# K-Factor and Bend Allowance for Sheet Metal Bending

> K factors by material and thickness for air bending, bottoming and coining per DIN 6935, with bend allowance and deduction worked examples for 90-degree bends.

https://www.sinbo-machining.com/wiki/reference/k-factor-bend-allowance/

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## On This Page

- K factors by material, thickness and bend method (calculated DIN 6935 + published ranges)
- What K-factor should I use for 2 mm mild steel in air bending?
- Why do CAD programs default to a K-factor of 0.44?
- How does K-factor change with inside radius?
- What is the difference between air bending, bottoming and coining in K-factor?
- How do I calculate bend allowance?
- How do I calculate bend deduction?
- What V-die opening should I use for 2 mm mild steel?
- What K-factor should I use for 304 stainless?
- What K-factor should I use for 5052 aluminium?
- Can I use one K-factor for all thicknesses?
- How do I measure the real K-factor for my press brake?
- Where is the neutral axis in a bend?
- Why does coining sometimes show a lower K than bottoming?
- How to Use These Numbers
- Frequently Asked Questions
- Standards & Sources
- Related Pages
- Download This Table

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# K-Factor and Bend Allowance for Sheet Metal Bending

K factors by material and thickness for air bending, bottoming and coining per DIN 6935, with bend allowance and deduction worked examples for 90-degree bends.

## K factors by material, thickness and bend method (calculated DIN 6935 + published ranges)

| Material | t (mm) | V die (mm) | R_i (mm) | R/t | K DIN 6935 | K air (DIN ↔ published) | K bottoming | K coining | y = K·t (mm) | Source |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| DC01 mild steel | 0.5 | 6 | 0.96 | 1.92 | 0.40 | 0.40–0.43 | 0.44–0.47 | 0.41–0.46 | 0.20 | S1/S2/S3 |
| DC01 mild steel | 0.8 | 6 | 0.96 | 1.20 | 0.34 | 0.34–0.43 | 0.44–0.47 | 0.41–0.46 | 0.27 | S1/S2/S3 |
| DC01 mild steel | 1.0 | 8 | 1.28 | 1.28 | 0.35 | 0.35–0.43 | 0.44–0.47 | 0.41–0.46 | 0.35 | S1/S2/S3 |
| DC01 mild steel | 1.5 | 12 | 1.92 | 1.28 | 0.35 | 0.35–0.43 | 0.44–0.47 | 0.41–0.46 | 0.53 | S1/S2/S3 |
| DC01 mild steel | 2.0 | 16 | 2.56 | 1.28 | 0.35 | 0.35–0.43 | 0.44–0.47 | 0.41–0.46 | 0.70 | S1/S2/S3 |
| DC01 mild steel | 3.0 | 25 | 4.00 | 1.33 | 0.36 | 0.36–0.43 | 0.44–0.47 | 0.41–0.46 | 1.07 | S1/S2/S3 |
| DC01 mild steel | 4.0 | 32 | 5.12 | 1.28 | 0.35 | 0.35–0.43 | 0.44–0.47 | 0.41–0.46 | 1.40 | S1/S2/S3 |
| DC01 mild steel | 6.0 | 50 | 8.00 | 1.33 | 0.36 | 0.36–0.43 | 0.44–0.47 | 0.41–0.46 | 2.14 | S1/S2/S3 |
| DX51D+Z galvanized | 0.8 | 6 | 0.96 | 1.20 | 0.34 | 0.34–0.43 | 0.44–0.47 | 0.41–0.46 | 0.27 | S1/S2/S3 |
| DX51D+Z galvanized | 1.5 | 12 | 1.92 | 1.28 | 0.35 | 0.35–0.43 | 0.44–0.47 | 0.41–0.46 | 0.53 | S1/S2/S3 |
| DX51D+Z galvanized | 3.0 | 25 | 4.00 | 1.33 | 0.36 | 0.36–0.43 | 0.44–0.47 | 0.41–0.46 | 1.07 | S1/S2/S3 |
| 304 stainless | 0.8 | 8 | 1.60 | 2.00 | 0.40 | 0.40–0.45 | 0.46–0.48 | 0.44–0.47 | 0.32 | S1/S2/S3 |
| 304 stainless | 1.5 | 16 | 3.20 | 2.13 | 0.41 | 0.41–0.45 | 0.46–0.48 | 0.44–0.47 | 0.61 | S1/S2/S3 |
| 304 stainless | 3.0 | 32 | 6.40 | 2.13 | 0.41 | 0.41–0.45 | 0.46–0.48 | 0.44–0.47 | 1.23 | S1/S2/S3 |
| 316L stainless | 1.0 | 10 | 2.10 | 2.10 | 0.41 | 0.41–0.45 | 0.46–0.48 | 0.44–0.47 | 0.41 | S1/S2/S3 |
| 316L stainless | 2.0 | 20 | 4.20 | 2.10 | 0.41 | 0.41–0.45 | 0.46–0.48 | 0.44–0.47 | 0.82 | S1/S2/S3 |
| 316L stainless | 3.0 | 32 | 6.72 | 2.24 | 0.41 | 0.41–0.45 | 0.46–0.48 | 0.44–0.47 | 1.23 | S1/S2/S3 |
| 5052-H32 aluminium | 0.8 | 6 | 0.96 | 1.20 | 0.34 | 0.34–0.40 | 0.42–0.46 | 0.38–0.44 | 0.27 | S1/S2/S3 |
| 5052-H32 aluminium | 1.5 | 12 | 1.92 | 1.28 | 0.35 | 0.35–0.40 | 0.42–0.46 | 0.38–0.44 | 0.53 | S1/S2/S3 |
| 5052-H32 aluminium | 3.0 | 25 | 4.00 | 1.33 | 0.36 | 0.36–0.40 | 0.42–0.46 | 0.38–0.44 | 1.07 | S1/S2/S3 |
| 6061-T6 aluminium * | 1.0 | 10 | 1.80 | 1.80 | 0.39 | 0.39–0.45 | 0.46–0.48 | 0.44–0.47 | 0.39 | S1/S2/S3 |
| 6061-T6 aluminium * | 2.0 | 20 | 3.60 | 1.80 | 0.39 | 0.39–0.45 | 0.46–0.48 | 0.44–0.47 | 0.78 | S1/S2/S3 |
| 6061-T6 aluminium * | 3.0 | 32 | 5.76 | 1.92 | 0.40 | 0.40–0.45 | 0.46–0.48 | 0.44–0.47 | 1.20 | S1/S2/S3 |
| C110 copper | 1.0 | 8 | 1.28 | 1.28 | 0.35 | 0.35–0.40 | 0.42–0.46 | 0.38–0.44 | 0.35 | S1/S2/S3 |
| C110 copper | 2.0 | 16 | 2.56 | 1.28 | 0.35 | 0.35–0.40 | 0.42–0.46 | 0.38–0.44 | 0.70 | S1/S2/S3 |
| C260 brass | 1.0 | 8 | 1.28 | 1.28 | 0.35 | 0.35–0.40 | 0.42–0.46 | 0.38–0.44 | 0.35 | S1/S2/S3 |
| C260 brass | 2.0 | 16 | 2.56 | 1.28 | 0.35 | 0.35–0.40 | 0.42–0.46 | 0.38–0.44 | 0.70 | S1/S2/S3 |

> **How to read this table.** These are calculated values plus published starting ranges, not measured K factors. K DIN 6935 = (0.65 + 0.5·log10(R/t))/2; V and R_i follow the 8×t rule and the 16% × V radius estimate (20–21% × V for stainless); thin sheet takes the smallest practical V6/V8 die instead of 8×t. K air pairs the DIN low end with the published t–3t tier high end; bottoming and coining are published starting values for R/t ≥ 1. y = K × t is measured from the inside surface. The 6061-T6 rows (*) borrow the hard tier as an envelope — DIN 6935 targets ductile medium-strength alloys, so test-bend 6061-T6 before committing. Do not quote 0.44 as a mild-steel K: it is a historical CAD default.

## What K-factor should I use for 2 mm mild steel in air bending?

At R/t = 1 (R = 2 mm), DIN 6935 gives K = 0.325 (0.33 rounded); the common CAD default 0.44 adds about 0.36 mm per 90° bend. Full flat-pattern check below.

| Step | Quantity | Value | Note |
| --- | --- | --- | --- |
| 1 | R/t | 2.0 / 2.0 = 1.00 | DIN 6935: k = 0.65 + 0.5·log10(1.00) = 0.65 |
| 2 | K = k/2 | 0.325 | Table value rounds to 0.33; this example carries the unrounded value |
| 3 | BA = (π/180)·A·(R + K·t), A = 90° | 4.163 mm | 1.5708 × 2.65 |
| 4 | OSSB = (R + t)·tan(A/2) | 4.000 mm | 4.0 × tan 45° |
| 5 | Flat legs from 50/30 outside | 46.000 / 26.000 mm | A_flat = 50 − OSSB, B_flat = 30 − OSSB |
| 6 | L = A + B + BA | 76.16 mm | BD = 2×4.000 − 4.163 = 3.837; 50 + 30 − 3.837 = 76.163 ✓ |
| 7 | If CAD default K = 0.44 | L = 76.52 mm (+0.361) | BA = 4.524 mm; four bends accumulate about 1.4 mm |

## Why do CAD programs default to a K-factor of 0.44?

It is a historical mild-steel default (0.44–0.45), not a universal value: for 5052 aluminium the same default is about 0.33–0.40, and for 304 stainless 0.40–0.45. DIN 6935 at R/t = 1 gives 0.325 — do not average the two calibers; test-bend and back-solve instead.

| Condition | Which K | Typical value | Next step |
| --- | --- | --- | --- |
| Ductile mild steel, air bending, R/t 0.5–3 | DIN 6935 Table / formula | 0.25–0.40 | Test bend, back-solve the real K |
| Mild steel, CAD default, V = 6–8t | Published medium air (t–3t) | 0.43 (defaults 0.44–0.45) | Use only when tooling and material match; otherwise switch to DIN |
| 304/316 stainless, air bending | DIN 6935 at R/t 2.0–2.2 | 0.40–0.41 (316 shop data up to 0.50) | Check springback, cracking and die wear |
| 5052-H32 aluminium, air bending | DIN 6935 at R/t 1.2–1.3 | 0.34–0.36 | Check cracking; soft temper takes the low end |
| 6061-T6 aluminium, air bending | Published hard air / DIN upper bound | 0.39–0.45 | Test bend mandatory; T6 cracks — prefer a larger radius |
| C110 copper / C260 brass | DIN 6935 at R/t 1.28 | 0.35 | Confirm annealed temper; hard temper takes the upper bound |
| Bottoming | Published t–3t tier of the class | 0.42–0.48 | Never reuse an air-bending K |
| Coining | Published t–3t tier of the class | 0.38–0.47 | Note coining sits below bottoming in the published table (see the coin-vs-bottom question) |
| R/t > 5 | K = 0.50 | 0.50 | Neutral axis back at mid-thickness |
| R/t < 0.5 | DIN Table low end | 0.15–0.25 | High cracking risk — open the radius or change material |
| New material / new tooling / no data | Test bend, back-solve | measured | Lock the measured K into the CAD material library |

## How does K-factor change with inside radius?

It rises toward 0.50 as R/t reaches 5: DIN 6935 gives K 0.25 at R/t = 0.5, 0.33 at 1.0, 0.40 at 2.0 and 0.45 at 3.0. Interpolate between rows with k = 0.65 + 0.5·log10(R/t), never linearly.

| R/t | k (DIN 6935) | K = k/2 | y/t | Note |
| --- | --- | --- | --- | --- |
| 0.05 | 0.00 | 0.00 | 0.00 | Neutral axis on the inside surface; cracking risk — do not design here |
| 0.07 | 0.06 | 0.03 | 0.03 | Source-table value; the DIN formula itself gives 0.07 (k) at this R/t — upstream document is internally inconsistent here |
| 0.10 | 0.15 | 0.08 | 0.08 | — |
| 0.20 | 0.30 | 0.15 | 0.15 | — |
| 0.25 | 0.35 | 0.18 | 0.18 | — |
| 0.33 | 0.41 | 0.21 | 0.21 | — |
| 0.50 | 0.50 | 0.25 | 0.25 | — |
| 0.75 | 0.59 | 0.30 | 0.30 | — |
| 1.00 | 0.65 | 0.33 | 0.33 | — |
| 1.25 | 0.70 | 0.35 | 0.35 | — |
| 1.50 | 0.74 | 0.37 | 0.37 | — |
| 1.75 | 0.77 | 0.39 | 0.39 | — |
| 2.00 | 0.80 | 0.40 | 0.40 | — |
| 2.50 | 0.85 | 0.43 | 0.43 | — |
| 3.00 | 0.89 | 0.45 | 0.45 | — |
| 3.50 | 0.92 | 0.46 | 0.46 | — |
| 4.00 | 0.95 | 0.48 | 0.48 | — |
| 4.50 | 0.98 | 0.49 | 0.49 | — |
| 5.00 | 1.00 | 0.50 | 0.50 | R/t ≥ 5: neutral axis at mid-thickness |

## What is the difference between air bending, bottoming and coining in K-factor?

Published starting values are 0.33–0.45 for air bending, 0.42–0.48 for bottoming and 0.38–0.47 for coining; never use an air-bend K for bottoming. These are starting values, not standard values.

| Material class | Bend method | R = 0–t | R = t–3t | R > 3t |
| --- | --- | --- | --- | --- |
| Soft (aluminium 5052, copper, brass) | Air bending | 0.33 | 0.40 | 0.50 |
| Soft (aluminium 5052, copper, brass) | Bottoming | 0.42 | 0.46 | 0.50 |
| Soft (aluminium 5052, copper, brass) | Coining | 0.38 | 0.44 | 0.50 |
| Medium (mild steel, galvanized) | Air bending | 0.38 | 0.43 | 0.50 |
| Medium (mild steel, galvanized) | Bottoming | 0.44 | 0.47 | 0.50 |
| Medium (mild steel, galvanized) | Coining | 0.41 | 0.46 | 0.50 |
| Hard (304/316 stainless) | Air bending | 0.40 | 0.45 | 0.50 |
| Hard (304/316 stainless) | Bottoming | 0.46 | 0.48 | 0.50 |
| Hard (304/316 stainless) | Coining | 0.44 | 0.47 | 0.50 |

## How do I calculate bend allowance?

BA = (π/180) × A × (R + K × t); for 2 mm mild steel, R 2 mm, K 0.325, 90° gives 4.163 mm. A is the bend angle — the angle the sheet travels: a 60° included angle means A = 120°.

| Term | Formula | Reading |
| --- | --- | --- |
| K factor | K = y / t, 0 ≤ K ≤ 0.5 | Neutral-axis offset from the inside surface over thickness |
| Neutral radius | R_neutral = R_i + K × t | The arc radius that actually keeps its length |
| Bend allowance BA | BA = (π/180) × A × (R_i + K × t) | Arc length of the neutral axis through the bend |
| Outside setback OSSB | OSSB = (R_i + t) × tan(A/2) | Tangent length from the apex to the bend tangent points |
| Bend deduction BD | BD = 2 × OSSB − BA | What the two outside corners lose versus the flat pattern |
| Flat length L | L = A + B + BA (flat legs); L = A_out + B_out − BD (outside dims) | Both give the same answer when K is right — mixing BA and BD is the classic flat-pattern error |
| Angle convention | A = 180° − included angle | 90° bend: A = 90; 60° included: A = 120; 120° included: A = 60 |

## How do I calculate bend deduction?

BD = 2 × OSSB − BA; the same 90° bend has OSSB 4.000 mm and BD 3.837 mm, so 50 + 30 mm outside flanges develop to 76.16 mm. The non-90 example cross-checks the same ledger at A = 120°.

| Case | Item | Value |
| --- | --- | --- |
| 90°, t 2.0, R 2.0 | OSSB = (2+2)·tan45° | 4.000 mm |
| 90°, t 2.0, R 2.0 | BD = 2×4.000 − 4.163 | 3.837 mm |
| 90°, t 2.0, R 2.0 | L = 50 + 30 − 3.837 | 76.163 mm |
| A=120°, t 1.5, R 3.0, 304 | K = (0.65 + 0.5·log10 2)/2 | 0.4003 |
| A=120°, t 1.5, R 3.0, 304 | BA = 2.0944 × (3 + 0.4003×1.5) | 7.541 mm |
| A=120°, t 1.5, R 3.0, 304 | OSSB = 4.5 × tan60° | 7.794 mm |
| A=120°, t 1.5, R 3.0, 304 | L = 52.206 + 32.206 + 7.541 | 91.95 mm |
| A=120°, t 1.5, R 3.0, 304 | BD check: 60 + 40 − (2×7.794 − 7.541) | 91.952 mm ✓ |
| A=120°, t 1.5, R 3.0, 304 | If hard-air K 0.45 instead | L = 92.11 mm (+0.156 vs DIN 0.40: 0.16 mm per bend) |

## What V-die opening should I use for 2 mm mild steel?

Start at V16 (8 × t); it gives an inside radius near 2.6 mm, a minimum flange over 11 mm and about 12 t/m in 60,000 psi mild steel. Stainless wants V = 10–12×t and a 20–22% radius factor.

| t (mm) | V (mm) | R_i (mm) | Min flange (mm) | Tonnage (t/m) | Source |
| --- | --- | --- | --- | --- | --- |
| 0.5 | 6 | 0.96 | — | — | practical min V6, calc |
| 0.8 | 6 | 0.96 | — | — | S3 method, calc |
| 1.0 | 8 | 1.3 | > 5.5 | 8 | S3 |
| 1.5 | 12 | 2.0 | > 8.5 | 10 | S3 |
| 2.0 | 16 | 2.6 | > 11.0 | 12 | S3 |
| 2.5 | 20 | 3.3 | > 14.0 | 14 | S3 |
| 3.0 | 25 | 4.0 | > 17.5 | 16 | S3 |
| 4.0 | 32 | 5.0 | > 22.0 | 22 | S3 |
| 5.0 | 40 | 6.5 | > 28.0 | 27 | S3 |
| 6.0 | 50 | 8.0 | > 35.0 | 32 | S3 |

## What K-factor should I use for 304 stainless?

Use 0.40–0.45: DIN 6935 gives 0.41 at R/t = 2.1, while the published hard-material starting value is 0.45; 316 runs higher, up to 0.45–0.50 in shop data — take the upper bound and test-bend.

| Caliber | K at R≈t | Applies to | Source |
| --- | --- | --- | --- |
| Shop-measured caliber | 0.46 | 304, air bending, R = t | S4 |
| Shop-measured caliber | 0.50 | 316, air bending, R = t | S4 |
| Published hard class (t–3t) | 0.45 | Generic 304/316 hard tier — does not separate 304 from 316 | S2/S5 |
| DIN 6935 at R/t = 2 | 0.40 | Geometry only — no work-hardening difference | S1/S6 |
| This page, Table 2 | 0.41–0.45 | 316 takes the upper bound; test-bend | S1/S2/S3 |

## What K-factor should I use for 5052 aluminium?

Use 0.34–0.40; 5052-H32 at R/t ≈ 1.2 sits near 0.33–0.35, while a generic soft-aluminium table gives 0.40 for R = t–3t. Keep 6061-T6 on its own row — do not mix it with 5052.

| Caliber | K | Applies to | Source |
| --- | --- | --- | --- |
| Shop-measured (R = t) | 0.33 | 5052-H32, air bending | S4 |
| Published soft air (t–3t) | 0.40 | Generic soft aluminium | S2/S5 |
| DIN 6935 (R/t = 1.2) | 0.345 | Geometry only | S1/S6 |
| This page, Table 2 | 0.34–0.40 | Soft temper low end, hard temper or wide V high end | S1/S2/S3 |

## Can I use one K-factor for all thicknesses?

Only if V/t and R/t stay constant; a 0.5 mm sheet on a practical V6 die runs R/t ≈ 1.9 and K ≈ 0.40, while 2 mm on V16 runs R/t ≈ 1.3 and K ≈ 0.35. Thickness moves K through the die, not through the formula.

| t (mm) | V (mm) | R_i (mm) | R/t | K DIN 6935 |
| --- | --- | --- | --- | --- |
| 0.5 | 6 | 0.96 | 1.92 | 0.40 |
| 0.8 | 6 | 0.96 | 1.20 | 0.34 |
| 2.0 | 16 | 2.56 | 1.28 | 0.35 |
| 3.0 | 25 | 4.00 | 1.33 | 0.36 |
| 6.0 | 50 | 8.00 | 1.33 | 0.36 |

## How do I measure the real K-factor for my press brake?

Bend a 200 mm strip at 90°, measure both outside flanges, get BD = 200 − (A + B), then back-solve K from K = [2(R + t)tan(A/2) − BD] / [(π/180)At] − R/t. Write the measured K back into the CAD library and stop reusing 0.44 for that material/tooling pair.

| Step | Rule | Value |
| --- | --- | --- |
| 1 | Strip | 200 × 50 mm, bend at 90° mid-length |
| 2 | Measure both outside flanges A_out + B_out | BD = 200 − (A_out + B_out) |
| 3 | Back-solve | K = [2(R+t)tan(A/2) − BD] / [(π/180)·A·t] − R/t |
| 4 | Write into CAD material library | per material + tooling pair |

## Where is the neutral axis in a bend?

At y = K × t from the inside surface: 2 mm mild steel with K 0.35 puts it 0.70 mm from the inside, so the neutral radius is R + 0.70 mm.

| Case | K | t (mm) | y = K·t (mm) | Neutral radius |
| --- | --- | --- | --- | --- |
| DC01 2.0 mm, R/t 1.28 | 0.35 | 2.0 | 0.70 | R + 0.70 mm (R_i 2.56 → neutral 3.26 mm) |
| DC01 3.0 mm, R/t 1.33 | 0.36 | 3.0 | 1.07 | R + 1.07 mm |
| 304 1.5 mm, R/t 2.13 | 0.41 | 1.5 | 0.61 | R + 0.61 mm |
| DC01 6.0 mm, R/t 1.33 | 0.36 | 6.0 | 2.14 | R + 2.14 mm |

## Why does coining sometimes show a lower K than bottoming?

The published table gives coining 0.44–0.47 versus bottoming 0.46–0.48 in the R = t–3t range, which is the opposite of the usual expectation that coining pushes K toward 0.50 — treat it as an unresolved conflict and verify with a test bend.

| Method | Soft K (t–3t) | Medium K (t–3t) | Hard K (t–3t) | Source |
| --- | --- | --- | --- | --- |
| Bottoming | 0.46 | 0.47 | 0.48 | S2/S5 |
| Coining | 0.44 | 0.46 | 0.47 | S2/S5 |
| Expectation | — | — | — | — |
| Second source confirming the ranking | none found | none found | none found | GAP |

## How to Use These Numbers

- Table rows are calculated values plus published starting ranges — never present them as measured K factors; the DIN column is a formula value and the published columns are CAD/shop starting values that differ from it by 0.03–0.09.
- 0.44 is a historical CAD default, not the K-factor of mild steel: DIN 6935 gives 0.325 at R/t = 1. If a CAD flat pattern will not fit the bent part, check which caliber the software used before touching the tooling.
- Both worked examples use the K values consistent with Table 1 (0.325 at R/t = 1, 0.40 at R/t = 2) — do not swap in 0.44 to make the arithmetic prettier.
- DIN 6935 targets ductile, medium-strength steels; the aluminium, copper and brass rows borrow only its geometry, with the published ranges covering the hardening difference. 6061-T6 is marked * because no DIN-caliber table exists for it — test-bend is mandatory.
- Titanium Grade 2, HSLA steels and Inconel have no citable K tables — back-solve K from a test bend instead of guessing; thicknesses above 6 mm, hems, curling and joggles are outside this table's scope.
- Springback is a separate compensation: K locates the neutral axis but does not give the springback angle; compensate per material and die, not from this table.
- The DIN 6935 original was not purchased; the formula and k table are verified against two independent public sources (a technical digest and a software implementation that agree value by value) — re-verify against the purchased standard before production-critical use.

## Frequently Asked Questions

### What K-factor should I use for 2 mm mild steel in air bending?

At R/t = 1 (R = 2 mm), DIN 6935 gives 0.325 (0.33 rounded); the common CAD default 0.44 adds about 0.36 mm per 90° bend.

### Why do CAD programs default to a K-factor of 0.44?

It is a historical mild-steel default (0.44–0.45), not a universal value: for 5052 aluminium the same default is about 0.33–0.40, for 304 stainless 0.40–0.45.

### How does K-factor change with inside radius?

It rises toward 0.50 as R/t reaches 5: K 0.25 at R/t = 0.5, 0.33 at 1.0, 0.40 at 2.0 and 0.45 at 3.0.

### What is the difference between air bending, bottoming and coining in K-factor?

Published starting values are 0.33–0.45 for air bending, 0.42–0.48 for bottoming and 0.38–0.47 for coining; never use an air-bend K for bottoming.

### How do I calculate bend allowance?

BA = (π/180) × A × (R + K × t); for 2 mm mild steel, R 2 mm, K 0.325, 90° gives 4.163 mm.

### How do I calculate bend deduction?

BD = 2 × OSSB − BA; the same 90° bend has OSSB 4.000 mm and BD 3.837 mm, so 50 + 30 mm outside flanges develop to 76.16 mm.

### What V-die opening should I use for 2 mm mild steel?

Start at V16 (8 × t): inside radius near 2.6 mm, minimum flange over 11 mm, about 12 t/m in 60,000 psi mild steel.

### What K-factor should I use for 304 stainless?

Use 0.40–0.45: DIN 6935 gives 0.41 at R/t = 2.1, the published hard starting value is 0.45, and 316 runs up to 0.45–0.50 in shop data.

### What K-factor should I use for 5052 aluminium?

Use 0.34–0.40: 5052-H32 at R/t ≈ 1.2 sits near 0.33–0.35 while a generic soft-aluminium table gives 0.40 for R = t–3t.

### Can I use one K-factor for all thicknesses?

Only if V/t and R/t stay constant: 0.5 mm on a practical V6 die runs R/t ≈ 1.9 and K ≈ 0.40, while 2 mm on V16 runs R/t ≈ 1.3 and K ≈ 0.35.

### How do I measure the real K-factor for my press brake?

Bend a 200 mm strip at 90°, take BD = 200 − (A + B), then back-solve K = [2(R+t)tan(A/2) − BD] / [(π/180)At] − R/t.

### Where is the neutral axis in a bend?

At y = K × t from the inside surface: 2 mm mild steel with K 0.35 puts it 0.70 mm from the inside, so the neutral radius is R + 0.70 mm.

### Why does coining sometimes show a lower K than bottoming?

The published table gives coining 0.44–0.47 versus bottoming 0.46–0.48 in the R = t–3t range — an unresolved conflict, so verify with a test bend.

## Standards & Sources

- S1 — Rainer Schlangen, Biegetechnik — Biegeverkürzung (2010): DIN 6935 digest — k = 0.65 + 0.5·log10(r/s), k table 0.05–5.0, K = k/2, neutral layer NT = (s/2)·k
- S2 — Protolabs Network (ex-Hubs), Sheet metal fabrication design guide — generic K-factors for air/bottoming/coining, soft–medium–hard, default 0.4468
- S3 — ADH Machine Tool, Press Brake V Die Opening Chart — 8×t start, R_i ≈ 16% V, stainless 10–12×t at 20–22%; rows for 1.0–6.0 mm verified against the published chart
- S4 — Lapas, K-Factor and Bend Allowance — material-at-R=t shop calibers: 5052 0.33, 6061-T6 0.40, A36 0.44, 304 0.46, 316 0.50; R/t > 3 → K ≈ 0.50
- S5 — MechCodex, Sheet Metal K-Factors by Material and Bend Method — cross-check of S2
- S6 — smlibpy, Sheet metal: what the functions share — independent DIN 6935 formula implementation, K = k/2, default 0.44
- S7 — DIN 6935 Kaltbiegen von Flacherzeugnissen aus Stahl (paid standard, not purchased; cited only as the upstream of S1/S6)

## Related Pages

- [/wiki/sheet-metal/bend-radius-and-bend-allowance/](/wiki/sheet-metal/bend-radius-and-bend-allowance/) — Long-form guide: inside radius selection and bend allowance practice
- [/wiki/sheet-metal/tolerance-and-surface/](/wiki/sheet-metal/tolerance-and-surface/) — Sheet metal tolerances and finish expectations formed parts are held to
- [/wiki/sheet-metal/design-for-sheet-metal/](/wiki/sheet-metal/design-for-sheet-metal/) — DFM rules for bends, flanges and reliefs that set R/t in the first place

## Download This Table

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