Bending and forming
Flat blank length for a bent part (equal bends)
- where
- bend allowance per bend (mm)
- bend angle (°)
- inside bend radius (mm)
- K-factor
- material thickness (mm)
- outside setback (mm)
- bend deduction per bend (mm)
- flat blank length (mm)
- sum of outside flange dimensions (mm)
- number of bends
Explain this equation
Works out the flat length to cut for a part with several identical bends, from its outside dimensions.
Add up the outside dimensions of every flange as drawn. Each bend makes the real part shorter than that sum by the bend deduction BD, which is found from the bend allowance and outside setback. Taking n deductions off gives the flat blank.
The neutral line keeps its length, so the blank equals the straight lengths plus the bend allowances, which is the same as the outside sum minus the deductions.
Sense check: a 50 × 50 × 50 channel (150 mm total) in 2 mm with two 90° bends and BD ≈ 3.57 needs a blank of about 142.9 mm.
Watch out: all bends must have the same angle, radius and thickness for this version. Mixed bends need adding one at a time.
Cut length of flat sheet or strip for a channel, angle or tray with several bends of the same angle and radius, from the outside dimensions on the drawing.
Enter your values to see the result.
Good to know
Add up the outside dimensions of every flange as drawn (for a 50 × 50 × 50 channel that is 150 mm), then subtract one bend deduction for each bend. This gives the same answer as adding the straight flats plus a bend allowance per bend. Dimensions to the inside faces need converting to outside dimensions first (add T for each bend at each end of that flange).
Common mistake
Measuring flanges to the inside of the bend on some legs and to the outside on others. Pick outside dimensions throughout.
Limits
All bends must share the same angle, radius and K-factor. For mixed bends work each bend separately with the bend allowance calculator.
Sources
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the welding procedure specification (WPS), BS EN ISO 15614/9606, BS EN 1090, BS EN 1993 and manufacturers’ data. Structural welds need qualified welders and engineered designs.