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newSchoolWorks.com › VoTechWorks › Transportation › Aviation Maintenance › Airframe Systems and Structures › Lesson 4
TRN-AVIA22-02 · Lesson 4 · 56 clock hours · lab

Sheet Metal Fabrication and Riveting

Objective. Students will fabricate a multi-piece aluminum assembly using hand-formed bends, drilled and countersunk holes, and driven solid rivets that meet aircraft airworthiness acceptance criteria.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Sheet Metal Fabrication and Riveting

Sheet metal work is one of the most fundamental skills in airframe maintenance. Aircraft skins, brackets, doublers, and fairings are largely fabricated from aluminum alloy sheet. In this lab you will carry a Z-channel bracket from raw stock through forming, drilling, riveting, and formal inspection—applying the same acceptance criteria an FAA airworthiness inspector would use.

Material Selection and Handling

The standard structural sheet for aircraft repair is 2024-T3 aluminum alloy. Its T3 temper means it was solution heat-treated and then cold-worked, giving it high strength with reasonable formability. Handle sheet stock carefully: nicks and scratches act as stress risers. Wear leather gloves when moving large sheets; their edges are razor-sharp. Store sheet flat or on edge racks—never lean it at a low angle where it can bow.

Laying Out and Forming Bends

Accurate layout is non-negotiable. Use a scribe and straightedge to mark bend lines. Before you bend, calculate the setback (the distance from the mold line to the tangent point of the bend) and the bend allowance (the length of material consumed in the curve itself). Both depend on the bend radius and the bend angle. Typical aircraft practice requires an inside bend radius of at least one material thickness for 2024-T3 to avoid cracking.

The cornice brake grips the sheet along the bend line and rotates a clamping leaf to form the bend. Set the gap between the upper and lower leaf to match material thickness. Bend in a single smooth motion to the target angle, then check with a protractor. Springback—the elastic recovery that opens the angle slightly after the brake is released—is expected. Over-bend a few degrees, release, measure, and repeat until the protractor confirms the correct angle. Check the inside radius with a radius gauge; it must meet the minimum specified on the drawing.

Hole Layout, Drilling, Reaming, and Countersinking

Rivet holes must be positioned within strict limits for edge distance and pitch (spacing between rivets). AC 43.13-1B specifies that edge distance (center of rivet to nearest edge) must be a minimum of two rivet diameters, and pitch must be a minimum of three rivet diameters. Lay out all 20 holes with a prick punch before drilling.

Drill sequence: (1) Start each hole with a small pilot drill to keep the bit on the punch mark. (2) Step up to the final drill size for the rivet selected. (3) Ream to final diameter for a snug fit—the rivet shank should slide in with light finger pressure, not loosely rattle. (4) Deburr both sides of every hole with a deburring tool before inserting clecos.

For AN426 flush (countersunk) rivets, machine-countersink the top sheet with a countersink cage set to the correct angle (100° for AN426) and depth. The rivet head must sit flush or very slightly above the surface—never below. Use a cleco clamp in every second hole to keep parts mated flat before driving begins.

PPE for drilling: safety glasses with side shields, hearing protection when using pneumatic tools.

Driving Solid Rivets

The pneumatic rivet gun drives the manufactured head while a bucking bar held firmly against the opposite side upsets the shank into the shop head. Match the rivet set in the gun to the rivet head style. The correct shop-head dimensions for AN426 rivets are found in AC 43.13-1B: shop-head diameter should be approximately 1.5 times the rivet shank diameter, and shop-head height approximately 0.5 times the shank diameter. Verify each driven rivet immediately with a rivet gauge.

Driving technique: hold the gun perpendicular to the sheet, use controlled bursts, and keep the bucking bar steady. A gun held at an angle produces a tilted shop head—a reject condition. Too many hammer blows work-harden the shop head and can crack the sheet around the hole.

Lockout/tagout the drill press and air supply when changing bits or sets, and when any person's hands are near the tooling.

Inspection Using AC 43.13-1B

AC 43.13-1B is the FAA's primary reference for acceptable methods, techniques, and practices for aircraft inspection and repair. After driving all rivets, inspect every one and the surrounding sheet for:

Classify each finding: acceptable (within limits), repairable (rivet can be drilled out and replaced), or reject (damage to the parent structure requiring a different repair scheme). Record every finding on the inspection record before submitting the assembly.

Completion Standard

Your completed assembly must pass a 10-point instructor checklist. At least 8 of 10 criteria must be rated satisfactory. Every shop head will be checked with the rivet gauge; any shop head outside AN tolerance is an automatic unsatisfactory on that criterion. Accurate recordkeeping on the inspection form is itself one of the 10 criteria.

Key terms9

AN426An Air Force-Navy standard specification flush (countersunk) rivet with a 100-degree head angle, commonly used where a smooth aerodynamic surface is required.
SpringbackThe elastic recovery of a bent metal part that partially opens the bend angle when clamping pressure is released; corrected by over-bending to a calculated angle before release.
Edge distanceThe measured distance from the center of a rivet hole to the nearest edge of the sheet; AC 43.13-1B specifies a minimum of two rivet diameters to prevent sheet-edge tear-out.
Shop headThe upset end of a solid rivet shank formed by the bucking bar during driving; its diameter and height must meet the tolerances specified for the rivet diameter.
Cleco clampA spring-loaded temporary fastener inserted into a drilled hole to hold mating parts in alignment while remaining holes are drilled and rivets are driven.
Countersink cageA drill-press or hand-drill accessory that controls countersink depth to a preset dimension, preventing over-countersinking of the sheet.
AC 43.13-1BFAA Advisory Circular 43.13-1B, 'Acceptable Methods, Techniques, and Practices—Aircraft Inspection and Repair'; the standard reference for structural repair acceptance criteria.
Bend allowanceThe length of sheet material that is consumed within the curved portion of a bend; calculated from the inside bend radius, material thickness, and bend angle.
2024-T3A high-strength aluminum alloy in the solution heat-treated and cold-worked temper; widely used for aircraft structural skins and formed parts because of its good strength-to-weight ratio.

Worked examples4

1. A Z-channel bracket requires a 90° bend in 0.040-inch 2024-T3 sheet using an inside bend radius of 0.0625 inch (1/16 inch). Calculate the bend allowance.
  1. Step 1 – Identify the formula: Bend Allowance (BA) = (0.01743 × R + 0.0078 × T) × Angle in degrees, where R = inside radius in inches, T = material thickness in inches.
  2. Step 2 – Substitute values: BA = (0.01743 × 0.0625 + 0.0078 × 0.040) × 90
  3. Step 3 – Compute the bracket term: (0.001089 + 0.000312) = 0.001401
  4. Step 4 – Multiply by the angle: 0.001401 × 90 = 0.1261 inch
Answer: Bend allowance ≈ 0.126 inch. This length of material will be consumed in the bend and must be subtracted from the flat-pattern layout to keep finished flange lengths correct.
2. You are using AN426AD4 rivets (shank diameter 0.125 inch). What are the minimum acceptable shop-head diameter and shop-head height after driving?
  1. Step 1 – Recall the shop-head rules from AC 43.13-1B: minimum shop-head diameter = 1.5 × shank diameter; minimum shop-head height = 0.5 × shank diameter.
  2. Step 2 – Calculate minimum diameter: 1.5 × 0.125 = 0.1875 inch.
  3. Step 3 – Calculate minimum height: 0.5 × 0.125 = 0.0625 inch.
Answer: Minimum shop-head diameter = 0.1875 inch (3/16 inch); minimum shop-head height = 0.0625 inch (1/16 inch). Use a rivet gauge to verify both dimensions on every driven rivet.
3. Your rivet pattern uses AN426AD4 rivets (shank diameter 0.125 inch). What is the minimum allowable edge distance and minimum rivet pitch according to AC 43.13-1B?
  1. Step 1 – Recall the AC 43.13-1B minimums: edge distance ≥ 2D; pitch ≥ 3D, where D = rivet shank diameter.
  2. Step 2 – Calculate minimum edge distance: 2 × 0.125 = 0.250 inch.
  3. Step 3 – Calculate minimum pitch: 3 × 0.125 = 0.375 inch.
Answer: Minimum edge distance = 0.250 inch (1/4 inch); minimum pitch = 0.375 inch (3/8 inch). Mark and measure all hole centerlines before drilling to confirm both limits are met.
4. When drilling a pilot hole before stepping up to the final rivet-hole diameter for an AN426AD4 rivet, the mechanic uses a No. 30 drill (0.1285 inch diameter) as the pilot. The final hole diameter for an AN426AD4 rivet is 0.1285 inch per standard drill charts. After reaming, the hole should allow the rivet shank to insert with light finger pressure. Describe how you would verify the fit is correct before countersinking.
  1. Step 1 – After reaming, clean the hole of chips and burrs with a deburring tool.
  2. Step 2 – Insert a sample AN426AD4 rivet shank (diameter 0.125 inch nominal) into the hole without force. A properly reamed hole allows the shank to slide in smoothly with light finger pressure and shows no lateral wobble.
  3. Step 3 – If the rivet shank drops through freely under gravity alone, the hole is oversize—flag and assess per AC 43.13-1B (may require next-size-up rivet if structure permits).
  4. Step 4 – If the shank will not enter without hammer force, the hole is undersize—ream again in small increments and re-check.
  5. Step 5 – Only when fit is confirmed correct proceed to countersinking with the cage set to 100°.
Answer: The correct fit is confirmed when the rivet shank slides in with light finger pressure, no force required, and no free lateral movement. This ensures the rivet will fill the hole correctly when driven and produce a sound, full shop head.

The math in this lesson2

Learners multiply rivet shank diameter by 1.5, 0.5, 2 and 3 to find shop-head size, edge distance and pitch.
6.RP.3A-SSE.1
Brush up in the math halls at newSchoolWorks.com:
In class: Form the Z-channel bracket laid out in Lesson 3 using a cornice brake; check flange angles with a protractor and bend radius with a radius gauge, correcting any springback before moving to hole layout. · Drill, ream, and countersink a 20-hole pattern in the assembly, cleco-clamp mating parts, then drive AN426 flush rivets to correct shop-head dimensions and check each rivet with a rivet gauge for head diameter and height. · Inspect the completed assembly for cracks, improper driven-head dimensions, sheet waviness, and edge distance using AC 43.13-1B criteria; classify each discrepancy as acceptable, repairable, or reject, and record results on an inspection record.
Materials: Cornice brake, drill press, pneumatic rivet gun, bucking bars, cleco clamps, AN426 flush rivets, 2024-T3 sheet stock, rivet gauge, protractor, radius gauge, countersink cage, deburring tool, AC 43.13-1B
Check: All shop-head dimensions within AN tolerance (checked with rivet gauge); no cracks at holes; edge distance and pitch meet AC 43.13-1B minimums; instructor rates the assembly satisfactory/unsatisfactory on a 10-point criteria checklist—minimum 8 of 10 criteria satisfactory

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Airframe Structures
1. What is the minimum inside bend radius generally required for a bend in 2024-T3 aluminum sheet to prevent cracking at the bend?
Airframe Repair
2. A mechanic drives an AN426 flush rivet and finds the shop head is tilted 15° from perpendicular to the sheet surface. According to AC 43.13-1B, this rivet should be classified as:
Airframe Structures
3. Using AN426AD4 rivets (shank diameter 0.125 inch), what is the minimum edge distance from the center of any hole to the sheet edge?
Airframe Structures
4. Which of the following best describes springback in sheet metal forming?
Safety and Ground Operations
5. Before using the pneumatic rivet gun and drill press in the sheet metal lab, which safety practice is required when changing drill bits or rivet sets?
Airframe Repair
6. The shop head of a driven AN426AD4 rivet measures 0.160 inch in diameter. The shank diameter is 0.125 inch. What is the correct assessment?
Airframe Structures
7. What is the primary purpose of reaming a rivet hole after drilling to pilot size?
Airframe Repair
8. During inspection of a riveted assembly using AC 43.13-1B, a mechanic finds two rivets whose flush heads are countersunk so deeply that the shop heads are recessed below the sheet surface. These rivets should be classified as:
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