What the ratio measures.
K is the distance from the inside surface to the neutral axis, divided by the sheet thickness. The neutral axis is the reference used for calculating the developed length through the bend.
K = neutral-axis offset / thicknessBA = (A × π / 180) × (R + K × T)These definitions follow Autodesk’s sheet metal rule reference and its Inventor unfold reference. A is the amount bent in degrees, T is thickness and R is the inside radius.
Recover an effective K from a test.
- Measure the initial flat blank and its actual thickness.
- Make a single test bend using the intended material, tooling and setup.
- Measure the resulting angle, inside radius and outside mold-line dimensions.
- Subtract the tangent-to-edge straight lengths from the flat length to obtain BA.
- Calculate K = (BA / α − R) / T and retain the test conditions with the result.
The K-factor calculator performs the dimension conversion and calculation. It also accepts a known BD using the same outside-dimension convention.
Keep the conditions with the number.
Record the material and condition, thickness, radius, tooling, process and test date. Recheck the value when relevant conditions change. A reverse-calculated K is an effective calibration for those measurements; it is not a universal constant for every sheet of the same material.
If the result is outside 0–1.
Check the length measurements, radius, thickness, angle convention and whether the data came from the same test. BendTab reports the mismatch rather than forcing the result into range. More decimal places cannot correct an unknown radius or mismatched dimensions.
When you have verified BA or BD values for the same conditions, the bend calculator can use those values directly without making you round-trip through a rounded K-factor.