When an aviation maintenance technician fabricates a sheet metal part — a bracket, a rib flange, or a repair doubler — the finished piece must match the engineering drawing exactly. That precision starts long before the first bend is made, during the layout phase, when the technician marks out the flat blank from which the part will be formed. Get the layout wrong and the finished part will be too short, too long, or cracked at the bend radius. Understanding bend allowance — how much material is consumed as metal curves around a radius — is therefore one of the most critical skills tested on the FAA AMT Airframe Knowledge Test and applied every day on the shop floor.
This article walks through the complete process: reading the relevant terms from a drawing, calculating bend allowance, determining setback, locating mold lines and bend tangent lines, and laying out the flat blank correctly so that the formed part matches its design dimensions.
Essential Terms and Geometry
Before numbers can be crunched, the vocabulary must be precise. Every bend has a geometry that the following terms describe:
- Bend radius (R): The radius of curvature measured on the inside of the bend. Engineering drawings specify minimum bend radii for each alloy and temper to prevent cracking. For most 2024-T3 aluminum, the minimum bend radius is approximately equal to the material thickness for thin sheets, but always check the specific chart in the aircraft structural repair manual or the applicable specification.
- Thickness (T): The actual measured thickness of the sheet stock, not the nominal designation.
- Bend angle (°): The number of degrees through which the metal is bent. A 90° bend is the most common. A 45° bend consumes less material than a 90° bend of the same radius.
- Mold line (ML): An imaginary extension of the flat surface of the metal beyond the bend, as though the bend did not exist. Mold lines intersect at the mold point, which is the theoretical sharp corner of the part.
- Setback (SB): The distance from the bend tangent line (where the flat surface ends and the curve begins) back to the mold line. Setback is subtracted from the mold-line dimensions on the drawing to locate the bend tangent lines on the flat blank.
- Bend tangent line (BTL): The actual line on the flat blank where the curve of the bend begins and ends. Material between two bend tangent lines on the same bend is the curved portion — the bend allowance region.
- Flat (leg length): The straight, unbent portion of the part between features. Flat lengths are calculated by subtracting setback from mold-line dimensions.
How Bend Allowance Is Calculated
When sheet metal bends, the neutral axis — a plane running through the thickness of the material — neither stretches nor compresses. Metal on the outside of the bend stretches; metal on the inside compresses. The neutral axis is located approximately 0.445 times the material thickness from the inside surface (often approximated as 44.5% of T from the inside), though many practical references simply place it at the mid-thickness as a close approximation for thin sheets.
The bend allowance is the arc length along the neutral axis through the bend angle. The formula is:
BA = (0.01745 × R + 0.0078 × T) × °
Where R is the inside bend radius in inches, T is the material thickness in inches, ° is the bend angle in degrees, and 0.01745 is the radian equivalent of one degree (π/180). The constant 0.0078 accounts for the neutral-axis offset (approximately 0.4444 × π/180 ≈ 0.0078). This formula is the standard one provided in the FAA Aviation Maintenance Technician Handbook — Airframe (FAA-H-8083-31).
Worked Example
Suppose you need to bend 0.040-inch 2024-T3 aluminum sheet to a 90° angle with an inside radius of 3/16 inch (0.1875 in).
- BA = (0.01745 × 0.1875 + 0.0078 × 0.040) × 90
- BA = (0.003272 + 0.000312) × 90
- BA = 0.003584 × 90
- BA = 0.3226 inches
That means roughly 0.323 inches of flat stock will be consumed by the bend itself and must be added to the flat leg lengths when cutting the blank.
Calculating Setback
Setback tells you how far back from the mold point to draw the bend tangent line. For a 90° bend:
SB = R + T
For angles other than 90°, the formula becomes:
SB = tan(°/2) × (R + T)
Using the example above: SB = tan(45°) × (0.1875 + 0.040) = 1.0 × 0.2275 = 0.2275 inches. For a 90° bend, tan(45°) = 1, so SB simply equals R + T, which is the shortcut most technicians memorize.
Laying Out the Flat Blank
Once bend allowance and setback are known, the flat blank length can be determined. The total blank length equals the sum of all flat portions plus the bend allowance for each bend. Here is the step-by-step process:
- Identify mold-line dimensions from the drawing. These are the outside dimensions to the theoretical sharp corners.
- Calculate setback for each bend using the formula above, then subtract one setback per leg end from the mold-line dimension to get each flat (leg) length. For a simple two-leg angle with one 90° bend: Flat A = ML-A − SB; Flat B = ML-B − SB.
- Sum all flats and all bend allowances. Total blank length = Flat A + BA + Flat B (for one bend). For multiple bends, add each BA and each flat in sequence.
- Mark the blank. On the cut sheet, measure from one edge to locate the first bend tangent line (equal to Flat A), then span the bend allowance distance to locate the second bend tangent line. The brake's nose is aligned to the first bend tangent line.
- Verify grain direction. Aluminum sheet is rolled, creating a grain direction. Bends made across the grain (bend line perpendicular to the grain) are stronger; bends made parallel to the grain are more prone to cracking. When possible, lay out parts so bends are across the grain.
Why Bend Allowance Matters for Airworthiness
A part fabricated without proper bend allowance calculation will not fit in its designed location, may stress adjacent structure when forced into place, or may develop cracks at under-radiused bends. Cracks in primary structure are an airworthiness concern addressed throughout FAA Advisory Circulars and the structural repair manuals. Over-bending to compensate for a short blank work-hardens the metal further and can cause immediate cracking or fatigue cracking in service. Under-bending leaves a part that does not mate flush with the skin, creating stress risers at fastener holes and compromising the load path the designer intended.
The minimum bend radius specified for each alloy-temper combination is non-negotiable from a structural standpoint. For example, softer tempers like 2024-O (annealed) can tolerate tighter radii than the hardened 2024-T3. The AMT must always reference the applicable aircraft structural repair manual or the bend radius chart in FAA-H-8083-31 before selecting tooling.
Key Numbers and Rules
- Neutral axis offset: approximately 0.445 × T from the inside surface.
- Bend allowance formula: BA = (0.01745 × R + 0.0078 × T) × degrees of bend.
- Setback (90°): SB = R + T (shortcut; exact formula uses tan of half the bend angle).
- Total blank length: Sum of all flats + sum of all bend allowances.
- Grain direction: Orient bends across the grain whenever possible to reduce cracking risk.
- Minimum bend radius: Always check the alloy-and-temper-specific chart; never assume a single value applies to all materials.
- Springback: Metal tends to partially unbend after the press brake is released. Overbend slightly (the exact amount depends on alloy and radius) to compensate; this is typically accounted for in tooling selection and experience rather than the flat-blank calculation itself.
Memory Aid
"Flats Plus Arcs Build the Part" — a simple phrase to remember the structure of the flat-blank calculation: every dimension on the finished part is either a flat (straight leg, found by subtracting setbacks from mold-line dims) or an arc (bend allowance, calculated with the BA formula). Add all the flats, add all the arcs, and you have the total blank length. Nothing else goes into it.
Common Test Traps
- Confusing inside radius with outside radius: The bend allowance formula uses the inside radius. If a drawing shows the outside radius, subtract one material thickness to get R before plugging into the formula.
- Forgetting to subtract setback twice for a single bend: Each 90° bend removes one setback from each adjacent leg. A two-leg part with one bend has two setback subtractions (one per leg end touching the bend), not one.
- Using the bend angle rather than half the angle in the setback formula: The formula is tan(half the bend angle) times (R + T). At 90° this equals tan(45°) = 1, masking the error — but at 45° bends the mistake produces a wrong answer.
- Ignoring grain direction: Test questions ask which orientation reduces the risk of cracking. The correct answer is that the bend line should be perpendicular to the grain (i.e., bending across the grain).
- Treating bend allowance as negligible for thin stock: Even for 0.020-inch aluminum, the bend allowance on a 90° bend with a 1/8-inch radius is about 0.22 inches — significant on a small bracket. Always calculate; never eyeball.