Time: 5 Hours
Yesterday I received a new tool from Aircraft Tool Supply to help with AN470 rivet removal. This tool is like a countersink cage with a drill bit and a centering head on the end. The bit and head can be changed to accommodate various sized rivets. The problem with drilling AN470 rivets is they had a round head and getting a drill bit to stay centered is very difficult, resulting in a usual oblonged hole.
Here you can see the forward spar and it’s rivets that tie the left to the right. All these get removed and a doubler with extra rivets will get added. The doubler extends from center outward and required some modifications of existing parts. For now I’m just removing all the parts and taking care of the mods that are required until my parts arrive.
First task is to drill all the skin/rib rivets that hold the forward and aft inboard ribs. I marked all the rivets that I needed to get. These AN426 rivets need no special tool since they are flat and much easier to drill out.
Once the skin rivets were done I turned to the 470’s and started with the easier accessible ones. I set the gauge on the rivet remover to drill just deep enough to cut through the rivets head but no the underlying material. Here you can see how the drill is perfectly centered.
Once I drilled that hole I used a 1/8″ pin punch inserted into the drilled hole and snapped off the head. I then drilled a smaller #40 hole in the center of the remaining rivet shaft for just a 1/16″ or so. I then used my modified automatic center punch. This punch has a spring internally that snaps the pin as you push pressure on it. Normally used to mark metal so that your drill bit does not wander during drilling. I took the sharp point off and squared it off, I also narrowed the end to more of a straight but rather than a tapered one.
For the 4 rivets that tie the flanges of the forward rib to the spar and aft rib are too close to the rib web that my new tool wouldn’t fit. I had to turn to a borrowed part, thanks Glen, which gives your drill a 90° turn.
This drill was no perfect and drilled at a slight angle since it was so tight. So I drilled just deep enough to be able to snap the head off. I then repeated the process of punching out the rivet. The holes turned out perfect with no damage.
I then grabbed my 1/4″ round file to make relief notches in the spar, top and bottom, as per the bulletin. These need to be 1/4″ wide by 5/16″ deep and will help alleviate stresses in this area avoiding future cracks.
You can see on the left spar the lower middle hole that I buggered up during the initial build and had to go up a size in rivet to fix it. The hole looks worse than it is and is a lesson for me to not try to drill out a rivet just because it has a smiley face on it! The next mod is to trim the ends on the spar flanges at an angle. I took measurements and marked these cut lines.