Rick6a
Well Known Member
It is a given that .032 is the minimum material thickness acceptable for machine countersinking aluminum sheet to accept an AN426AD3 rivet which is set in a standard 100 degree countersink. Still, with .032 it is all too easy to generate the dreaded knife edge...that is...no bearing surface if the countersink is made the slightest bit too deep. A knife edge is a bad thing and does nothing for structural integrity. That is why we routinely dimple thin material. That said, some of you may remember the late Tyler Feldman's Grand Champion RV-6 of a few years ago. He elected to machine countersink the .032 wing surfaces to provide the smoothest distortion free surface possible. He knew a dimpled hole simply cannot match the smooth look of a machine countersink. It is that kind of attention to subtle detail that raises the bar and takes home the top trophy. I did something similiar on the upper tank skin surfaces on my RV-6A. For the most part, I dimpled most of the wing skins per plans but did machine countersink the upper surfaces of the .032 tank skins....with a small difference. To insure that I produced adequate bearing surface for the rivets set into the skins, I intentionally made slightly shallow the countersinks for the AN426AD3 rivets. After setting the rivets, I used a high speed rivet shaver to flush the rivets to the skin. The picture attempts to emphasize the point. The rivets to the right of the tank have been dimpled normally, all the holes on the tank itself have been machine countersunk and most of the rivet heads slightly shaved. This is a common sheet metal technique routinely used in aerospace production. A 2D picture simply cannot convey the obvious differences between the two styles of countersinks which is especially noticeable with changing light and viewing angles, but I hope you get the idea.
Rick Galati RV-6A "Darla"
Rick Galati RV-6A "Darla"
Last edited: