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Spar nutplate countersinks

georgemohr

Well Known Member
Hi team 14!

Please check me. I'm getting ready to countersink the spar for the center screw holes in the fuel tank and access panel nutplates. I created a test sample to try and get the depth of this countersink right using some rather thick bar stock and a K1000-06 nutplate.

I'm aware of the .007 rule for countersinking below a skin dimple for rivets but less clear about the approach for this special case. The instructions offer that for the #6 nutplate the max outer diameter of the dimple should be 7.8mm and the max inner should be 4mm.

On my test sample I snuck up on about 7.5mm OD and test fit a piece of scrap skin with a #6 dimple applied to it. The result is attached. I think the skin to spar gap is something like .008 on average all around (I can't quite get a .010 feeler gauge into the gap). Pls ignore the oversqueezed rivets, this bar stock is thicker than the real spar.

What do you all think. Am I on the right track here?

THE SPAR LIFE YOU SAVE MIGHT BE MINE :)

Thanks,
G
 

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Trust the process. Do your best job one step at a time and things will come together (99%). These kits line up very well when assemblying it. As far as your pic im not seeing anything glaring.
 
Should this Dimpling be any Different?

Scott McDaniels has repeatedly stated and shown with some awesome pictures why countersinks at flush plus 0.007" are preferred. (I thought I had a link to the first time posted them, but can't find it.)

The reason I bring this up is, because in these discussions, Scott pointed out that there is almost NO way to maintain structural integrity and have the dimpled skin outside of the dimple sit flush with countersunk piece. What makes these dimpled/countersunk holes any different? Am I missing the point?

Please correct anything I've misunderstood, I'm not an expert at any of this.

Also, if you run into any of Scott's posts where the discussion and pics are located, please let me know where.

Good luck!
Mike
 
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When I did this a couple years back, I followed the plans as you seem to be doing and it worked out fine. I find a digital caliper useful for this. If you don’t have one yet, pick one up. You’ll use it a lot during the build. When I fitted my tanks to the spar I r&rd them several times to get the tank skin to lay (almost) flush to the top skin and the spar on bottom. Each time marking the pillowed areas and then breaking the skin edge a little at a time, usually between the screws, until it looked good. Buy extra screws and boelube. During the build your going to come across situations where structural integrity could be compromised for the sake of aesthetics. Always error on the side of structural integrity. Stick to the plans.
 
Remember that when countersinking for a dimple you need to be using a 120degree countersink, not the standard 100 degree.
That could be the source of your problem.

Jim Frisbie
RV 9A (950 hrs)
RV 10 under const.
 
Scott McDaniels has repeatedly stated and shown with some awesome pictures why countersinks at flush plus 0.007" are preferred. (I thought I had a link to the first time posted them, but can't find it.)

The reason I bring this up is, because in these discussions, Scott pointed out that there is almost NO way to maintain structural integrity and have the dimpled skin outside of the dimple sit flush with countersunk piece. What makes these dimpled/countersunk holes any different? Am I missing the point?

Please correct anything I've misunderstood, I'm not an expert at any of this.

Also, if you run into any of Scott's posts where the discussion and pics are located, please let me know where.

Good luck!
Mike

Lots of info and the photos you mentioned here


https://vansairforce.net/community/showthread.php?t=159343

It is a sticky thread at the top of the builder tips section.
 
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Remember that when countersinking for a dimple you need to be using a 120degree countersink, not the standard 100 degree.
That could be the source of your problem.
.

???

Not what we recommend in any of our construction manuals other than specific cases for CS4-4 blind rivets on the RV-12, but in those instances the dimple is also 120 deg.
 
I think nearly every reply missed the point entirely. In the plans we are told the maximum dimensions of these particular holes, not a general rule like "countersink for a dimpled skin", which is where the 0.007" would apply. The OP is almost at the limit and sees a significant gap. In other words, the skin is not going to sit anything approaching flush with the spar like it normally would in most cases, and that is the main concern.
 
nesting

I suspect the dimpled part is not nesting closely with the machine countersunk part because of the sharp shoulder of the machine countersink, versus the rounded shoulder of the formed dimple.

This assumption could be tested by rounding over the shoulders on the machine countersink using some fine sandpaper or scotchbrite wheel.

It also may be that the dimple has not been formed with enough force - try hitting it harder!
 
Hi all, thanks for the feedback. I think I'm on the right track with these countersinks so I'm going to build on! I have a digital calipers so I'll use them to get the hole to the max OD spec and roll with it.
 
I think nearly every reply missed the point entirely. In the plans we are told the maximum dimensions of these particular holes, not a general rule like "countersink for a dimpled skin", which is where the 0.007" would apply. The OP is almost at the limit and sees a significant gap. In other words, the skin is not going to sit anything approaching flush with the spar like it normally would in most cases, and that is the main concern.

You are correct that the .007 rule of thumb doesn't apply to countersinks for screw dimples but the principle for why there is a specific limit and not a statement of "cut until the dimpled skin lays flush" is the same.
Bottom line is, that the dimension limit is given for a reason, whether it be .007 deeper or a specific maximum cut diameter, and it sounds like the OP has resolved to use that as a guide.

As a side note, the quality / crispness of the dimple is a large factor in how flush a skin appears to lay when a dimple and countersink (or even two dimples) are involved. This becomes even more pronounced in cases of larger dimples for screws. I believe many builders have been fooled by this in the past, and pressed on making their countersinks deeper and deeper in an attempt to get a flush fit, only to destroy their spar in the process.
 
Nutplate

Did Vans switch to #6 nutplates. I seem to remember #8 on the 7 wings. Big difference in dimensions.
 
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