Tank skin countersinking...

[rvbuilder2002]

The chances that knife edging on an RV will cause anything other than a few smoking rivets years down the road is slim.

In the context of machine countersinking on the vert. and horz. stab skins, it is not just a chance but pretty much a certainty at some point in the future.
This used to be (20 years ago) the recommended process but service history proved it was a bad choice.
That is why for the most part the only recommended machine countersinking on .032 is on the fuel tanks, which has never been a problem because of the sealant bonding etc.

Bottom line, the very specific recommendation in Section 5 of the construction manual is to dimple countersink any skin material .040 or less thick unless specifically instructed too machine countersink.

 

[tgmillso]

Tank tested

As discussed previously in this thread, I countersunk the skin baffle holes to 0.002 shy of knife edge (i.e. 30 thou deep), leaving the rivets 0.006" proud of the surface. After completing the assembly, I then shaved the rivets using my makita drill, $14.50 countersink cage and Avery bit as described in the thread below.

http://www.vansairforce.com/community/showthread.php?t=124442&page=3

Anyway, today I tested the tank using the manometer method and pressuring to 24" of water, leaving and additional 3" of tube above that so that I could track the changes with temperature and atmospheric pressure but had pressure relief at a 27" maximum (i.e. the tube was cut at this level). Anyway I ran the test for two hours, in which time the atmospheric pressure remained constant, but the temperature raised from 15.9 to 16.7deg. The manometer lifted in that time to 24.75". So in summary, I'm very satisfied with the results.
Just as an additional reference point, I attached a balloon the first time I pressurized the tank. When the manometer was about halfway up to the target level, the balloon was approximately 9", then it burst. I guess the moral of the story is that a 9 to 10" balloon is only about 0.5psi.
It feels good to have this finally complete.
Cheers,
Tom.

 

[jonelford]

Lots of good info here! I just dimpled mine...