18 gauge galvanized steel bend allowance and bend deduction
At a 90° bend with an inside radius of one thickness (0.0516 in / 1.31 mm) and K = 0.43, 18 gauge galvanized steel (0.0516 in / 1.31 mm) has a bend allowance of 0.1159 in (2.94 mm) and a bend deduction of 0.0905 in (2.30 mm).
| Bend angle | Included angle | BA (in) | BD (in) | BA (mm) | BD (mm) |
|---|---|---|---|---|---|
| 30° | 150° | 0.0386 | 0.0167 | 0.98 | 0.42 |
| 45° | 135° | 0.0580 | 0.0275 | 1.47 | 0.70 |
| 60° | 120° | 0.0773 | 0.0419 | 1.96 | 1.06 |
| 90° | 90° | 0.1159 | 0.0905 | 2.94 | 2.30 |
| 120° | 60° | 0.1545 | 0.2030 | 3.93 | 5.16 |
| 135° | 45° | 0.1739 | 0.3244 | 4.42 | 8.24 |
| Inside radius | R / T | K-factor | BA (in) | BD (in) | BA (mm) | BD (mm) |
|---|---|---|---|---|---|---|
| 0.0516 in | 1 | 0.43 | 0.1159 | 0.0905 | 2.94 | 2.30 |
| 0.0774 in | 1.5 | 0.43 | 0.1564 | 0.1016 | 3.97 | 2.58 |
| 0.1032 in | 2 | 0.43 | 0.1970 | 0.1126 | 5.00 | 2.86 |
| 0.1548 in | 3 | 0.43 | 0.2780 | 0.1348 | 7.06 | 3.42 |
Practical notes
The angle in these formulas is the bend angle — how far the flange turns away from flat. The included angle between the two flanges is 180° minus the bend angle, so a 90° bend is also 90° included, but a 135° bend leaves a 45° included angle.
K-factor values here come from a generic air-bending table that depends on R/T (about 0.38 below R = T, 0.43 from T to 3T, 0.50 above 3T for galvanized steel). They are approximations: tooling, die width, grain and temper all shift the neutral axis. Bend a test strip, measure the flanges and back-calculate K before cutting a production run.
A V-die opening of about 8 × T (0.4128 in / 10.49 mm) is the usual starting point. In air bending the inside radius forms at roughly 15–17% of the die opening for galvanized steel, so the radius you actually get depends on the die more than the punch — recompute with that radius if it differs from the R = T used above.
Springback for galvanized steel at modest radii is typically about 0.5–2°, increasing as R/T grows; overbend to compensate. It changes the angle, not the bend allowance calculation.
Galvanized gauge thickness includes the zinc coating, so it is thicker than uncoated steel of the same gauge number (18 ga galvanized 0.0516 in vs 0.0478 in uncoated). Use the measured coated thickness in the formula. About 1 × T is a common minimum; tighter bends risk cracking or flaking the zinc on the outside of the bend.
FAQ
What is the bend deduction for 18 gauge galvanized steel at 90°?
With R = T (0.0516 in / 1.31 mm) and K = 0.43, the bend deduction is 0.0905 in (2.30 mm). For a part with two 2 in flanges measured to the outside, the flat blank is 2 in + 2 in − 0.0905 in = 3.9095 in.
What K-factor should I use for 18 gauge galvanized steel?
For air bending galvanized steel at R = T, a K-factor of about 0.43 is a typical starting value; below R = T it drops to about 0.38, and above 3 × T it approaches 0.50. Real values depend on tooling and material lot, so confirm with a test bend.
How thick is 18 gauge galvanized steel?
18 gauge galvanized sheet is 0.0516 in (1.31 mm) including the zinc coating, versus 0.0478 in for uncoated 18 gauge steel.
Is the angle in the bend allowance formula the bend angle or the included angle?
It is the bend angle, the amount the flange is turned from flat. If your drawing gives the included angle between flanges, use 180° minus that value: a 45° included angle is a 135° bend.
Sources: Galvanized sheet gauge · Bend allowance formula BA = A·(π/180)·(R + K·T); bend deduction BD = 2·tan(A/2)·(R + T) − BA (Machinery's Handbook, sheet metal bending) · Generic air-bending K-factor table by R/T (approximate)
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