Author: "lindbergs52" <lindbergs1@...> Time: Tue Feb 10, 2004 3:22 am PST Link
Got this today straight from Bruce at Van's. When machine
countersinking a thick skin (fuel cap flange in this case) to accept
a thinner dimpled skin, machine countersink the thick skin only to
the same depth as you would to accept a rivet. In other words,
contrary to what is stated in the plans, do not bother to make a test
coupon with a dimple and machine countersink until the test dimple
fits comfortably into the machine countersink. That will make the
countersink too deep. Machine countersink only to the depth that you
would for a rivet or, sez Bruce, two or three thou. more. Why? This
is what happened to me. I machined away until the fuel tank flange
fit the skin dimples. When I shot the rivets the shop heads just
about disappeared into the hole. The rivets had expanded into the
extra space between the dimple and the machine countersink and the
rivets were now too short. I tried longer rivets and made a hash of
the shop heads. Whatta mess. I switched to NAS 1097AD4-4 "goof
rivets" and managed to save the day. I was sweating for a while,
though. (BTW, I back riveted with the fuel tank skin resting on a
steel plate. Worked great.) Bruce says they just hired someone from
Boeing who says it is Boeing's policy in this situation to machine
countersink only to the same depth as one would for a rivet. Bruce
says that new plan revisions will reflect this change. Steve
Lindberg.