Drilling Nutplate Rivet holes for the Fuel Tank Attach Bracket Section of the Subskin

As stated in the previous post, I had some options for how I was going to locate and drill the #40 nutplate rivet attach holes. These are the 14 nut plates - 7 on top and 7 on the bottom, that allow screws to be used to join the inboard LE to the outboard fuel tank skin when they are butted up next to each other on the wing spar to make one contiguous leading edge. All of these holes are on one side of the subskin. I had purchased a #8 nut plate jig long ago but had not really used it just yet. So I decided to give it try.

The way that it works is that you must have already located and drilled and deburred the center screw hole with a #19 drill bit first, and then use the jig to start locating the two remaining #40 rivet holes for the rivets that attach the nutplate to the skin. Using the jig properly requires some planning. You basically need to be able to position the part so that there is clearance for the nutplate jig, and the real trick is that you are supposed to drill each of the two rivet holes into a piece of wood to minimize the burs that occur on the opposite side of the hole. The problem with this is that the jig has a center lug the same size of the #19 center screw hole that inserts into the drilled hole in the skin, and it becomes a bit awkward to try and hold the jig in position and position the drill to drill the holes. IN short, I fond this to be a rather monumental clamping effort that required some creative thinking.

I started by placing the subskin on top of two 3/4 inch drill boards on my bench to give it enough height to clear the tools. I drilled each hole on the inside edge of the subskin for starters, and started at the rear of the subskin since this is where I had the most clearance for the drill and the clamps. I changed this setup as I got closer to the forward edge of the subskin where space for the drill is almost non-existant. Here is the pic that does a much better job of showing how I set up for this:

This pic also shows two thin strips of 3/4 inch partical bard along the edge of the bench, shown toward the bottom right of the above pic. And then the nutplate jig is inserted into the first hole in the subskin, and I used a small metal clamp to secure the nutlplate jig to the subskin to keep it from moving as I drilled the rivet holes. The jig has this long handle on it that frankly causes the jig to be heavy on the end and this causes the jig to tilt upward just a bit in the screw hole. If left in this position then your #40 drilled holes might end up off center and out of position with the screw hole. So, as I have stated many times before - rule #1 is SECURE THE WORK!!!!! You want to set this all up so that NOTHING moves and you can keep your hands as free as possible for other tasks while drilling each new hole through the jig.

Here are some blurry pics and some good pics of the nutplate drill jig on both sides. On one side you get the #19 lug only, and on the other side is another #19 lug plus another #40 lug, which you use after you have drilled the first hole. The trick is that as long as you have the drill jig aligned properly for your fist hole, then it will all come out pretty nice. 

Getting ready to drill the first rivet hole:

The single lug side of the jig is inserted into the screw hole and the jig is clamped into position using the skin edge as a reference to ensure that holes are drilled as straight as possible along that edge. The small pieces of wood are inserted under the skin in the area where the new hole is to be drilled, which is through the hole in the jig located closest to the yellow clamp face.Whe you drill this first hole, you flip the tool over and insert both lugs into their respective holes and drill the rivet hole on the other side.

A view from underneath showing how the small wood drill board is positioned under the skin:

Here the first set of rivet holes has been drilled for the first hole, and now the jig is in position to drill the second rivet hole for the second screw hole. The first rivet hole has already been drilled:

I had to keep switching the position of the small clamp from one side to the other as I switched the jig position, because the drill board wood also had to be repositioned each time as well - a major pain in the ass. This is the main reason why a job that you might think will only take a few minutes to perform can actually take a couple of hours.

The handle on this ool just keeps getting in my way and makes this process more difficult, but I am hesitant to remove it because I am sure it may come in handy later on.

As I got down to the most forward holes of the subskin, the tool clearance for the drill became an issue, so I flipped the subskin over and drilled the final two sets of holes from the outer surface of the skin:

The real challenge was trying to use the jig to position and drill the most-forward hole, because this hole is located on the curved portion of the subskin where it forms the curved tip of the leading edge. My method for this also followed some advice I got from Vans a long time ago about this very problem where I was told that I could either bend the tabs of the nut plate to confirm with the curvature of the skin, or flatten the skin as much as possible and drill the rivet holes normally, and then allow the parts to confirm the nutplate as the parts are joined together later. I chose the latter method and attempted as best I could to flatten out the skin so that the rivet hole locations still matched the holes in each nut plate.

IN the pic above you can see the gap between the nut plate and the subskin. The theory is that when I finally get around to securing the fuel tank to the LE with the screws, the nut plate flanges will bend to confirm the curvature of the skins that are being attached together with the screw. I hope that works, but I won't know for certain until I get to that step. I might be able to take the nutplates for these locations and use the inner circumference of a steel tube and a clam to pre-form the flanges of the nut plate to the desired curvature. Will have to experiment with that and see how it goes.

Here is a pic that shows the truly creative nature of this process. I came up with a plan to use clamps and a heavy can full of clecoes to help flatten out the skin as much as possible before I drilled the rivet holes using the jig. The can opens up the top side of the skin, while the bench clamps pull the skin down onto the work bench and drill boards, and then one last clamp is cleverly used to pull the curved skin out to help flatten it in the localized area where the holes need to be drilled. This actually seemed to work quite well.

To that end, I have to rant just a bit now. I watched a series of videos on the EAA web site and other sites that supposedly show you how to use the nut plate jig tool. The problem with most of these videos is that they are only designed to show a demonstration, and almost always do apply the "real life" situation by using an actual part where all the holes MUST be aligned perfectly for edge distance and other considerations. SO they just apply the jig any old way to the test part and start drilling holes. SO they do show you how the tool is supposed to used, but they don't do a very good job of showing you how you need to use it in a REAL application. It basically boils down to the fact that they do not tell you or show you how you must align the tool properly so that all your rivet holes are in the desired line along the part, and they do not discuss how you need to position and secure the tool so that the alignment is maintained as you drill. I have found that I really hate that about many of the how to videos, because most of them are like - look, in just 5 minutes you do all this and you are done, when that could not be further from the truth in most cases. In reality the steps to position the work, clamp I down, figure the flow of the work, etc. take consider planning and thought if you want to do it correctly.

I have more pics to post about this process but will finish later as it is late now. I am also  a lot further ahead than this step but will finish posting each step in the process so that everything I have had to do to finish this up is properly documented. Till then, KPR....