The neutral layer is not necessarily at mid-thickness during bending. K-factor is its distance from the inside surface divided by thickness; it determines the developed length of the bend region. A wrong K value creates error at every bend and accumulates on multi-bend parts.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
FORMULA AND EXAMPLE
Bend allowance BA=(π/180)×A×(R+Kt). For 90°, R=2 mm, t=2 mm and K=0.35, BA≈1.5708×2.7=4.24 mm. With an incorrect K=0.50, BA≈4.71 mm—a 0.47 mm error in one bend. Bend deduction BD=2×OSSB−BA, where OSSB=(R+t)tan(A/2). Software may use K, BA, BD or a bend table, but do not mix uncalibrated rules on one part.
BUILD A BEND TABLE
1. Group by material, temper, thickness, grain, V opening and target angle; do not use one K-factor plant-wide. 2. Bend coupons of known length, measure external legs, angle and actual R, then back-calculate BA/BD. 3. Use at least five parts per group and record mean/range; monitor lot and tool wear through first-off data. 4. CAD unfold settings, brake program and inspection drawing must use the same bend-table revision.
PROJECT CHECKLIST
Final dimension scheme; material/thickness; R/V; angle; K/BD source; bend-table revision; measurement temperature and method.