BUCK SEASON? - why yes it is

Limited time at the shop this week as I concentrated on some home/cabin maintenance that needed to be done.

Pulled the right wing spar assembly apart again for deburring and prime.  Took almost an hour just to scuff everything with Scotchbrite, and clean down with lacquer thinner in preparation for prime:

The self etching primer dries fast, but I decided to let everything cure for a couple of days.  When I came back to the shop, I was ready to reassemble the spar.

I cleaned off the bench completely in order to make room for the assembly process. 

Like the stab, elevator and flapperons, it's important to have a flat "surface" to assemble on.  I placed the steel angles on the bench edges and lined them with painters tape to keep their surfaces smooth.  Before adding the cross members again, I started the process of riveting the spar back together:

Acceptable practice for rivet placement is that the head of the rivet should be on the surface of the thinnest material being assembled.  The centre spar doubler is 063 and the spar web is 032, so I started by first completing all the pulled rivets called out in the plans for the spar.  To do this, I flipped the spar over and pulled the rivets from the aft side.  I also attached the wing spar tip while in this position:

The root doubler and spar web are both 032, so it doesn't matter which way rivets are pulled and the plans don't have directions on this.  I decided for consistency to pull them in the same direction the driven rivets will be along the spar caps: 

Driven rivets are called out in the plans for most of the spar and depending on their location they have different lengths before being formed.  The length is dependent on what thickness of materials are being joined.  We don't have any rivets in stock that are correct length for some of the spar cap/spar web/centre spar doubler interfaces.  There aren't many of them, so instead I used a rivet cutter to shorten a few longer ones (made -9 rivets which we have lots of into -7 rivets).

There are very specific standards with regards to properly formed driven (bucked) rivets.  The formed tail of the rivet MUST be 1.5 times the diameter of the rivet tail once bucked.

I originally thought I'd be able to use the hand squeezer to form the rivets along the spar caps but I decided to test that theory first on some scrap material.  This also confirmed I had the correct length of rivet for the thickness of the material (trust the plans but verify!).  I used a piece of aluminum angle and 032 sheet to simulate the spar cap/spar web and discovered the A5 driven rivets are much too hard to squeeze by hand - I couldn't squeeze hard enough on the tool to get the correct formed head dimensions.

Putting the hand squeezer away, I got the air rivet set out and attempted to drive the first rivet.  It went much easier than I expected and once I developed a feel for it, I got good at estimating the amount of time on the trigger to set the rivet correctly.  The shank of an undriven A5 rivet is 4 mm in diameter, so the formed head needs to be 6mm in diameter. 

Pulled rivets in the web/doubler. Looking from bottom to top on the left - clecos in the spar cap, formed rivet, cleco then an undriven rivet

Three complete driven spar cap rivets between the clecos

It takes a bit more time than pulled rivets, but the evolution is the same.  Place the rivet in open holes between the clecos, drive/form with the rivet gun/bucking bar then repeat on the next empty hole.  I started with the top spar cap and measured each formed rivet for conformity as I went along. Once I had alternating holes done, I removed the clecos and filled in the rest.  Next was the spar doubler and strut pickup angles.  These were driven from the other side to respect thickness/rivet standards.

Then I repeated the process for the bottom spar cap:

Laying the spar down on the cross tubes of the bench confirms the spar is completely straight and true - very happy as everything that attaches to the spar is relying on this.  It's amazing how stiff the assembly is and that without all the clecos it weighs much less too.  The wing attach point still needs to be anodized before it gets riveted onto the spar, I'll be doing that this coming week.

Once I have the anodizing done on the wing and strut pick ups done, I'll add them to right wing spar - these have much longer rivets.  Then I'll get all the spar cap holes on the left wing spar upsized to A5.  Then the process of disassembly, debur, clean, prime and reassembly begins, followed by doing some more buckin' rivets!
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Getting closer every day.  Thanks for following along!