Definition
A cold-forming process used on thin aluminum aircraft skin in which a male and female die set presses a shallow conical depression around a rivet hole, allowing a flush rivet to seat flat with the surface of the skin. Coin dimpling differs from standard radius dimpling in that the dies are designed to coin (compress and flow) the metal slightly, producing a sharper, more accurate dimple shape that mates precisely with the countersunk rivet head.
Plain English
A way of pressing a small dished shape into thin aircraft metal around a rivet hole so that a flat-headed rivet sits flush with the surface instead of sticking up. The dies squeeze the metal a bit as they form the shape, giving a cleaner, more exact fit than ordinary dimpling.
Context Anchor
Seen in aircraft sheet-metal repair and construction when preparing holes for flush rivets.
Derivation
Coin' here comes from the metalworking term 'coining,' which describes pressing metal between dies hard enough to make it flow and take on the exact shape of the dies — the same process used to stamp coins. Applied to dimpling, it means the metal is not just bent into shape but slightly compressed, producing a more precise dimple.
Why Pilots Care
Flush rivets must sit perfectly flat on the skin to keep the surface aerodynamically smooth and to avoid stress concentrations. A poorly formed dimple can cause the rivet head to sit proud or crack the surrounding metal, which matters for both drag and structural integrity.
Intuition Check
“Coin” does not mean using a coin as a tool here. It means forming metal by pressing it between shaped tools.
Example Sentence 1
The technician used coin dimpling on the thin wing skin so the flush rivets would sit perfectly level with the surface.
Example Sentence 2
Coin dimpling avoids thinning the metal compared with cutting a countersink in thin aluminum sheets.