Bend Allowance & K-Factor.
Calculate flat blank size for sheet metal press brake forming. Bend allowance, bend deduction, K-factor, and outside setback for any angle, thickness, and inside radius.
OSSB = tan(α/2) × (R + T)
BD = 2·OSSB − BA
Flat = A + B − BD
Typical K-factor reference
| Material | Soft (R<T) | Medium (R≈T) | Hard (R>2T) |
|---|---|---|---|
| Mild steel — CRCA | 0.33 | 0.42 | 0.50 |
| Mild steel — HR | 0.38 | 0.45 | 0.50 |
| Stainless steel 304/316 | 0.40 | 0.45 | 0.50 |
| Aluminium — soft (5052, 6061-O) | 0.33 | 0.40 | 0.45 |
| Aluminium — hard (6061-T6) | 0.38 | 0.42 | 0.50 |
| Copper / brass — annealed | 0.35 | 0.40 | 0.45 |
Sheet Metal Bend Allowance
When sheet metal is bent, the metal stretches on the outside and compresses on the inside. The neutral axis (where length is unchanged) shifts toward the inside. Bend allowance accounts for this when calculating the flat blank size.
Bend Allowance Formula
BA = (π/180) × A × (R + K × T)
Where A = bend angle (degrees), R = inside bend radius, T = material thickness, K = K-factor (location of neutral axis as fraction of T).
Typical K-Factor Values
- Soft brass / copper: 0.35
- Aluminum (annealed): 0.40
- Mild steel: 0.42-0.45
- Stainless 304: 0.45
- Hard / cold-rolled: 0.50
Worked Example
1.5 mm mild steel, 90° bend, inside radius 2 mm, K = 0.44:
- BA = (π/180) × 90 × (2 + 0.44 × 1.5) = 1.5708 × 2.66 = 4.18 mm
- For an L-bracket with arms 50 mm and 30 mm, flat blank = 50 + 30 + 4.18 = 84.18 mm
Springback Compensation
After release, sheet metal springs back partially. Typical springback:
- Mild steel, 90° air bend: ~2°
- Aluminum 5052-H32: ~3°
- Stainless 304: ~3-4°
- High-strength steel: ~5-8°
Solution: overbend by springback amount, or use coining (high force, plastic deformation throughout).
Bend Radius Limits
Minimum inside bend radius depends on material ductility:
- Soft aluminum, copper: R = 0 (zero bend)
- Mild steel: R = T (1× thickness)
- Stainless 304: R = T to 1.5T
- High-strength steel: R = 2T to 4T
Related Tools
For material weight estimation, see Material Weight Calculator. For dimensional verification, Tolerance Stack-Up.