R+T)tan(A/2)), then solve for
K. (5) Average 3–5 coupons and store K by material-thickness-tooling
combo in your CAM library.
Sources
FAQ
What K-factor should I use for air bending 0.090" mild steel?
For mild steel (1018, A36) in the 0.040–0.125" band, use an air-bend K-factor of 0.38; bottoming is 0.42 and coining 0.44. With no data, start at K = 0.44 for mild-steel air bending and calibrate down using test coupons on your production setup.
What are the bend allowance and bend deduction formulas for a press brake?
Bend allowance: BA = (π / 180) × (R + K × T) × A. Bend deduction: BD = 2 × (R + T) × tan(A / 2) − BA. Flat blank length: L_flat = Leg1 + Leg2 − BD. Here T = thickness, R = inside radius, A = bend angle, K = K-factor. Subtract one BD per bend on multi-bend parts.
What is the minimum inside bend radius for 6061-T6 aluminum?
For Aluminum 6061 in T6 condition, the minimum inside bend radius is 3.0 × T to 5.0 × T (transverse to rolling). By comparison, 6061-T4 is 1.5 × T to 2.5 × T, and as-rolled mild steel 1018/A36 is 0.5 × T to 1.0 × T. Parallel-to-grain bends need about 1.5× these values.
How much springback overbend does stainless 301 half-hard need?
For 90° air-formed bends in an 8×T V-die, Stainless 301 ½-hard needs 6°–10° overbend, versus 1°–2° for mild steel and 3°–5° for annealed 304/316. Coining nearly eliminates springback but costs 5–10× the tonnage.
Log every pass before the CWI signs off
Welding & Sheet Metal Toolkit bands kJ/in against your WPS, stacks the pass log, and prints the day-sheet citing AWS D1.1 / ASME IX. Offline. Pay once.
Related
K-factor values are empirical averages — starting points, not production numbers. Machinery's Handbook formulas reproduced here are public engineering knowledge. Always validate with test coupons on the production setup for ±0.010" or tighter tolerances.