Wings: Ribs. A cracked rib scare that wasn't.

I've been waiting to post this for 7 months.  What was holding me back was the final resolution, which was completed today.

On 4-Dec-13, I decided to remove several rivets along a rib line as their shop heads were less than perfect.  This was a bad day:  I botched the removal of four rivets ending up with slightly elongated holes.  On the fifth rivet, I removed the factory head just fine.  Thinking I was on a skin-spar joint, I gave the rivet shank a good smack with my punch.  Out popped the rivet, like it should.  I was  then horrified to find that I was actually on a skin-spar-rib joint!  Meaning, now I bent the rib flange out.  I took my low quality boroscope to the inboard side of the rib to get a closer look at what I had done.

Oh no!  It must be a cracked rib!  How can I fix this?  The top and bottom skins are already on.  The rib can't realistically be replaced.  Maybe I could run a rat tail file in the rib notch and work out the crack then re-radius the edge.  This isn't good!  I was ready to give up and start saving a few decades for a Cirrus.

To investigate how feasible my filing fix might be, I went in to get some better pictures to check clearances and angles for running the rat tail through there.  On the outboard side of the rib (not shown), I noticed the surface of the rib in the area of the alleged crack was completely undisturbed.  That was perplexing.  If there was a crack, I should've seen it on both sides.  With a simple Canon SD1100IS consumer camera, I adjusted the camera angle and lighting and it turned out it wasn't a crack at all!  It's actually bucking bar damage from when the rivets were bucked, as shown below.  My bucking bar is rectangular and the edge got slightly driven into the rib since it was resting on the spar flange.

You can see why I went in to remove the rivets:  Of those in focus, the one on the left is damaged and the two on the right are over-driven.  Though it looks like the dimples are cracked on the right, it's actually the primer unable to conform to the metal expansion when the rivet is driven.  Ya, it scared me too when I first saw these popping up.  Turns out it can be a good indicator of an over-driven rivet.

Anyway, how does that tab look after being pulled in by a cleco?  Compare the left and right sides below.  Clearly, this tab won't sit flush on its own and certainly not with the pulling power of a cleco.

So, to pull it flush, I could use a strong pull rivet like a CCR-264SS-3-2, but I really wanted to have a normal AN426AD3-4.5 in there.  The challenge then becomes how to keep the flange flush whilst riveting.  Well, I had a "moment of clarity" and devised the following methodology:  Use a normal mushroom set on the factory head but instead of a bucking bar on the shop end, use a back rivet set with the shank of the set inserted into a bucking bar with a hole.  The spring loaded collar of the rivet set will push the rib's flange flush onto the spar flange, whilst the bucking bar it's inserted in will provide the mass for forming the shop head.  As the rivet expands, it should pull in and lock the flange tab in as flush as it can be.  Here's what that looks like in concept.

So, I tested how far the tab would be pushed down onto the spar flange with the back rivet set, as shown below.  Turns out, it has a way to go.  I looked around for stronger springs, but decided on another method instead.

I've read that some folks use a rubber grommet between the bucking bar and the shop head to help keep two parts flush against each other.  What kind of grommet would be strong enough to withstand the brutality of a bucking rivet gun and of the appropriate elasticity to allow the bucking bar to push everything flush?  Well..uhhh...these will...

So here's the bucking bar with the..uhhh..."ring".  The Popsicle sticks were attached so the bucking bar could rest on the spar.  The shop head of the rivet will go into the ring and the bucking bar is then pushed very hard into the flanges, keeping everything flush.  Then the rivet is set with the gun.

Bucking bar in position.

And here's the result.  Not perfect, but far, far better than had I just bucked it normally.  And it's clearly better than had I used my back rivet set as the bucking bar, as the image four above suggests.

Whew.  Ya, I ain't so smart.  But managed to get myself out of this jam and still feel okay trusting my life to these wings.