Sheet Metal Bend Radius & K-Factor Chart

The dominant variable for K is R/t, not the material. The neutral axis shifts inward as the bend gets tighter, from K ≈ 0.21 at R/t = 0.1 to the 0.50 asymptote at very large R/t. Pick K from R/t first, then calibrate against a test bend on the actual stock and tooling. A 0.05 error in K produces roughly 0.3 mm of deviation per bend on 3 mm stock — four bends in series and the part is 1.2 mm off.

K-factor by material is genuinely contested. Machinery's Handbook gives cold-rolled steel K = 0.45; production fabricators use 0.38 for the same material. Both are published in good faith: the difference is bending method (bottoming and coining versus air bending) and R/t. Both columns are shown rather than averaged.

Minimum bend radius is the other place the sources conflict. For mild steel one fabricator source says 0.5 t while two design guides say 1.0 t; for 304 stainless the spread is 0.5 t to 1.5 t; for copper it runs from 0 t (soft) to 1.0 t. Rather than average — which would produce a number no source supports and no fabricator would honour — both positions are shown. The one place a design guide was rejected is 6061-T6, where it claims 1.5 t against 2.0 t from two other sources; 8–10% elongation makes 2.0 t the defensible figure.

Terminology trap. Some guides label the bend-radius multiplier itself "k factor" (min radius = k × t). That is not the K-factor. The K-factor is the neutral-axis ratio and is always ≤ 0.5. The two are unrelated quantities that happen to share a letter.

Grain direction: bending parallel to the rolling direction stretches the weak axis. Add 50–100% to the minimum radius for with-grain bends, or rotate the flat pattern. Feature clearance: keep holes and slots at least 2.5 × t + bend radius from the bend line, measured to the near edge, or they distort. Springback: expect 1–3°, so a 90° bend is typically overbent to about 87° in cold-rolled steel or 88° in 5052.

Thickness matters too. Minimum radius grows with thickness even in the same alloy. A working rule for steel: up to about 6 mm use 0.8–1.0 t, 6–12 mm use about 1.2 t, above 12 mm expect 1.5 t or more. And the bend takes the punch tip radius plus springback correction — if the theoretical minimum is 1.0 mm but the smallest tip on the floor is 1.5 mm, you get 1.5 mm.