Sheet Metal Structures: Theory and Layout

Why Sheet Metal Layout Matters

Before a single cut is made or a rivet is driven, every aircraft sheet metal repair or fabrication job begins with careful layout and calculation. Errors made on paper cost only time; errors cut into expensive 2024-T3 aluminum — or, worse, installed in an aircraft — cost money, airworthiness, and potentially lives. AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, Chapter 4, is the authoritative FAA reference for sheet metal work and must be consulted throughout this lesson.

Material: 2024-T3 Aluminum

2024-T3 is the most common aircraft structural aluminum alloy. The designation means: 2024 alloy (copper-based), T3 temper (solution heat-treated, cold-worked). It is strong, relatively lightweight, and has predictable forming characteristics. Because it work-hardens quickly, it must not be over-bent or re-bent without proper annealing. Always check the drawing for the correct alloy and temper before ordering or cutting stock.

Bend Allowance

When sheet metal is bent, the material on the inside of the bend compresses and the material on the outside stretches. Somewhere through the thickness is the neutral axis — the line that neither compresses nor stretches. The bend allowance (BA) is the length of material consumed by the bend along the neutral axis.

The K-factor describes the location of the neutral axis as a fraction of the material thickness. For most aircraft aluminum bent in the shop, a K-factor of 0.4173 (approximately 0.4) is used for bends with a bend radius equal to or greater than the material thickness. Your textbook or AC 43.13-1B tables give K-factors for other conditions.

Bend Allowance Formula:

```

BA = (0.01745 × R + 0.0078 × T) × Degrees of bend

```

Where:

• R = inside bend radius (inches)
• T = material thickness (inches)
• 0.01745 = π ÷ 180 (converts degrees to radians)
• 0.0078 ≈ 2π × K ÷ 360 (the K-factor constant for typical aircraft aluminum)

For a 90° bend this simplifies to:

```

BA = (0.01745 × R + 0.0078 × T) × 90

```

Setback

Setback (SB) is the distance from the bend tangent line (where the flat portion ends) back to the mold line (the imaginary extension of the flat surface). For a 90° bend:

```

SB = R + T

```

For bends other than 90°, use:

```

SB = tan(angle ÷ 2) × (R + T)

```

Setback is subtracted from the mold-line dimensions to locate the bend tangent lines on the flat pattern.

Flat-Pattern Development

The flat-pattern length is the total developed length of the part before bending. For a part with multiple flanges:

```

Flat pattern = (sum of all flat portions) + (sum of all bend allowances)

```

Flat portions are measured between bend tangent lines, not between mold lines. The process:

1. Identify each flat leg dimension from the drawing (mold-line to mold-line).

2. Calculate setback for each bend; subtract one SB per bend from each adjacent leg to get the flat leg length.

3. Calculate bend allowance for each bend.

4. Add all flat legs and all BAs.

Rivet Identification

Rivets are identified by their head style, material, shank diameter, and shank length using AN (Air Force-Navy) and MS (Military Standard) numbering systems.

Common head styles:

• AN470 — Universal head (most common structural rivet)
• AN426 — 100° countersunk (flush) head
• AN430 — Round head
• AN441 — Flat head

Material codes (dash letter after the AN number):

• AD — 2117-T4 aluminum (most common; driven cold)
• D — 2024-T4 aluminum (must be refrigerated or "ice-box" rivet)
• DD — 2024-T3 aluminum
• B — 5056 aluminum (for magnesium structure)
• A — 1100 or 3003 aluminum (non-structural)

Shank diameter is given in 1/32-in increments as the first dash number (e.g., -4 = 4/32 = 1/8 in).

Shank length is given in 1/16-in increments as the second dash number (e.g., -6 = 6/16 = 3/8 in).

Example: AN470AD4-6 = universal head, 2117-T4 aluminum, 1/8-in diameter, 3/8-in long.

Rivet Spacing Rules

Two critical spacing dimensions govern structural integrity:

• Edge distance (ED): The distance from the center of a rivet to the nearest edge of the sheet. Minimum ED = 2× the rivet diameter; preferred = 2.5×D.
• Pitch (spacing): Distance center-to-center between rivets in a row. Typical range: 3D minimum to 12D maximum for structural joints.
• Transverse pitch (gage): Distance between rows, typically 2.5D to 5D.

Always verify spacing requirements against AC 43.13-1B tables and the specific repair drawing.

Safety

• Wear safety glasses at all times when scribing, cutting, or drilling sheet metal — chips and scribe tips are eye hazards.
• Deburr all cut edges before handling; freshly cut aluminum is razor-sharp.
• Clamp stock securely before scribing to prevent movement and inaccurate layout.
• Never cut material until an instructor has verified the flat-pattern layout — material is expensive and the aircraft structure depends on accuracy.
• Apply lockout/tagout procedures on any power tools or equipment before changing blades or tooling.