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  #1  
Old 09-28-2019, 11:54 AM
Surfdoc Surfdoc is offline
 
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Default Tank Skin Countersinking

I am sure this has been posted before but couldn't find anything. Where the tank skin attaches to the spar with the #8 screw. Did you dimple the skin or countersink? I dimpled one tank and it looks bad so I was told to countersink the #8 hole instead. I did one but seems like it removed way to much material and there is a lot of play with the screw. Now I do realize that the plate-nut carries the load but I would like to see what others have done.
Thanks in advance.
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  #2  
Old 09-28-2019, 11:56 AM
chaskuss chaskuss is offline
 
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That tank skin is to thin to machine countersink. Use the dimple dies.

Charlie
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  #3  
Old 09-28-2019, 11:58 AM
lr172 lr172 is online now
 
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Not a good idea. As the hole gets bigger, you lose the pilot hole and the cutter will drift. This will create problems when you attach the tank to the wing. If you're committed to this approach, make a backing strip for pilot holes. It will be more secure with dimples though.

Larry
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Last edited by lr172 : 09-28-2019 at 12:23 PM.
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  #4  
Old 09-28-2019, 12:02 PM
FredMagare FredMagare is offline
 
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Countersink it so that the screw grips the metal of the tank to help hold it in place.

Good luck!
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  #5  
Old 09-28-2019, 12:25 PM
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rmartingt rmartingt is offline
 
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My recollection for the -7 spar is you machine countersink the anodized spar flange (where the nutplate is) and then dimple the tank skin. At least, that's what I did. The -9 is probably almost identical in method here.
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  #6  
Old 09-28-2019, 02:43 PM
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rvbuilder2002 rvbuilder2002 is offline
 
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Location: Hubbard Oregon
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Quote:
Originally Posted by Surfdoc View Post
I am sure this has been posted before but couldn't find anything. Where the tank skin attaches to the spar with the #8 screw. Did you dimple the skin or countersink? I dimpled one tank and it looks bad so I was told to countersink the #8 hole instead. I did one but seems like it removed way to much material and there is a lot of play with the screw. Now I do realize that the plate-nut carries the load but I would like to see what others have done.
Thanks in advance.
Let me guess.... the poor dimples on the first tank were probably done with a hand squeezer?
The fuel tank skin is pretty thick by RV standards, but still no where thick enough to machine countersinking for a screw. The thickness of the skin also takes quite a bit of force to form a nice dimple. Only a hammer with a c-frame tool should be used. If that is what you used, then you just didn’t hit hard enough. If not, you can form the dimples again ( on a finished tank it is useful to have a helper).

BTW, it is not the nutplate that takes the load (In fact in most fastener joints on an RV, the nutplate is never the main load path). The primary load path is the shear load of the dimple within the counter sink of the spar flange. If you machine countersunk all of the holes, the shear strength of all the screw joints would be severely compromised.
It might be good to review section 5 of the construction manual regarding dimpling and machine countersinking.
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  #7  
Old 09-29-2019, 05:47 AM
Surfdoc Surfdoc is offline
 
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Thank to all. I am glad I did not listen to all the "helpers" who came over to the shop and fed that garbage to me.
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  #8  
Old 09-29-2019, 12:15 PM
sblack sblack is offline
 
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Advice is always worth exactly what you pay for it. Buyer beware!
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