Sun Aug 15 2010

Adding nutplates to lower longeron

Well it's hard to see in this photo, but where the floor panel rests on the lower longeron, I need to install three nutplates. Here I've got the joggle strip drilled to #40 at the nutplate locations. Unfortunately, it looks like the forward-most screw is going to be blocked by the diagonal gear weldment arm. I probably should have drilled that one about 1/8 further forward. Well, maybe I can cheat the hole forward a bit when I drill it up to #19. On top of that, the weldment is going to be in the way of installing these nutplates. Looks like the weldment has gotta come back out.

After an hour or so of removing all the bolts that hold the weldment on, I still couldn't get this thing out. It turned out that this nutplate was hanging it up. No prob...just drill out the two rivets and...

...the weldment is out!

I made a little drill bushing out of a 1/8 inch thick piece of 6061-T6 scrap. The aft two holes on this bushing/strap are clecoed into the two mating holes on the lower longeron, but I drilled the forward hole (shown here) about 3/32 further forward than the original #40 hole. Now when I drill the lower longeron up to #19, the center of the hole will have moved forward about 3/32.

Since I didn't have a #19 long bit, I went ahead and ordered one from Brown Tool. While I was at it I also ordered a #27 (for #6 screws), a #12 (for AN3 bolts), and a 1/4 and 5/16 long bit.

The drill bushing / strap worked great, although I did have to clamp it to the longeron close to the hole to keep the bit from flexing it around.

After borrowing Brent and Greg's #8 nutplate jig which I used to drill the rivet holes, it was time to countersink them. Because of the limited space, this is pretty much what I resorted to. I knew it would be hard (impossible) to accurately control the depth of the countersink, but I wasn't too worried since these rivets are just for nutplates. Looking at this photo after the fact though, it occurs to me that I could make up a spacer out of some tubing which would fit around the countersink and act as a non-adjustable microstop. Maybe I'll do that when I countersink the left side lower longeron.

Here you can see the depth of the countersinks isn't well controlled. Pretty much all of them ended up being over-countersunk to one degree or another...

...which actually worked out great. It was really difficult to get the pop rivet tool in the confined space and hold it straight, so some of the pop rivets wern't square. Having the countersinks too deep though allowed any non-square installation to still be below the surface of the angle. I did have to drill out and replace a few that were too crooked, hence the shavings on the skin.