Bend allowance and K-factor
Bend allowance, flat length and bend deduction for folded sheet, using the K-factor of your material.
K-factor guidance · Process dependent · unit: mm
K-factor values are published guidance. They change with material, temper, grain direction and tooling, so confirm them with the forming supplier.
| Material family | K (R/t < 0.1) | K (R/t ≥ 0.1) | BA t=1 R=1 90° (mm) | BA t=2 R=2 90° (mm) | BA t=3 R=3 90° (mm) | Note |
|---|---|---|---|---|---|---|
| Aluminium (5052, 5754, 6061, 7075) | 0.4 | 0.44 | 2.262 | 4.524 | 6.786 | Verify with the forming supplier – K changes with temper, grain direction and tooling |
| Carbon steel and low-alloy steel | 0.38 | 0.42 | 2.231 | 4.461 | 6.692 | Verify with the forming supplier – K changes with temper, grain direction and tooling |
| Stainless steel (304, 316L) | 0.38 | 0.42 | 2.231 | 4.461 | 6.692 | Verify with the forming supplier – K changes with temper, grain direction and tooling |
| Copper and brass | 0.34 | 0.38 | 2.168 | 4.335 | 6.503 | Verify with the forming supplier – K changes with temper, grain direction and tooling |
| Titanium (Ti-6Al-4V) | 0.4 | 0.44 | 2.262 | 4.524 | 6.786 | Verify with the forming supplier – K changes with temper, grain direction and tooling |
| Engineering plastics (ABS, POM, nylon, PEEK) | 0.4 | 0.45 | 2.278 | 4.555 | 6.833 | Verify with the forming supplier – K changes with temper, grain direction and tooling |
K-factor guidance · Process dependent · unit: mm
Values assume a 90° bend with the inside radius equal to the thickness (R = t). The deduction is 2 × set-back − allowance.
| Material thickness (mm) | Steel K=0.42 BA | Steel K=0.42 BD | Aluminium K=0.44 BA | Aluminium K=0.44 BD | Copper K=0.38 BA | Copper K=0.38 BD |
|---|---|---|---|---|---|---|
| 1 mm | 2.231 | 1.769 | 2.262 | 1.738 | 2.168 | 1.832 |
| 1.5 mm | 3.346 | 2.654 | 3.393 | 2.607 | 3.252 | 2.748 |
| 2 mm | 4.461 | 3.539 | 4.524 | 3.476 | 4.335 | 3.665 |
| 2.5 mm | 5.576 | 4.424 | 5.655 | 4.345 | 5.419 | 4.581 |
| 3 mm | 6.692 | 5.308 | 6.786 | 5.214 | 6.503 | 5.497 |
| 4 mm | 8.922 | 7.078 | 9.048 | 6.952 | 8.671 | 7.329 |
| 5 mm | 11.153 | 8.847 | 11.31 | 8.69 | 10.838 | 9.162 |
| 6 mm | 13.383 | 10.617 | 13.572 | 10.428 | 13.006 | 10.994 |
| 8 mm | 17.844 | 14.156 | 18.096 | 13.904 | 17.342 | 14.658 |
| 10 mm | 22.305 | 17.695 | 22.619 | 17.381 | 21.677 | 18.323 |
| 12 mm | 26.766 | 21.234 | 27.143 | 20.857 | 26.012 | 21.988 |
The K-factor depends on the material, the temper, the grain direction and the tooling, so the tabulated values are starting guidance only. The bend deduction table assumes a 90 degree bend with the inside radius equal to the thickness. Trial bend a first-off part and measure it before releasing a batch.
Reference source
Bend allowance and K-factor
The K-factor describes how much of the bend allowance comes from stretching the material outside the neutral axis rather than from rotating it around the bend centre. It is a dimensionless number, usually between 0.33 and 0.50, and it is the part of the bend calculation that depends most on the material.
The bend allowance is the bend angle in radians multiplied by the sum of the inside radius and the K-factor multiplied by the thickness. For a 90 degree bend with a 2 mm radius, a 1.5 mm thickness and a K-factor of 0.42, the allowance is about 3.35 mm. The flat length is then the two straight legs plus the allowance.
The bend allowance is the arc length you must add to the flat blank. The bend deduction is the difference between the outside dimension and the flat length, so it is the number you subtract when you work from the outside dimensions. They describe the same geometry from two directions.
Use the radius that the drawing specifies, and never smaller than the minimum inside radius of the material and thickness. A tighter radius needs more allowance, stretches the material more, and is much more likely to crack the outside of the bend, especially in aluminium.
Steel and stainless steel sit around 0.38 to 0.42, aluminium around 0.40 to 0.44, copper and brass lower at about 0.34 to 0.38, and engineering plastics the highest at about 0.40 to 0.45. The value also shifts with the bend radius, the temper and the grain direction.
Springback is the elastic recovery that makes the part open up after bending. Compensate by over-bending, by using a tighter radius than the final geometry, or by choosing a material and temper with a lower springback. Always trial bend a first-off part and measure it before the batch.
The published tables assume a 90 degree bend with the inside radius equal to the thickness. If your part uses a different angle or a different radius to thickness ratio, calculate from the formula instead, or use a bend allowance table that matches your geometry.
Send the drawing and the requirements to Evefab’s engineering team. We reply with manufacturability feedback, not just a price.
