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Tank Attach Nut Plates

July 1, 2011

The first task on the wings is getting the tank attach nutplates on. There are a few more steps here, than normal. This is the first place where there is a lot of counter sinking. Ugh…I’m not very good at it. I guess its time to figure it out. About the only place to mess this up is to make your counter sink (cs) to deep. This is a common mistake, but I’ve read enough build sites and the RVator articles to be ready for it.

One of the first things I wanted to do was make a “jig” of sorts to allow me to improve my cs skills. Part of the problem with counter sinking is that sometimes the cs is so deep that the hole at the bottom of the cs is enlarged. To counter this you can place a sacrificial piece of aluminum behind your work. When you final drill your holes, you bore through the sacrificial piece. When you start to cs the work piece, the sacrificial piece has a hole in it, the proper size, to evenly guide the bit. Theoretically this will keep the hole even, without the chatter. Chatter is hard to explain. Like Justice Potter Stewart said about porn, “I know it when I see it”.

Before drilling you have to tape off quite a bit of area. The spars are made by a contractor for Vans. They are VERY EXPENSIVE. The last thing you want to do is goof up. You need to manage the aluminum chips and shavings that are produced while drilling. To do this I’ve masked off anywhere that I dont want to have chips wandering.

Roll of paper that I salvaged from the painting supplies in the attic.

I found this roll of paper in the painting supplies box. Once I’ve taped it in place, it does a good job of deflecting chips. Why do we do this? I’ll show you in a minute.

The paper hopefully keeps the chips out.

Here’s a shot with my paper in place and you can see my sacrificial aluminum stock clamped in place.

Once this is all setup it is time to drill. Right off the bat I find a problem with my taping.

See the shiny little chip in that slot?

There is a tiny little piece of aluminum in that groove in the picture above. What you cant see in the photo is there are a bunch of chips. The one you can see just happens to be the largest one. As promised here’s why this is a problem. Imagine if I left that chip there and assembled the plane and went flying. That chip is an piece of untreated aluminum alloy. It will likely start to corode in a year or so. Once it starts coroding the corrosion will find any weakness in the alodine coating (the gold stuff) and start to work on the spar. Make no mistake, there is no more important part on a plane than the wings spar. If corrosion weakens it you may get no warning of the failure. It could be instantaneous and end badly. Where ever two pieces of metal come together you want to keep things clean. Normally before we rivet two pieces together there is a deburring, dimpling or counter sinking, cleaning and priming cycle. However the spars are pre-assembled. Policing the chips is just another thing you have to watch out for. Using a split toothpick and some compressed air I was able to evacuate all the chips. A long strip of painters tape will keep any further chips from getting in there.

So the cycle here is drill all the #40’s. Drill all the #19’s which are the large holes in the center of the pair of #40s. Deburr the underside of the holes. I dont care about the tops because I’m going to be counter sinking all the holes. Cs the #40’s and then rivet the nutplates in place. Last but not least cs large center hole. That will require a bit of finesse which I’ll describe tomorrow.

The faded X is to remind me not to do anything to this hole.

Vans warns you to be very careful and know exactly where your drill is going before you pull the trigger. See the faded blue X in the middleish of the picture. That is a hole that has nothing to do with the nutplates. I have mark it to remind me not to mess with it. Next time I think that I’ll place a bit of tape over it.

See that blue, goopy, greasy mess in the pictures. That is called Boelube. Boelube is for aluminum what drilling oil is to steel. It helps keep the work lubed which helps preserve the drill bit and the counters sink bit. It also has the added benefit of keeping the chips isolated and in one place.

Great stuff. Must have for this project.

It is slightly greasy but wipes away very easily. I dab it on the holes that I’m drilling. Generally if I’m only drilling a handful of holes I dont mess with it. But for a lot of holes like this, I don’t mind the added mess.

A little dab'll do ya.

I had a good session tonight. It felt really good getting back into the garage. I hope to get a bunch done this long weekend.

Close up of counter sinks.

The nutplate attach holes have been counter sunk. Still have to do the middle hole for the screw.

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From → Build Log, Spars, Wings

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