Deburred the tank rib flange to skin holes

Today I managed to start deburring the rivet holes for the tank rib flanges and the tank skin. The first order of business was to start removing the ribs one at a time with the tank in the cradle. This fuel tank has been clecoed together for over 3.5 years. This is the first time (aside from the removal of the inboard and outboard ribs to drill the tank baffle to the Z brackets) that the fuel tank has been completely disassembled since that time.

After removing all the tank ribs, I was bit surprized by the amount of "residue" sitting in the bottom of the tank skin. This is the result of all the match drilling and deburring that had taken place so far:

If only that were gold dust in there!

The next shot was simply to show the removal of the blue vinyl along the rivet lines for the ribs. I am leaving most of the vinyl covering over the holes for the stiffeners until those have been match drilled.

And of course this next pic has to come out blurry. I was trying to get a shot of a problem area that I will actually have to call Vans about tomorrow. Unfortunately, long ago when I was first wrestling the tank together, this resulted in some unsightly scratches that extend a bit wider than the typical inch or so on either side of the rivet hole center lines where I scuff and clean the line for primer. This usually does not phase me too much because I can always scuff the scratches out and prime over then since the alcad layer is removed during the scuffing process.

For the fuel tanks, no primer or other anti-corrosion treatment can be applied because it might not ply well with 100LL avgas. So now I need to find out just how wide I nee to scuff everything, because I HAVE to do it to prepare for proseal, but what I don't know is if I am supposed to ONLY scuff enough material to be as wide as the rib flanges, but no wider than that. Most other builders logs the that I review show that they have scuffed quite a bit more material, much wider than the width of the rib flange, but I don't think that proseal is applied to the area that is wider than the rib flange. Anyway, I have to check this because worst case is I will need a new tank skin.

Next is the stack of ribs that were already marked for each position inside the skin. As I pulled then out of the skin it was obvious that I need to do a bit more deburring and maybe touch up a few edges here and there on certain ribs.But the main task for tonight was to get as many holes deburred as possible.

I was able to debur the both sides of the fuel tank skin for each rivet line of each rib. I did the outer holes with the skin still in the cradle. For the inner holes, I wanted to try to continue to use my standard deburring bit in my electric screw driver, but to do so meant that I needed to remove the skin frm the cradle and place it on top of the dimpling table on my work bench. This would allow the skin to open up enough to be able to use the screwdriver to debur the holes.

I also was able to finish deburring all of the #19 screw holes for the joiner plate attachment on the outboard edge of the tank skin as well as the holes along the top and bottom edges of the skin where they attach to the main wing spar flange. To do this I used a combination of my standard deburring bit, and my edge deburring tool. These screw holes are about the same size as the tooling holes that are used to hydroform each of the ribs at the factory. I found that they were large enough that I could use the edge deburring tool to go around the edges of each hole to make sure they are truly smooth and free of burs.

Why is this so important? If the edges of these holes are not extremely smooth, when you try to dimple them with the #8 dimple die they can crack or end up having rough edges that could lead to stress cracks later on. So it is vital that these holes are super smooth. Here are the tools I am talking about:

The one on the left is my standard deburring bit from Cleaveland tools. It fits in my small electric screwdriver (NOT DRILL!!) DO NOT use this in a drill - only a screw driver with a slow setting. Usually 2 squeezes of the trigger does the job for #30 and #40 holes. It only has one single flute on the tip.
The center tool is the edge deburring tool with a fine hardened small blade. It is normally used for deburring straight edges in skins, but works pretty well in these larger sized bolt and screw holes.

The tool on the right is a speed deburring bit, similar to the one on the left, but with 3 cutting flutes instead of one. All I can say about this tool is to AVOID using it all costs. It tends to chatter when used, leaving uneven edges around the hole, and it can also take off too much material too quickly, which will leave the hole basically countersunk instead of deburred if you are not careful. Needless to say I don't like this tool very much and therefore do NOT use it very often.

I'm sure I have reviewed the use of these tools before, but it never hurts to do a refresher now and then. I managed to debur the outer flanges of all 7 tank ribs, so tomorrow I still need to debur the inner flanges, which, unfortunately, I have found I need to do by hand with my standard deburring bit. This is because I have not found a good reliable method for using an angle drill that would actually fit in the small space without marring up the rib web in the process. So I just do these holes by hand, painful as it sounds.

Lastly, I did some research on the T-405 tank attach bracket. Turns out there are some tricks to this assembly that also need to be taken into account when this is fabricated and positioned inside the nose section of the inboard tank rib.

Questions for Vans that need to be asked tomorrow are:

1. Scratches in the tank skin beyond the edges of the rib flanges - can I scuff those out without compromising the skin?
2. Z bracket prep for proseal and primer - do I plan to proseal the entire T-712 Z bracket flange that will sit against the rear baffle plate, or do I prime this flange, but only in the areas where proseal is not applied, i.e. about 1/2 inch around each rivet hole.
3. Method for clearing rivets when fabricating and positioning the T-405 tank attach bracket? Notches? Reduce the edge enough for dimple and rivet shop head clearance where the skin attaches to the rib?

After I get all the inner rib holes deburred, I need to put everything back together so I can countersink the rear baffle plate holes where they attach to the tank skin. I kind of did this out of order from the instructions because I wanted to remove all the burs first before I countersink these skin holes, to make sure everything is fitting together properly. I probably should have just countersunk the holes in hindsight before I removed the ribs - oh well - so it goes.

So deburring the inner rib flange holes and hopefully rough cutting the stiffeners are next on the list for tomorrow.

KPR