9.1 Sheet Metal Layout: Bend Radius, Setback, Bend Allowance & Sight Lines

9.1 Sheet Metal Layout: Bend Radius, Setback, Bend Allowance & Sight Lines

[!NOTE] Why an inspector needs the layout math: The IAR test does not ask you to bend metal. It asks you to check someone else's bend. A bracket fabricated with the wrong setback comes out of the brake with its flanges the wrong length, its holes out of position, and its bend radius potentially below the minimum for the alloy and temper — which is a structural defect, not a cosmetic one. Sheet metal layout questions on the IAR test are pure arithmetic with an FAA-published K-chart, and they are among the most reliably scoreable items on the exam.

Sheet metal work sits inside AC 43.13-1B, Chapter 4, Metal Structure, Welding, and Brazing. Layout is covered by paragraph 4-56 (Setback), riveting by 4-57, and the K-factors by Table 4-7. Chapter 4 also carries Table 4-6, Recommended radii for 90-degree bends in aluminum alloys, which is the source you check before accepting any bend radius on a repair.

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The Four Lines of a Bend

Every layout problem is a statement about four imaginary lines on the flat sheet. Confuse two of them and the arithmetic will be right while the part is wrong.

A useful mental picture: the mold line dimension is what the part would measure if you could fold it with an infinitely sharp corner. Real metal will not do that — it needs a radius, and forming that radius consumes material. Setback and bend allowance are the two bookkeeping entries that account for the difference.

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Setback: The AC 43.13-1B Formula

AC 43.13-1B paragraph 4-56 defines setback as the distance the jaws of a brake must be set back from the mold line to form a bend, and gives:

$\text{Setback} = K \, (BR + MT)$

where BR is the bend radius, MT is the metal thickness, and K is taken from the K-chart in Table 4-7 for the number of degrees through which the metal is bent.

The 90-Degree Case

For a 90-degree bend, K = 1.0, and the formula collapses to the rule most technicians memorize:

$\text{Setback}_{90^\circ} = BR + MT$

Paragraph 4-56 states this directly: for a 90-degree bend, the point is back from the mold line a distance equal to the bend radius plus the metal thickness.

Bends Other Than 90 Degrees

For any other angle you must go to the K-chart. Two anchor values from AC 43.13-1B worth carrying into the test room:

Notice the asymmetry: K grows without bound as the bend angle approaches 180°, because a nearly flat-folded piece of metal consumes an enormous amount of material at the corner.

The Closed-Angle Trap

This is the single most common way an IAR layout question is made hard. A "closed angle" is stated as the angle between the finished flanges, but the metal is bent through the supplement of that angle.

• Form a closed angle of 45 degrees and the metal has been bent through 135 degrees.
• So you do not use K = 0.41421. You use K = 2.4142.

AC 43.13-1B works this exact example. With a bend radius of 0.25 inch and material 0.064 inch thick:

• Open 45-degree bend: Setback = 0.41421 × (0.25 + 0.064) = 0.41421 × 0.314 = 0.130 inch
• Closed 45-degree angle (bent through 135°): Setback = 2.4142 × 0.314 = 0.758 inch

Same radius, same material, same stated "45 degrees" — and a setback that differs by nearly six-tenths of an inch. If an answer choice sits suspiciously close to the value you would get by ignoring the closed-angle rule, that choice is the trap.

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Worked Setback Problem

A mechanic fabricates a bracket from 0.032-inch 2024-T3 aluminum alloy, bent to a closed angle of 37 degrees, using a bend radius of 0.156 inch. How much setback should have been used?

1. Convert the closed angle to the bend angle: 180° − 37° = 143°.
2. Look up K for 143° in Table 4-7 (it is a large number, well above 2.4142, because 143° is closer to flat than 135° is).
3. Apply the formula: Setback = K × (0.156 + 0.032) = K × 0.188.

The examinable skill is not the arithmetic — it is step 1. Any candidate who reads "closed angle of 37 degrees" and reaches for the K-factor at 37° will produce a setback roughly an order of magnitude too small and will pick the wrong answer with confidence.

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Bend Allowance and Flat Development

Bend allowance is the length of material actually consumed in the curved portion of the bend, measured along the neutral axis. It is a function of bend radius, metal thickness, and the number of degrees of bend, and it is read from the bend allowance tables in AC 43.13-1B Chapter 4 rather than derived at the bench.

The development of a flat pattern proceeds in one direction:

   Mold line dimension  (from the drawing)
        MINUS  setback for each bend the flat is bounded by
        =====  FLAT (the true length of that straight portion)

   Total developed length  =  sum of the flats  +  bend allowance for each bend

Two consequences follow directly, and both appear as test questions:

• To find the length of a flat, subtract the setback from the mold line length. Not the bend radius; not the bend allowance. The setback is the quantity that locates the bend tangent line relative to the mold point.
• Total developed length is never the sum of the mold line dimensions. That sum is always too long, because it counts the corner material twice.

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The Sight Line

The sight line is a layout aid, not a dimension of the finished part. It is drawn one bend radius from the bend tangent line, on the side of the tangent line that will pass under the nose of the radius bar. The operator positions the sheet so the sight line is directly under the leading edge of the radius bar; when the brake closes, the bend then begins exactly at the bend tangent line.

Do not confuse the three candidate distances:

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Minimum Bend Radius: The Limit That Governs Everything Else

None of the above matters if the bend radius itself is illegal. Bending an aluminum alloy tighter than its minimum radius cracks the outer fiber, and a cracked bend in a structural bracket is a rejectable defect regardless of how neat the layout was. AC 43.13-1B Table 4-6 gives recommended radii for 90-degree bends in aluminum alloys by alloy, temper, and thickness. Two practical rules follow:

• Temper drives the radius. The same alloy in the annealed condition bends far tighter than in the -T3 or -T4 condition. A repair that substituted heat-treated stock for annealed stock and kept the original radius is suspect.
• Grain direction matters. Bending across the grain is more forgiving than bending parallel to it; a part cracked on one flange and sound on another is often a grain-direction failure, not a radius failure.

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What the IA Actually Inspects

When a fabricated part is presented on a major repair, the IA should be able to reconstruct the layout:

1. Is the bend radius at or above the Table 4-6 minimum for that alloy, temper, and thickness?
2. Do the flange lengths match the drawing once setback is accounted for? A flange short by exactly one setback is the signature of a mechanic who used mold line dimensions directly.
3. Is the bend free of cracking, orange-peel, and thinning on the outer fiber?
4. Are hole positions correct after forming? Holes located on the flat before bending shift if the setback was wrong.
5. For a closed angle, was the supplement used to pick the K-factor?

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High-Yield Exam Traps

• Closed angle means the supplement. A closed angle of 45° is a 135° bend, K = 2.4142.
• Setback for a 90° bend is BR + MT, because K = 1.0 — but only for 90°.
• The sight line is one bend radius from the bend tangent line, not one setback.
• Flat = mold line dimension − setback. Subtracting bend allowance instead is the classic wrong turn.
• Check the radius before checking the arithmetic. A perfectly calculated part bent below minimum radius is still rejectable.