Subskin Nutplate Rive Holes Part 2

I continued to clamp and drill the nutplate rivet holes for all 14 fuel tank attach nutplates using the methods shown in the previous post. Once those were done I decided to mark and drill the #40 nutplate rivet holes along both edges in the subskin that will be used to attach the removable outer skin to the subskin. I used a different method for these holes by taking a nutplate and inserting a #30 cleco in the center screw hole to secure it to the skin. I had to use this method instead of the nut plate jig because I am not ready to increase the size of the screw holes with the #19 drill just yet. So this was the only option for locating and drilling the rivet holes for each of these nut plates.

The problem with it is that you end up using the nutplate rivet holes to match drill the $40 holes in the skin, and that cleco does not exactly tightly secure the nutplate to the skin, so it is possible for the nutplate to shift a bit when drilling the rivet holes. How do I know that? You will see in a bit...….

Step 1 was to use a sharpee and a straight edge to draw lines next to each #30 hole that I had already drilled so that I had a reference line to align each nutplate. At the end of the day it does not matter too much if the rivet holes are all nicely aligned with each other, as long as the center screw hole is aligned properly. However, it does look better if the rivet holes are aligned with each other.

IN this pic I had already drilled the holes after adding the lines, but you can see how it looked.

Each nut plate was mounted on the outside of the skin, and the #30 clecoes were inserted from the other side as shown here:

I thought  had snapped some pics when I started match drilling the #40 rivet holes through each nut plate, but apparently I did not. How I did that was to align each nut plate with the lines that I drew so that the rivet holes would align  properly, and I simply held each nut plate with my fingers on my left hand while I drilled the holes through each nut plate with my right. As you will notice, this lack of securing the work properly ended up causing the drill to slip when I got to the most forward nut plates along each edge.

You can see how "off" the rivet holes are from the line that was drawn. At the end of the day this will not affect anything, but it really pissed me off when the drill slipped.After drilling the first hole I put a #40 cleco in from the top side, and then drilled the second hole.

When all the rivet holes were drilled it looked like this:

After these holes were drilled I removed all the clecoes and the nut plates and deburred all the new holes. Then it was time to mark and drill the holes along the rear area of the removable section of the subskin. I basically used the same process for these holes that I did along bot sides:

I had to get creative once again about how to clamp everything down so that each of the holes was being drilled into a piece of wood underneath the metal:

Here is a series of pics showing the clamping arrangement of the other side of the fuel tank attach nut plate holes. The wood and clamps had to be repositioned each time to drill each hole, and yes, it was just as much of a pain in the ass as it was for the other side:

After that was done it was time to dimple the #8 screw holes in the center of each nut plate position o the skin for all the fuel tank attach nut plates along one edge of the subskin. This time I made sure that the male AND female dimple die were installed on the C-Frame. The C-frame was used instead of a squeezer because the dimple are much crisper. I have seen many airplanes where the builder obviously used a squeezer instead of a C-frame to dimple these holes, and there is a very noticeable difference. ALL the screw holes had been deburred long ago, but I went over then one more time to be sure, because this is a step where cracks can form in the dimple. I read one post from a builder's kit log where he was using a squeezer and he ended up ordering new skins and parts because his dimples kept cracking. Had he used a C-frame he might have had those problems.

Here is the photo sequence:

The most forward holes closest to the bend in the skin were tricky to dimple. I basically used my head, hands, and whatever else I could to try to straighten the skinor at least orient it with the bend as it sat on the male dimple die so that the dimple would be properly oriented with the bend. I think this came out OK.

Next I needed to dimple the #40 rivet holes for each of the fuel tank attach nutplates. I did these with my pneumatic rivet squeezer. Once the skin dimples were done I now needed to dimple the #40 rivet holes in each nut plate so that it will sit flush with the dimpled skin. I should also back up a bit to note that prior to dimpling the fule tank attach rivet holes I did quite a bit of research on other build sites and on VAF, and ultimately called Vans, to find out if I should dimple those holes and use normal sized rivets for each nut plate, or if it was acceptable to use the NAS 1097 "oops" rivets with a smaller head so I could countersink the subskin holes instead of dimpling. The reason for using the NAS method s to avoid having to dimple the nut plate flanges by countersinking the skin.

I had seen builders do it both ways, but I wanted to be sure, since this is a potentially structural area of the aircraft where several parts of the wing are under stress. The general rule is that in that case you should dimple the holes and nut plates, because a dimpled hole is stronger than a countersunk one. Vans ended up telling me to dimple the holes and also dimple the nut plate holes using a squeezer.The problem with that is that the plans do show to use an AN426 rivet, but they do not show the dimple, which implies that the joiner plate is countersunk. They also state to use an AN426AD3-3.5 rivet, which is probably correct for a countersunk hole, but according to my rivet gauge the correct size rivet to use for a dimpled hole is an AN426AD3-4 rivet, so that is what I used.

Now, the problem with having to dimple the K1100-8 nut plate rivet holes, is that the center screw hole is already countersunk, so the area of the nut plate around he center expands outward quite a bit toward the rivet holes. If you use a standard sized #40 dimple die the female die is too wide for the nut plate center hole, and you will damage or deform the nut plate or mis-align the dimple in the #40 rivet holes. SO the solution for this is to use a reduced radius female die, or you can take a standard sized die and grind down one side so that it clears the countersunk center hole of the nut plate and cleanly dimples the #40 rivet holes on either side.Here it is installed in my squeezer:

Notice how the female die on the right has a smaller radius than the male die on the left.This allows you to dimple the flanges of the nut plates without damaging or deforming the center hole. I'll continue with more pics and info about this process tomorrow. Tonight I actually got tote point where I started setting rivets for the nutplates, and I had not set any rivets for quite a while, so that was refreshing. Eventually I will get the blog caught up with my actual progress, but my focus right now is to stay on track to finish all the remaining steps necessary to complete this mod. The closer I get to that point the more I want to finish it. Anyway, more later.