Sheet Metal Bend Calculator: Compute Flat Pattern & Setback

Calculate sheet metal bend allowance, setback adjustments, and total flat layout lengths for aircraft structural fabrication.

The Mechanics of Sheet Metal Bending

In aircraft structural repair and sheet metal fabrication, flat sheet metal changes its physical properties when bent in a brake. The material on the outside of the bend stretches, while the material on the inside compresses.

Between these two zones lies a theoretical line called the neutral axis, which retains its original length without stretching or shrinking. Because the metal expands around the outside of the bend radius, you cannot simply add the final outside dimensions (mold line flanges) together to cut your stock. Doing so results in a part that is physically too long. Fabricators use a flat pattern layout to calculate the exact amount of metal consumed during the bend before making a single cut.

Key Sheet Metal Terms Explained

To prepare an accurate flat pattern layout, you must track five key mechanical variables:

• Material Thickness (T): The thickness of the sheet metal stock.
• Inside Bend Radius (R): The radius of the bend curvature measured along the inside compressed face of the metal (determined by the radius of the brake nose bar).
• Mold Line Flange Length: The total outside distance measured from the edge of a flange to the intersection point of the opposing flange (the apex or mold line point).
• Setback (SB): The distance from the mold line intersection point back to the tangent line where the metal begins to curve.
• Bend Allowance (BA): The physical length of the material contained within the curved area along the neutral axis. This is the exact amount of metal needed to form the bend.

How It’s Calculated

The calculator processes your material dimensions through these precise plaintext steps:

1. Dimension Unit Synchronization

The system matches the calculator logic to your chosen metric scale. If you select millimeters, the input placeholders adjust to standard metric stock parameters to prevent data entry confusion:

• Inches to Millimeters = Inches * 25.4
• Millimeters to Inches = Millimeters / 25.4

2. Setback Derivation

The calculator finds the setback value by adding the inside radius to the material thickness, then multiplying that sum by the tangent of half the total bend angle:

Setback = (Inside Radius + Material Thickness) * Tangent(Bend Angle / 2)

3. Bend Allowance Extraction

To find the amount of metal required to create the curve, the tool uses the standard civil aviation empirical formula (found in FAA Advisory Circular 43.13-1B). This calculation establishes the neutral axis line at roughly 44.7% of the material thickness:

Bend Allowance = (0.01743 * Inside Radius + 0.0078 * Material Thickness) * Bend Angle

4. Total Developed Flat Length Compilation

Finally, the tool calculates the true flat layout length. It subtracts the setback distance from each outside mold line flange length to isolate the flat, unbent sections, then adds the bend allowance value to join them:

Flat Length = (Flange 1 – Setback) + (Flange 2 – Setback) + Bend Allowance

Scope and Limitations

• Fixed K-Factor: The bend allowance formula uses a fixed empirical K-factor baseline (0.447) calibrated for standard aviation aluminum alloys (like 2024-T3 and 7075-T6). It does not adjust for high-temper steels, soft coppers, or specialized materials.
• Single-Axis Bends: This formula handles single-axis flat bends only. It cannot calculate compound multi-axis drawing steps or hydroformed aircraft part layouts.
• Flange Size Overlap Guardrail: If your inputs generate a setback value that is physically larger than either of your input flange lengths, the calculator will reject the entry to prevent an impossible layout.