Sheet metal and rivets

Most aluminum repairs are a patch or a new part riveted in. The structural repair manual (SRM) gives the exact repair; when it doesn't, AC 43.13-1B's rules of thumb apply. A repair uses the same alloy and thickness as the original, or the next thicker.

Laying out rivets

• Diameter (D): about three times the thickness of the thicker sheet.
• Edge distance: at least 2D from the center of the rivet to the edge of the sheet (2.5D for countersunk rivets).
• Pitch (between rivets in a row): at least 3D, usually 6 to 8D.
• Transverse pitch (between rows): about 75% of the pitch.
• Length: grip (the total thickness being joined) plus 1.5D, so there's enough shank to form the shop head.

1/8 inch rivet (D = 0.125) through two 0.040 sheets
Grip = 0.080, length = 0.080 + 1.5 × 0.125 = 0.2675
Rivets come in 1/16 inch steps: round up to 5/16 (0.3125)
Edge distance at least 2 × 0.125 = 0.25 inch

Holes are drilled with number drills a little larger than the rivet: a #40 for a 3/32 inch rivet, #30 for 1/8, #21 for 5/32 and #11 for 3/16. Deburr the holes, hold the sheets with clecos, and drive the rivet with a rivet gun and bucking bar. A good shop head (the end you form) is about 1.5D across and 0.5D tall. Where you can't reach the back, use blind rivets (like Cherry or CherryMAX) as the SRM allows.

Bending sheet

Every alloy and thickness has a minimum bend radius; tighter bends crack. To lay out a bend you need the setback, the distance from the mold line (where the outside surfaces would meet) to the bend tangent line: SB = K × (R + T), where R is the bend radius, T the thickness, and K = 1 for a 90° bend. Where two bends meet at a corner, drill a relief hole first so the corner doesn't crack.