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Purpose of countersinking

A

AviatorJ

Well Known Member
I've always thought the purpose of countersinking was purely aesthetic. The main reason being to have a smooth surface once screwed, riveted or otherwise. Is this the case or is there some added bonus of strength?

I'm asking because I did my rivets on the read spar of the VS last night. Onbe of the previous parts I had countersunk was the rear spar doubler. Here's a picture from someone elses log;

IMG_20141224_212704.jpg


This particular piece gets riveted to the rear spar and wouldn't be visible on the final build, it also doesn't have anything directly butt up to it. Only thing I could think of is they wanted to reduce the rivet length, so they had it countersunk...
 
AviatorJ said:
This particular piece gets riveted to the rear spar and wouldn't be visible on the final build, it also doesn't have anything directly butt up to it. Only thing I could think of is they wanted to reduce the rivet length, so they had it countersunk...
Click to expand...

Actually it butts up against the aft end of tailcone when the VS is installed. Take a look at Section 11, page 11-5, step 5 and you'll understand why the plan calls flush rivets in this area.
 
Many times flush rivets are used because another piece must be installed directly to the surface. Flush rivets are not stronger than universal rivets.
 
Justin -
Take a look at page 11-1 where the vertical stab gets mounted to the empennage. That piece is forward facing and mates flat against the back of the tail cone - which is why its countersunk.

Tim
 
Mel said:
Many times flush rivets are used because another piece must be installed directly to the surface. Flush rivets are not stronger than universal rivets.
Click to expand...

Technically correct, but *dimpled joints* (not countersunk) using flush rivets have been tested as stronger in shear than non-dimpled joints using the same diameter universal head rivets. Including the dimple apparently increases the effective shear area of the joint.

At least, that's what I read on the interwebs....

Charlie
 
That picture brings up a question. I cannot think why one would drill and countersink holes with the blue plastic still attached to the piece. That plastic came off as soon as possible for me. Once you find your first sign of corrosion underneath some of that plastic you will understand why.
 
I was thinking the same thing. It would interfere with deburring as well. You will have burnt plastic pieces possibly melted to your Al and your tools. Also, I especially primered all my non-Alcad pieces such as this anyway, so no worries about scratching it.
 
RVbySDI said:
That picture brings up a question. I cannot think why one would drill and countersink holes with the blue plastic still attached to the piece. That plastic came off as soon as possible for me.
Click to expand...
Can't take credit for the picture, I just wanted to have a picture for the talking point. I personally take the plastic off the entire piece.

One thing to note though, I went outside to look at the plans to see where that piece ends up going. When running my hand over my rivets I feel a bit of an edge, some more than others... but now I'm not happy with my work.

I ordered more rivets from Vans and am going to redo about 20 of the rivets. This will remove the primer I put on the piece when I start making deeper countersinks. Should I reprime each hole after drilling and deburring? Or leave it since the new rivet will smash it down. Also is it better to have the rivet more 'sunk' than 'raised'? Fun times!
 
rv7charlie said:
Technically correct, but *dimpled joints* (not countersunk) using flush rivets have been tested as stronger in shear than non-dimpled joints using the same diameter universal head rivets. Including the dimple apparently increases the effective shear area of the joint.
At least, that's what I read on the interwebs....
Charlie
Click to expand...

I believe we were talking about machine countersinking here, not dimpling.
 
Justin,
If it were me (and it was a few times) I would not reprime your countersink holes. In fact, I wouldn't prime the holes in the first place. My concern is that you cannot control the primer thickness exactly, therefore some rivets might end up a bit "proud" and some be a little low even though your conical holes should be very close to the same if you use a Micro-stop c-sink tool. Also, I don't know about the "hardness" of primers as a material to rivet against or into. In any case, I'll prefer the metal to metal i.e. direct rivet to spar contact for strength and resistance to shear.
2-cents,
Dave
 
You can always dip your rivets in primer and set them wet. Another school of thought says to simply prime after riveting - the rivets are alodined so just keeping moisture out should be enough corrosion prevention. As for the 'sunk' vs. 'raised' question, generally it does not matter. But if the top material is thin, too deep leaves less contact area. Saving grace here: the top material gets sandwiched later when the part is bolted to the tailcone, so rivets that are within the usual tolerances (don't have the published figures to hand) will gain some extra support from the clamping action of the bolts. Don't obsess too much over getting a perfectly smooth surface; the parts will go together fine even if the rivets are raised (within tolerance).
 
If they sit too proud, I would just shave the heads flush with the surface. Replacing them brings up the argument of your initial rivet being stronger. There are times where the rivet should be replaced, but it sounds like right now you have a good rivet... just with a sharp, proud head.
The problem with overcountersunk rivets, is the head may not make solid contact with the countersunk surface. The shear & moment forces on this rivet can allow it to move in the hole. For an extreme example, if you were to install a rivet with no butt on it at all, it would eventually work it's way out of the hole.
 
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