Thursday, April 15, 2010

Too clever by 62.5%

Why not "too clever by half?" Rest assured, we'll get to that. We always do.

I posted a question on the ever-helpful Vans Air Force dot Net site to see if anyone could shed some light on the problem that I had with a gap being created between the baggage floor flange and the Center Section (which I am still capitalizing as a proper noun because I'm on a first name basis with that part, as in "You ^#%*$# piece of %$@# Center Section") when I riveted them together. I received a very accurate answer:

Looks to me like you are waaaaaay over driving the rivets.
If unable to do it with the gun, why not just do it the recommended way with a hand squeezer?

To which I replied:

With the #4s, I use a Cleveland Main Squeeze. It has an unfortunate bulge that precludes use close to another skin. I might have to try it with a "regular" squeezer if I can't get a handle on driving them.

Which is also accurate, as far as it goes. But we'll get to that. First, about the over driving. If you look at the shop head on the rivet as measured by the rivet gauge, you will see that it quite plainly is not over driven. If fact, it's perfect!

Or so I thought. But then I saw this reply:

If a rivet is slightly too short to start with and an attempt is made to squeeze it extra hard to achieve the correct diameter shop head, then the rivet will expand too much inside of the hole and cause the skin to pucker.

Well, there ya go! That's exactly what happened! You learn something new every day, it seems, even if it's sometimes painful to do so.

No time for self recriminations, though, as the immediate problem at hand was the removal of the over-driven rivet. I stewed about that most of the night last night, literally losing sleep over it. It was also a nagging itch in the far aft reaches of my brain as I tried to concentrate on a difficult piece of recursive code at work; I ultimately failed at that too and punted to a less elegant but more comprehensible solution. You see, if the location of the rivet makes it hard to get at with a rivet squeezer, it's going to be just as hard, if not harder, to get to with a drill bit. I had to come up with a plan!

I thought the best approach would be to get a 90 degree angle attachment for the drill, but the only ones I could find locally were big, bulky affairs that were larger than the actual drill. No help there. That left me with the only other option I could think of: use the super-long #30 drill bit. I wasn't overly excited about using it to remove a rivet since 1) I usually use a bit one size smaller than the hole to avoid enlarging the hole, and 2) it's hard to be precise with that much bit wobbling around. As it was the only choice, however, I resigned myself to it.

Once I tried to line it up on the hole with the Center Section lying flat on the table, I could see that it would be more accurate to put the Center Section up on edge and drill straight down. I couldn't do that, though, because the "feet" were keeping the baggage floors rigid and I couldn't push them back a little bit to give the drill some room. The crowding problem was exacerbated by the fact that the floors are at a less than 90 degree angle with the Center Section. I would have to remove the "feet" to allow the baggage floors to lean back out of the way.

Did a light bulb just illuminate over your head? Don't look up, I meant that figuratively.

"If I can lean these floors back to allow a drill to get in there," I asked myself (rhetorically - I might ask myself questions out loud, but it would be weird to answer them too, right? I avoid the temptation by only asking myself questions that I don't have to answer), "wouldn't that also have worked with the rivet squeezer? Even if it did suffer from an inconvenient bulge?"

Yes. Yes it would. And it also would have kept the clecos from scratching up the surface of the baggage floors, an event the instructions warned me about with the suggestion to put some tape on the rivet squeezer. I'm so smart that I decided that since I wasn't going to use no stinkin' rivet squeezer, I didn't need no stinkin' tape. But with the floor pushed past the 90 degree angle by the "feet," it came into contract with the edges of the clecos and got all scratched up.

That brings us up to "too clever by half."

I removed the baggage ribs, pushed the floors a little bit back to get them out of the way, and started drilling on the rivet.

Eventually, after slow and diligent drilling, the head of the rivet popped off.

Now on some rivets, that's all you need; the remainder of the rivet will pull right out from the other side. But an over-driven rivet is a different beast indeed. You see, what caused the gap in the first place was some of the rivet spilling out from the top of the hole between the two pieces of material, in effect forming another head. The new head was still trapped between the baggage floor flange and the Center Section. The other three rivets would have to come out too.

I hadn't counted on that. When I was leaving the house, I remembered that I hadn't brought a fresh battery for the drill. Wouldn't need it, I thought, since I was only drilling out one rivet.

It died halfway through drilling out the third rivet. I was 62.5% done with drilling out the rivets when I had to stop because I was too clever to go back and get the fresh battery. Finally! Too clever by 62.5%.

Rather than head back home to get the battery and run the risk of getting trapped, I thought I'd try squeezing a few rivets into the other baggage floor to see if it was going to go any smoother than the first has.

It did. Although... the rivets are still a little too short. I don't think I'm going to be able to squeeze a regulation size shop head onto them. Maybe with the rivet gun....

Nah, not a chance.

This his how the flange is supposed to look:

They all went in fairly easily, although I found that there was a trick to getting the first little squeeze started with the floor leaning forward to keep the flange flush, then pushing the floor back to allow a straighter squeeze after the rivet was set enough to hold the flange in place.

Note the scratching from the clecos. Bummer, that. I think mine is by far the ugliest center section in the brief history of RV-12 building. That's kinda like my golf trophy: Worst in the League. I won that one pulling away. Of course, I couldn't hide my shame by painting the trophy - the same is not true for the baggage floors.

You think that side is ugly? You ought to see the other guy!

One of the effects of over driving the rivet is that it creates a kind of reverse dimple in the flange where the rivet material pushes it out of the way. I took the floor back home to try to work it back to flatness. After escaping an hour later to get back to the hangar and cleco the floor back onto the Center Section, I found that it still needs work.

I'm not sure I'll ever get it as nice and tight as the other side, but I'm going to give it at least one more try. I think it's always going to be at least a little ugly, though.

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