SO the next step was check the instructions again. They basically take you through the entire process to finish the fuel tanks, but because of the mod to the LE I really needed to finish that first. After the tanks are all but finished, the instructions tell you to rivet the LE together with it in the LE cradle. So the reality is that the LE can be assembled while in the cradle at any time. It cannot be riveted to the Wing spar, however, until the fuel tanks are done, because the screw holes in the tank skin that slide over the top of the joiner plate attached to the LE need to be final drilled, deburred, and dimpled for the #8 screws, and the assembly fit-checked with the LE again to make sure everything is AOK. One of the things that also gets in the way of this is excess proseal on the end rib of the fuel tank that may interfere with the joiner plate flange. Sometimes this has to be trimmed away to prevent this interference, which can cause the screw hole alignment to get messed up if this is not resolved. SO this is all interlinked, but for now I can go ahead and rivet the LE together, shich also means that the subskin is finally going to become a permament part of the LE.
The instructions say to cleco it all together and then "ensure that the rear holes are perfectly aligned with each other, which I guess means the holes on the rear flanges of each LE rib. SO to reverify this I think I clecoed it all to the wing spar one last time to reset all the ribs after tugging and pulling on all that vinyl, then I carefully removed the LE again, making sure that I did not twist or bend it so that the hole alignment would not be disturbed. Then I carefully placed it back in the cradle.
Next, there were some scratches on some of the rear rib flanges and the area of the joiner plate where the tank skin edges dug into the primer and the metal a little bit, so I sanded those areas out to remove the stress risers. Then the instructions say to start with the rear-most rivet hole with a squeezer on the top and bottom of each LE rib to secure them into place. I was able to do this easily with the pneumatic squeezer and a flange yoke. As part of the mod, the two inboard-most LE ribs also have two additional holes that I needed to add to the skin and each rib and the subskin. These "second" to the last rivet holes are relatively close to the rearmost holes, so I thought I could set those rivets with the air squeezer as well. The thing that I forgot was that the #2 inboard rib, which is a modified 408-1 rib, has the shop heads of the AN470 rivets protruding a bit from the rib web, and when I tried to set these with the air squeezer the rivet set hit the edge of those shop heads and caused the set to jump, which resulted in a badly set rivet, which I had to drill out and replace. After drilling out the bad one, I decided that in this situation I needed to use the hand squeezer to maintain positive control of setting the rivet so that I could ensure that both the yoke AND the rivet set would clear the shop heads of those AN470 rivets holding the rear rib flange extensions onto the rib web. I put the 3 inch yoke on the hand sqeezer and got those rivets set as well.
Now that the rear-most rivets were set on the top and bottom sides of each LE rib, it was time to get out the rivet gun and the bucking bar. More decisions to make and prep to do. For example:
I had to decide the pattern I would use to set the rivets on each rib
I had to get towels and place them inside the LE skin to prevent accidental dings in case the bucking bar slips out of my hand
I had to tape up the edges of each rib web to help prevent gouges from the bucking bar if it slides off the rivet shaft
I had to figure out how to clamp down the table/cradle assembly to ensure that NOTHING would move or slip during the riveting process
I had to check the air pressures on the compressor and at the adjustable pressure regulator on the gun with the new slightly heavier rivet set.
I had to determine all the rivet sizes I would need to attach various parts together.
ON the last, the plans call for AN426AD3-3.5 for he main rib flanges, and AN426AD3-4 for the rivets that attach the skin, rib flange, and joiner plate assembly. However, I found that the 3.5s are OK for the main rib flanges on the main ribs, but on the two ribs that are involved with the subskin assembly, I found that the 3-4s were good for the skin-to-subskin rivets with no rib flange involved, but I found that a 3-4.5 was the correct length for the rib flange, subskin, and outer skin assemblies. A 3-4 just is not quite long enough for some reason.
SO the plan was to start on the center ribs and then work outward toward each end rib. I also started on the bottom side of each rib because it is flatter than the top - this is something that many builder advise to do when building the fuel tanks because you can set the tank down on the bottom afterward to make work on the top a bit easier. I worked from the rear-most hole to the front, as stated in the instructions.
When I started, the one thing I forgot to do was check and reset the pressure on the air compressor, which had been tuned down to 75 psi for the air squeezer. Since the pressure on the rivet gun is controlled by separate regulator attached to the air inlet of the gun, the pressure at the compressor needs to be at 90 psi. Since I forgot to do that, the first several rivets I set seemed to take forever, and the shop head was not being formed as quickly as I was expecting. A quick check and reset of the pressure at the compressor basically fixed that problem.
Time for the pics. Here are some of the new "rubber boot" flush mushroom rivet set showing the before pic, the disassembled pic before trimming down the rubber, and the "after" pic showing the trimmed down rubber on the reassembled tool:
I took the rubber boot and ran it by hand across some aluminum oxide 220 grit sandpaper after marking it with a sharpee line. I did not want to sand too much of it away because it is still supposed to be a little proud of the set to provide some gripping ability on the skin. Then I put the tape on the rivet set - another important little trick to prevent damaging the metal with this pesky tool:
Here are some pics of the tape prep and towel prep to minimize or prevent other possible damage:
And finally some pics of te rivet gun in place and ready to set a rivet, and another pic of the bucking bar, using the angled side of the bar, showing how all this has to be positioned to set each rivet:
And here are some pics showing the progress - basically not more clecoes and a clean looking skin. the reality is that I have several rivets to drill out, mostly because they were overset, but a few are badly set rivets that need to come out. Overall I was pleased with the rivet process, and the rubber mushroom seems to do well at preventing skin damage. But the one thing that it does NOT do is ensure that the set cannot "walk" across the skin. I had numerous events where this set decided to have a mind of its own and start walking up, down, and to either side. Luckily it did not seem to damage the skin at all, thanks to the rubber edges I guess. It is designed to allow it to swivel a small amount so that you can presumably hold the gun not quite perpendicular to the skin and still get a good set. Personally I found that I still had to figure out what angles to use both up and down and side to side and how much pressure to apply to hold the rivet gun in place.
You will notice that I left the bottom/forward-most rivets for later. These are too low to be bucked - both from a position standpoint of the gun and bucking bar, and a gravity standpoint where the rivet cannot stay in the hole at the curved angles of the these holes - it falls out. SO these will be set with the cradle turned on its side after all other rivets have been set. These forward rivets are the ones I dread the most, because the potential for skin damage due to a slipping rivet gin or bucking bar is extreme, and I already have enough bad experiences with that to prove it. The trick, once again, is securing the work so that it cannot move, and so that you can easily position the gun and the bar to set those rivets. it usually also means that you have to set these "blind", meaning that if you do this solo, you will either be able to view the bucking bar on the rivet, or the rivet gun, but you will not be able to see both at the same time. I hate setting those rivets. Curved surface riveting sucks big time.
Anyway, this was the longest work session I have had on the place for a very long time several hours today. I only have about 3 more ribs and the final forward rivets to go, as well as the swiss cheese rib, subskin, and outer skin assembly for the LE mod, so there are still a large number of rivets to set. Hopefully I will get most, if not all of them done tomorrow.
Pounding rivets again using every method on the planet and lots of tricks you learn along the way - so far so good...….
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