Why calibrate it
The K-factor locates the neutral axis inside the bend, and it determines the bend allowance and the flat length of every bent part. A small error repeats on every bend: on a part with four bends, a 0.3 mm error per bend is more than a millimeter on the finished part.
What you need
- Strips of the same material, thickness and lot you will produce, cut in the same direction.
- The punch and die you normally use for that thickness.
- A caliper, and a radius gauge and protractor to check the result.
The test
- Cut a strip and measure its flat length L precisely (for example, 150.00 mm).
- Bend it to 90° near the middle with your usual tooling and settings.
- Measure the angle actually achieved and the inside radius R of the bend.
- Measure both outside flange lengths A and B, from each end to the outside of the bend.
- Repeat with at least three strips and average the results.
Use the result
Enter the K-factor, thickness, radius and angle in the bend allowance calculator to get the bend deduction and the flat length of new parts. Record the value for each combination of material, thickness and tooling: it is not a single number for the whole shop.
In air bending, the inside radius depends mainly on the die opening, not only on the punch radius. That is why measuring R on the test piece matters.
Formula
- BD = A + B − L
- OSSB = tan(θ ÷ 2) × (R + T)
- BA = 2 × OSSB − BD
- K = (BA × 180 ÷ (π × θ) − R) ÷ T
- L
- flat length of the strip
- A, B
- outside flange lengths after bending
- T
- thickness
- R
- measured inside radius
- θ
- bend angle in degrees
- BD, BA
- bend deduction, bend allowance
Worked example
Illustrative example: 2 mm sheet bent to 90° (measurements are made up for the explanation)
- 1L = 150.00 mm, T = 2.00 mm, measured R = 2.00 mm, θ = 90°, A = 78.83 mm, B = 74.83 mm
- 2BD = 78.83 + 74.83 − 150.00 = 3.66 mm
- 3OSSB = tan(45°) × (2.00 + 2.00) = 4.00 mm → BA = 2 × 4.00 − 3.66 = 4.34 mm
- 4K = (4.34 × 180 ÷ (π × 90) − 2.00) ÷ 2.00 ≈ 0.38
- 5New part with 40 mm and 60 mm outside flanges, same setup: flat length ≈ 96.34 mm
K ≈ 0.38 for this material, thickness and tooling. Check it with a first part before a production run.
The example is illustrative. Its numbers are checked by automated tests.
Assumptions and limitations
- The formulas assume a constant radius and a measured angle; springback must already be included in the angle you measure.
- Measurements in the example are illustrative and do not come from a real test.
- The result is valid only for the material, thickness, grain direction and tooling you tested.