A sheet metal panel came out 2 mm short across three bends. The part was 1.5 mm steel, 90-degree bends, 12 mm bend radius on the tooling. The press brake operator compensated by adjusting the back gauge. The next batch was 2 mm short the other way. The operator adjusted again. The parts were inconsistent. The customer blamed the operator and the machine. The problem was the K-factor in the flat pattern.
The K-factor is a guess until you measure it
When you unfold a bent part, the flat length depends on where the neutral axis sits. The K-factor is the ratio of the neutral axis position to the material thickness. Textbook values: 0.33 for air bending, 0.44 for bottoming, 0.5 for a perfect 50% neutral axis. The actual K-factor depends on the material, the bend radius, the tooling, and the grain direction. A 1.5 mm steel with 12 mm radius (a large radius relative to thickness) has a K-factor closer to 0.4, not 0.33. If the CAD model used 0.33, every bend is calculated 0.7 mm short. Three bends, 2 mm short. The math was wrong in the model.
The CAD package’s default K-factor is a starting point, not a truth. It’s the value the software vendor chose for a typical case. For this material (1.5 mm CRS), the correct K-factor was 0.38. We measured it: bend a 100 mm test piece at 90 degrees, measure the flat length, work the K-factor back out of the bend allowance formula. The corrected K-factor fixed the 2 mm error in one batch.
The measurement that settles it
To measure the K-factor for your material and tooling:
- Cut a test strip 100 mm wide, 200 mm long. Mark it at 100 mm from one end.
- Bend it 90 degrees at the mark.
- Measure the outside dimension from the bend line to the end (the “outside leg”).
- Calculate the bend allowance: BA = outside dimension – (100 mm – bend deduction).
- Solve for K = (BA – π × R × θ/180) / (2 × T × θ/180) … or just use the software’s K-factor calculator with the measured bend allowance.
The result for this material: K = 0.38. The CAD was set to 0.33. The 15% difference in K translates directly to the flat length error. Correcting it in the model fixed every future batch.
The grain direction matters
Sheet metal bends differently with and across the grain. Bending across the grain (perpendicular to the rolling direction) cracks more easily but holds a tighter radius. Bending with the grain gives a more consistent bend allowance. If the part has bends in both directions (this panel did — three bends, two directions), the K-factor varies by direction. The difference is usually 2-4% of the bend allowance. For a ±0.5 mm tolerance part, it matters. The fix: orient the part so the critical bends are across the grain, and set the K-factor for the dominant direction.
The springback curve
The K-factor controls the flat length. Springback controls the bend angle. They’re separate. The panel’s 2 mm error was a length error (K-factor). The operator’s adjustment was trying to fix it with angle compensation (springback) — the wrong tool. If a part comes out short, check the flat length first, not the bend angle. Measure the outside dimension against the drawing. If the outside dimension is right but the angle is 92 degrees, it’s springback. If the outside dimension is 2 mm short, it’s the K-factor. Don’t adjust the angle for a length error.
For production consistency, use a bend deduction chart for your specific material and thickness. Print it, tape it to the brake, and use it. The chart turns the K-factor question into a lookup. The operator stops guessing, the parts come out right the first time, and the rework disappears.
The K-factor is a measured value, not a software default. The 2 mm short panel was unfolded with K=0.33 when the real material ran at 0.38. Measure a test bend, set the K-factor in the model, and separate springback from length error. The operator’s back gauge adjustments were fixing the wrong variable. Get the flat pattern right and the brake follows.
