Glossary term
K-Factor
A sheet-metal bending factor that locates the neutral axis through the material thickness for bend allowance calculations.
Definition
metricIn sheet-metal bending, the K-factor is the ratio between the distance from the inside bend surface to the neutral axis and the material thickness.
The K-factor is used to calculate bend allowance and flat-pattern length before forming. During bending, the inner surface is compressed and the outer surface is stretched. Between them lies a neutral axis whose length is assumed not to change. The K-factor locates that neutral axis as a fraction of thickness. Its value depends on material, thickness, bend radius, tooling, forming method, grain direction, lubrication, and plastic strain distribution.
The K-factor is used in sheet-metal flat-pattern development. When sheet metal is bent, material near the inside of the bend is compressed and material near the outside is stretched. Somewhere through the thickness is a neutral axis where the length is assumed to remain unchanged. The K-factor defines the location of that neutral axis:
where d is the distance from the inside bend surface to the neutral axis and t is material thickness. If K = 0.33, the neutral axis is one third of the thickness from the inside surface.
Bend allowance
The K-factor appears in bend allowance calculations. A common form is:
where BA is bend allowance, \theta is bend angle in radians, R is inside bend radius, t is thickness, and K is the K-factor. The bend allowance is added to flange lengths to estimate the flat blank length needed before forming.
Why it varies
The K-factor is not a universal material constant. It changes with inside radius-to-thickness ratio, material ductility, yield strength, work hardening, tooling, forming process, grain direction, lubrication, and whether the bend is air formed, bottomed, coined, roll formed, or stretch formed. Tight bends tend to shift the neutral axis toward the inside surface. Larger bend radii often produce values closer to the middle of the thickness.
Production shops often determine K-factor empirically from test bends because real forming includes springback, tool compliance, anisotropy, and local thinning. CAD defaults are useful starting points, but they are not a substitute for calibrated process data.
Common mistakes
A common mistake is using one K-factor for every material and bend geometry. That can produce flat patterns that miss flange length, hole position, edge distance, and assembly tolerance. Another mistake is confusing K-factor with bend deduction or bend allowance. They are related, but they are not the same quantity. Good manufacturing documentation states material, thickness, bend radius, tooling, bend angle, grain direction, and the source of the K-factor.