The calculator
Runs in your browser. Inches or millimetres at the top. Dimensions are mold line dimensions, to the outside corners the part would have with no radius, as drawings usually give them. Start from the left edge of the part.
How the layout works
Mold lines, setback and flats. A drawing dimensions a formed part to its mold lines — the outside corners it would have if it bent to a sharp edge. The metal actually turns on a radius, so each bend shortens the straight part of the flanges either side of it by the setback: SB = K(R + T), with K = tan(½ × bend angle). At 90°, K is 1 and the setback is simply R + T. Each flat is its mold line dimension minus a setback for every bend at its ends — the middle flat of a channel loses two.
Bend allowance. The curved part needs its own length of material: the length of the neutral axis through the bend. The FAA Airframe Handbook gives an empirical formula that holds “for any degree of bend from 1° to 180°”: BA = (0.01743R + 0.0078T) × N, N in degrees. Its constants put the neutral axis about 0.45 of the thickness in from the inside of the bend. The alternative assumes it at mid-thickness. Or measure your own.
Total developed width is the sum of the flats and the bend allowances: the size to cut the blank. The Handbook's sanity check: it must come out smaller than the sum of the mold line dimensions, because the metal follows the radius instead of the corner. Bend deduction, 2 × SB − BA, is the same fact for one bend.
The sight line. The bend has to start exactly on the bend tangent line. The Handbook's method: draw a line one bend radius inside the tangent line that goes under the brake's nose bar, and line it up with the edge of the radius bar. The common mistake, it says, is drawing it in the middle of the bend allowance instead. The calculator gives the position for either way round in the brake.
Two things called “K-factor”. In the FAA Handbook and AC 43.13-1B, K is the setback factor, tan(½ × angle) — 1 at 90°, 2.414 at 135°. In CAD software and most online calculators, “K-factor” means the neutral-axis position as a fraction of thickness — typically 0.3 to 0.5. Same name, unrelated numbers. This page calls the second one k, and never mixes them.
Worked example — the Handbook's U-channel
FAA-H-8083-31, pp. 4-62 to 4-67: a channel in 0.040" 2024-T3, mold line dimensions 1", 2" and 1", two 90° bends.
- Radius: the Handbook takes 0.16" from the manufacturer's minimum-radius chart. AC 43.13-1B Table 4-6 recommends 3t to 5t for 2024-T3 at this thickness, so 0.12" or more: 0.16" passes.
- Setback: K = 1 at 90°, so SB = 0.16 + 0.040 = 0.200".
- Flats: 1.000 − 0.200 = 0.800"; 2.000 − 2 × 0.200 = 1.600"; 0.800" again.
- Bend allowance: (0.01743 × 0.16 + 0.0078 × 0.040) × 90 = 0.279" each.
- TDW: 0.8 + 1.6 + 0.8 + 2 × 0.279 = 3.758", under the 4.000" of mold lines.
3.758". The Handbook rounds the bend allowance to 0.27" and prints 3.74"; the 0.018" difference is that rounding, twice. Sight lines: with the first flange clamped, one radius in from the first tangent line, at 0.800 + 0.160 = 0.960" from the edge.
Measure your k from a test bend
The Handbook formula is an average. A strip of the same sheet, bent on the same brake and nose bar, gives the real figure: cut it to a known length, bend it once, and measure both legs to the outside.
Limits — when NOT to use this
Flat-pattern geometry for straight bends in sheet.
- The drawing or SRM governs. Where a repair drawing gives a flat pattern, radius or bend sequence, use it.
- The radius check is AC 43.13-1B Table 4-6, read at the next tabulated thickness up. Manufacturers publish their own minimum-radius charts, sometimes stricter: the Handbook's example uses 0.16" for 0.040" 2024-T3 where the AC's lowest recommendation is 0.12".
- Straight bends only. Joggles, flanged holes, stretched or shrunk flanges are other calculations.
- Bends past about 150° are where the formulas are weakest, and where the setback grows fastest. Make a test piece.
- The formulas assume constant thickness. The Handbook notes the bend area thins 10 to 15%. The empirical formula allows for that; the mid-thickness assumption does not.
- Grain and relief holes. AC 43.13-1B par. 4-55: drill relief holes at the ends of bend lines and at corners, and bend across the grain where possible.
Questions people actually ask
What is the bend allowance formula?
The FAA Airframe Handbook's: BA = (0.01743 × R + 0.0078 × T) × N, where R is the inside radius, T the thickness and N the bend in degrees. For 90° with R = 0.16" and T = 0.040" it gives 0.279".
What is the difference between bend allowance and bend deduction?
Bend allowance is the length of material in the bend itself. Bend deduction is how much shorter the blank is than the two mold line dimensions added together: twice the setback, minus the bend allowance. Same geometry, two bookkeeping methods.
How do I find the setback for a bend that is not 90°?
Multiply (R + T) by K = tan(½ × bend angle). AC 43.13-1B Table 4-7 lists it: 0.41421 for 45°, 2.4142 for 135°. Its own example: 0.064" sheet, R = 0.25", bent 45°, setback 0.130".
Is the bend angle the angle between the flanges?
No. The bend angle is how far the metal turns from flat. A bracket with 120° between its flanges has a 60° bend. The calculator takes either; choose which you are entering.
Where do I draw the sight line?
One bend radius inside the bend tangent line that goes under the brake's nose bar. Not in the middle of the bend allowance — the Handbook calls that the common mistake.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4, pp. 4-59 to 4-67 — terminology (Figure 4-120), setback SB = K(R + T) with K = tan(½ angle), flats, the two bend allowance formulas, total developed width and its check, the U-channel example, and the sight line (Figure 4-131).
- FAA AC 43.13-1B CHG 1, Chapter 4, par. 4-55 and 4-56, Tables 4-6 and 4-7 — recommended radii for 90° bends by alloy, temper and thickness; the K-chart for setback, with the 45° and 135° examples; relief holes and grain direction.