- Flat (blank) length
- 76.4 mm (3.01 inch)
- Bend allowance
- 4.4 mm (0.173 inch)
- Bend deduction
- 3.6 mm (0.142 inch)
- Outside setback
- 4 mm (0.157 inch)
- K-factor used
- 0.4
Air bending, mild steel, 2 mm: a V-die of about 16 mm gives an inside radius near 2.56 mm; keep flanges at least 11.2 mm.
How the flat length is worked out
When sheet bends, the inside is squeezed and the outside stretched. Somewhere in between is a neutral line that keeps its length. The K-factor says where it is, as a fraction of the thickness from the inside. The length of that line around the bend is the bend allowance.
- Bend allowance BA = angle × (R + K·T)
- Outside setback = (R + T) × tan(angle ÷ 2)
- Bend deduction BD = 2 × setback − BA
- Flat length = sum of outside flanges − BD for each bend
Questions
- What is the formula for bend allowance?
- Bend allowance = bend angle (in radians) × (inside radius + K × thickness). For a 90° bend in 2 mm (0.0787 inch) sheet with a 2 mm (0.0787 inch) radius and K = 0.4: 1.571 × (2 + 0.8) = 4.40 mm (0.173 inch).
- What is bend deduction?
- The length to subtract from the sum of the outside flange lengths to get the flat length. Bend deduction = 2 × (R + T) × tan(angle ÷ 2) − bend allowance. In the example above it is 3.60 mm (0.142 inch), so 50 + 30 mm (1.18 inch) flanges need a 76.4 mm (3.01 inch) blank.
- What K-factor should I use?
- About 0.33 when the inside radius is smaller than the thickness, about 0.4 for a radius of 1 to 3 times the thickness, and up to 0.5 for large radii. Bend a test piece and measure it to find your own shop's K-factor.
- Which V-die opening should I use?
- For mild steel, about 8 times the sheet thickness (6–12 times). The inside radius in air bending comes out at roughly 0.16 times the die opening.
Need parts bent? See sheet metal bending, the press brake and laser cutting for the blanks.
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Get startedResults are for planning. The real K-factor depends on the material, the tooling and the bending method; check with a test bend before cutting many blanks.