Flat Length: Why Your Blanks Come Out Short

The cause: during bending, metal in the bend zone stretches. The total flat blank length does not equal the sum of the leg lengths on the drawing.

The formula

Arc length at each bend:

BA = π/180 × bend angle × (R + K × t)

R = inside radius
t = sheet thickness
K = neutral-axis position factor

Flat length = sum of straight sections + sum of all BA values.

Choosing K

K is not a constant. It shifts with the R/t ratio:

R/t < 1 K ≈ 0.33 small radius, soft material
R/t = 1 – 3 K ≈ 0.40 the most common case
R/t > 3 K ≈ 0.45 large radius, hard material

These are empirical values. For parts with tight tolerances, the reliable route is still to bend one test piece and back-calculate K from it.

On the shop floor

• Parts with several bends: error accumulates. Three bends off by 0.5 mm each is 1.5 mm.
• Changing dies or steel grade: K changes, so run a new test bend.
• Production runs: sacrificing one blank to a test costs far less than scrapping the batch.