Sheet metal K-factor locates the neutral axis through the material thickness during bending. It is the neutral-axis distance from the inside surface divided by material thickness, using the conventional definition applied by the MfgWorkbench calculator.
K = neutral-axis offset ÷ material thickness
The conventional range is greater than 0 and no more than 0.5. It is an empirical process input, not a universal property of an alloy name.
What K-factor represents
Fibers near the inside of a bend compress while fibers near the outside stretch. Between them is a layer whose developed length is treated as unchanged: the neutral axis. If its offset from the inside surface is 0.8 mm in 2.0 mm material, K = 0.8 ÷ 2.0 = 0.40.
How K-factor changes bend allowance
For a 90° bend with R = 2 mm and T = 2 mm, K = 0.30 gives BA = 4.084 mm; K = 0.40 gives 4.398 mm. A change of 0.10 in K changes this example by about 0.314 mm.
A K-factor chart is a starting point
Charts often group values by radius-to-thickness ratio or bending method. They can be useful for estimating a first flat pattern, but the achieved result depends on the actual material and tooling. Changing V-opening can change inside radius and the flat pattern. Grain direction, material temper, thickness tolerance, punch radius, lubrication, and machine setup also matter.
Avoid copying a value without recording its convention. Some shops use bend deduction tables, bend allowance tables, or software-specific factors instead of exposing K directly. Confirm that the number is defined as neutral-axis offset divided by thickness.
Back-calculate from a controlled bend
- Cut a flat coupon of known length and record material, thickness, temper, and grain direction.
- Bend it with the production punch, die, angle, and method.
- Measure the finished outside legs and achieved inside radius with an appropriate method.
- Determine the observed bend allowance or deduction.
- Rearrange the bend-allowance equation to solve K, or tune the value in validated bend software.
- Repeat enough samples to understand process variation before releasing production data.
Store the validated factor with the whole setup—not only the material name. A value derived for 2 mm mild steel over one V-opening should not automatically be applied to another thickness, radius, die, or alloy.
Technical references
Published and formula-checked by MfgWorkbench. Last reviewed September 15, 2026. See our editorial method.
Back-calculate K from a test-bend record
The SOLIDWORKS definition uses BA = Arad(R + KT). Rearranging gives K = (BA/Arad − R)/T. You need the achieved inside radius and angle, not only their CAD targets.
Here is an invented measurement example to demonstrate the workflow, not a shop-validated material factor. Suppose a 100.00 mm flat coupon is bent through 90°, with measured thickness 2.00 mm, inside radius 2.00 mm and outside legs totaling 103.55 mm.
- Observed bend deduction: BD = 103.55 − 100.00 = 3.55 mm.
- Outside setback for each leg: OSSB = tan(45°)(2 + 2) = 4.00 mm.
- Observed bend allowance: BA = 2(4.00) − 3.55 = 4.45 mm.
- Convert angle: Arad = π/2. Then K = (4.45/(π/2) − 2)/2 = 0.4165.
The answer belongs to that recorded setup. It is not a recommendation to enter 0.4165 for every 2 mm sheet. Repeat specimens and compare the resulting flat-length errors before adopting a production table.
| Record | Why it is needed |
|---|---|
| Actual thickness and achieved radius | Both enter the equation directly |
| Flat length, outside legs and angle | Establish the measured developed-length correction |
| Alloy, temper, grain, punch and die | Define when the result may be reused |
| Measurement method and repeats | Expose uncertainty and process variation |
If the calculated K falls outside the supported range of your model or software, investigate angle convention, datum selection and measurement error before clipping it to a convenient value. Compare the dimensions in the BA-versus-BD guide.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.