Free Engineering Tool
Reverse-calculate the sheet-metal K-factor from a measured flat blank and formed flanges, or use a known K-factor to predict the flat length for a bend.
This is the upstream calibration tool for bend allowance and bend deduction. A measured trial bend is more reliable than applying one material table to every tool and radius.
The K-factor in sheet metal, also called the neutral-axis factor or K-value, describes where the unstretched neutral axis sits inside the material as a fraction of sheet thickness. It is used in bend allowance: BA = (π/180) × α × (R + Kt), where R is inside radius, t is thickness and α is bend angle. Typical air-bending starting values are about 0.33 for mild steel and aluminium and 0.35 for stainless steel, but these ranges depend on temper, radius-to-thickness ratio, tooling, bottoming and springback. To reverse-calculate K, measure a flat blank and both formed flanges to the mold line, derive BD = flange 1 + flange 2 − flat, then BA = 2 × OSSB − BD and K = ((BA ÷ angle in radians) − R) ÷ t. Zhejiang Balford Mechatronics Co., Ltd. reviews formed-part projects at its 12,000 m² facility and uses trial parts to confirm critical flat patterns.
The calculated K-factor is within the common 0.10-0.50 range.
K-factor is process-specific. It changes with material, temper, radius-to-thickness ratio, bending method and tooling. Use the reverse mode on a measured trial part when accuracy matters.
The reverse mode derives bend allowance from the measured blank and formed flanges, then solves for the neutral-axis position.
| Material | Starting K | Minimum radius guide | Use note |
|---|---|---|---|
| Mild / low-carbon steel | 0.33 | About 1t | Verify with the actual V-die |
| Stainless steel 304 | 0.35 | About 1.5t-2t | Springback and work hardening matter |
| Aluminium 5052 | 0.33 | About 1t | Temper and grain direction matter |
| Aluminium 6061-T6 | 0.33 | About 2t-3t | Higher cracking risk at tight radius |
| Copper, soft | 0.35 | About 1t | Confirm temper with a trial bend |
| Brass | 0.35 | About 1t | Temper dependent |
These values are starting points for air bending, not production constants. Reverse-calculate K from a representative part whenever flat-pattern accuracy matters.
A 2 mm mild-steel test bend uses a 3 mm inside radius and a 90° angle. The formed flanges measure 50 mm and 30 mm to the mold line, and the measured flat blank is 75.76 mm. BD = 80.00 − 75.76 = 4.24 mm. OSSB = 5.00 mm, so BA = 10.00 − 4.24 = 5.76 mm. K = ((5.76 / 1.5708) − 3) / 2 = 0.333.
The K-factor is the position of the neutral axis as a fraction of sheet thickness. A value of 0.5 places the neutral axis at mid-thickness; typical air-bending values are lower because the neutral axis shifts toward the inside of the bend as the outer fibres stretch.
Measure the flat blank length and both formed flange lengths to the mold-line intersection. Find bend deduction as BD = flange 1 + flange 2 − flat. Then BA = 2 × OSSB − BD and K = ((BA / bend angle in radians) − R) / t.
Common air-bending starting values are around 0.33 for mild steel and aluminium and 0.35 for stainless steel. These are not constants. Material, temper, radius-to-thickness ratio, tooling, bottoming and the press-brake controller convention can change the effective value.
Check the flange measurement convention first. Using tangent-point dimensions in a formula intended for mold-line dimensions will shift the result. Also verify inside versus outside radius, actual bend angle, blank measurement and whether the part was bottomed.
No. K-factor locates the neutral axis within the thickness. Bend allowance is the arc length of that neutral axis through the bend. Bend allowance therefore depends on K-factor, radius, angle and thickness.
Send the drawing, material, tooling, formed-leg measurements and blank measurement from the trial part. Send the drawing to shawn@balford.net. We normally reply within 24 hours, China Standard Time.