When an aviation maintenance technician bends a sheet metal part on a brake, the metal does not simply fold along a sharp crease. Instead, the material stretches on the outside of the bend and compresses on the inside, with a neutral layer somewhere in between that neither stretches nor compresses. Unless the technician accounts for this shift in material behavior, every bent part will come out slightly wrong — either too long or too short. Two closely related calculations, the K-factor and setback, bring that reality under control and allow precise, repeatable bends that match engineering drawings on the first attempt.
These concepts appear throughout the FAA Aviation Maintenance Handbook — Airframe (FAA-H-8083-31) and are fundamental knowledge for anyone preparing for the AMT Airframe written test or Oral and Practical examination. More importantly, getting these numbers right the first time saves material, reduces scrap, and ultimately means aircraft structures are fabricated to the tolerances the design engineer intended.
The Neutral Axis and Why Metal Shifts
To understand K-factor and setback, start with what happens inside a piece of sheet metal during a bend. The outer surface of the bend is placed in tension — the metal fibers are stretched longer than they were originally. The inner surface is in compression — those fibers are squeezed shorter. Somewhere through the thickness of the sheet there is a plane where the metal experiences neither tension nor compression. This is the neutral axis (sometimes called the neutral line or neutral plane).
If the neutral axis ran exactly down the center of the sheet thickness, calculations would be straightforward. In practice, the bending process causes the neutral axis to shift toward the inside of the bend. The K-factor is the ratio that describes exactly where the neutral axis sits relative to the total material thickness. It is expressed as a decimal between 0 and 0.5, where 0.5 would mean the neutral axis is at the exact center of the sheet. In real bending operations, the K-factor typically falls between about 0.25 and 0.50, depending on the material, its temper, and the bend radius used.
Bend Allowance: Putting K-Factor to Work
The K-factor feeds directly into the calculation of bend allowance (BA), which is the length of material that is actually consumed by the curved portion of the bend. Knowing the bend allowance lets the technician calculate the total flat-pattern length needed to produce a finished part with correct leg dimensions.
The bend allowance formula used in FAA-referenced sheet metal work (AC 43.13-1B, Chapter 4) is:
BA = (N/90) × (0.01743 × R + 0.0078 × T)
In this formula, R is the inside bend radius in inches, T is the material thickness in inches, N is the bend angle in degrees, and the constants 0.01743 and 0.0078 are derived from converting degrees to radians and from the empirically established K-factor for typical aircraft aluminum alloy. The N/90 term scales the result for bend angles other than 90 degrees, since the constants are derived for a 90-degree reference bend. The constant 0.0078 is essentially 2π × K-factor / 360, reflecting where the neutral axis actually sits for standard aircraft sheet. When a different K-factor applies — for example, when working with harder alloys or very tight radii — the formula is adjusted accordingly.
A worked example helps: suppose a technician must bend 0.040-inch 2024-T3 aluminum to a 90-degree angle with a 3/16-inch (0.1875-inch) inside radius. Plugging into the standard formula:
BA = (90/90) × (0.01743 × 0.1875 + 0.0078 × 0.040)
BA = 1 × (0.003268 + 0.000312)
BA = 1 × 0.003580
BA ≈ 0.0036 inches
Because this example uses exactly a 90-degree bend, the N/90 term equals 1, so the arithmetic here mirrors a direct sum of the two terms. For bend angles other than 90 degrees, the N/90 scaling factor must be applied to get the correct arc length that wraps through the bend. That arc length must be added to the flat leg dimensions to get the correct total blank length.
Setback: Locating the Bend Tangent Lines
While bend allowance tells you how much material the bend consumes, setback (SB) tells you where the bend begins relative to the finished edge of a flange. More precisely, setback is the distance from the mold line — the theoretical intersection of the two flat leg surfaces extended — back to the bend tangent line (the point where the curved section begins).
For a 90-degree bend, the setback formula is simple:
SB = R + T
where R is the inside bend radius and T is the material thickness. Using the earlier example (R = 0.1875 in, T = 0.040 in):
SB = 0.1875 + 0.040 = 0.2275 inches
For bends other than 90 degrees, a trigonometric correction is applied:
SB = (R + T) × tan(N/2)
where N is the bend angle. This tangent function accounts for the geometry of non-right-angle bends, where the mold point shifts farther or closer to the tangent line depending on the included angle. A 45-degree bend, for instance, produces a smaller setback than a 90-degree bend for the same radius and thickness.
Flat Layout and the Sight Line
Once the technician knows setback and bend allowance, the flat blank can be laid out precisely. The process works as follows:
- Draw the mold lines corresponding to the finished outside edges of each flange on the flat blank.
- Measure inward from each mold line by the setback distance to locate the bend tangent lines. These mark where the curved bend zone begins and ends.
- Verify that the distance between the two bend tangent lines equals the calculated bend allowance. If the layout is correct, this distance should match.
- Mark a sight line — typically located half the bend allowance inward from the first bend tangent line. This is the line that lines up with the front edge of the brake die when the sheet is inserted, ensuring the brake applies pressure in exactly the right location.
The sight line is critical in practice. Many technicians make the mistake of aligning the bend tangent line with the brake instead of the sight line, which shifts the bend and produces incorrect leg dimensions.
Why It Matters in Aircraft Fabrication
Aircraft structures are designed to extremely tight tolerances. A spar web flange that is even a few hundredths of an inch too short or too long can create misalignment during assembly, require rework, or — in worst-case scenarios — introduce stress concentrations that the original design did not account for. The K-factor and setback system gives technicians a mathematically reliable way to produce correct parts from flat stock on the first attempt.
Beyond accuracy, these calculations protect the material itself. If a technician guesses at bend locations and makes a test bend that is wrong, the sheet must be scrapped — bending work-hardens aluminum, and trying to unbend and rebend risks cracking the material, especially in heat-treated alloys like 2024-T3 or 7075-T6. Getting the math right before touching the brake is therefore both economically and structurally important.
Key Numbers and Rules
- K-factor range: Typically 0.25–0.50 for aircraft sheet metal; 0.50 would mean a perfectly centered neutral axis (theoretical maximum).
- Minimum bend radius: Each alloy and temper has a minimum bend radius (expressed as a multiple of material thickness) below which cracking is likely; always consult the manufacturer's data or AC 43.13-1B.
- Standard formula constant 0.01743: Derived from converting degrees to radians (π/180 = 0.01745, rounded in practice).
- Setback for 90°: SB = R + T — the most frequently tested formula on the AMT written exam.
- Sight line position: Placed at one-half the bend allowance from the first bend tangent line, aligned with the nose of the brake die.
- Bend allowance increases with larger radius and larger bend angle; it decreases as the radius tightens toward the minimum bend radius limit.
Common Test Traps
- Confusing setback with bend allowance: Setback is the distance from the mold line to the tangent line; bend allowance is the arc length consumed by the bend itself. They are related but not interchangeable, and the exam tests both.
- Using the wrong setback formula: The simple SB = R + T applies only to 90-degree bends. For any other angle, the tangent function must be included. Students who memorize only one formula will miss non-right-angle problems.
- Forgetting to account for both setbacks on a multi-bend part: Each bend has its own setback. When a part has two or more bends, the flat blank length equals the sum of all flat leg dimensions plus all bend allowances, minus all setbacks where they overlap with the leg dimensions — careful layout prevents double-counting material.
- Aligning the brake to the bend tangent line instead of the sight line: This is the most common shop error and a known exam topic. The brake nose must align with the sight line, not the tangent line, for the bend to land in the correct location.
- Assuming the K-factor is always 0.5: The neutral axis is almost always shifted inward from center, making the K-factor less than 0.5 in real bends. Using 0.5 as a default overestimates bend allowance slightly and produces parts with legs that are a fraction too short.