June 28, 2007

Rear Spar Work Continues - 4.5 hrs.

I started this session by match-drilling the outboard stiffener to the right spar. I did the left one earlier, but this one was waiting for me.

Now I can get back to finishing those holes that were drilled into the center stiffeners. Here, the stiffener is clecoed to the spar and then flipped over to see how close my drilling came to the hole in the spar. You can see the ends of the clecos sticking up through the spar, and the small bit of blue sharpie marks remaining on the stiffener. I still had some roughing-out to finish, but not too bad. I ended up using a round file to finish the rough cutting.

Next, I found that a small sanding drum in my dremel tool was very effective in bringing this to a medium grade of finish, with both parts now perfectly matched.

Nice! The two holes are perfectly matched and ready for a final scotchbrite polish. But then I found out that my smallest scotchbrite wheel, the 1" size, won't fit in this hole. At least not in the smaller end of the hole. I had planned on using my die grinder with it to polish and finish the job. Looks like this job will have to be done by hand since I don't have a scotchbrite wheel smaller than 1".

Once again, this deburring tool is coming in very handy. I changed the "V" bit to this one, that swivels freely. It's inserted in the hole and twirled around one direction, then the other, and it puts a very nice bevel on the sharp edges of the hole. Note that the stiffener has been removed for this deburring operation, allowing me to deburr the spar's hole on both sides, and then the same for the stiffener's hole. Repeat for the right spar; four holes total, and you're done.

The final step called for in the rear spar assembly is to drill and dimple the inboard holes in the top flange of the rear spars, where the wing skins will be riveted on. The reason for this is because when those two inner stiffeners, the fork-shaped 707G and the 707D, get riveted to the spar, they are thick enough that you won't be able to get a dimpling tool under the flange and dimple the flange for the wing skin rivets. The stiffeners will be in the way. So you're supposed to drill them out to full size and dimple them now. Then the stiffeners get riveted on. I'm showing them all done here, but it turned out to be no easy matter.

The reason is, the Z-shaped spar has odd angles on the flanges. The lower flange is bent at less than 90-degrees, but the upper one is bent more than 90-degrees. It's an acute angle. I soon found that even without the stiffeners in place, I did not have a tool that would reach under the flange and allow me to dimple these holes. Not without scratching the heck out of the spar web, or bending the flange out. So all of a sudden, a simple drilling and dimpling operation was turning into a major hassle. I tried every one of my yokes on my squeezer, and none of them would reach under the flange without interference. The thin-nose yoke will work for riveting, but there's no hole in it for a dimple die. I tried my DRDT-2 and even a C-Frame but they won't work either. I even tried my pop-rivet dimple dies. I can get the dies on and pull hard enough to break the nail, but these flanges are thick enough that it doesn't make a good dimple. Hardly enough to even notice. I thought about just countersinking these holes, but that removes metal and isn't as strong when finished as a dimpled rivet. That worried me because this is the inboard part of the wing where the stresses are the greatest. I don't want to fudge on this! Besides that, there will be the rest of all the holes on the top of the rear spars to deal with later when the skins are riveted on. So I have to find a solution for this problem now.

This wasn't easy for me, but after much deliberation and hesitation I decided to grind away part of one of my favorite yokes. It is, after all, only a tool. And in the grand scheme of things, even if I ruined an expensive tool it's only a drop in the bucket compared to what a well-built airplane is worth. So I ground down the tip of this yoke as close to the dimple die as I felt I could. I also ground down the top so it's thinner out at the very end. By the time I finished polishing it on my scotchbrite wheels, I think it actually looks kind of cool! Best of all, it fits under the overbent flange perfectly! So I was able to finish dimpling those holes now, as well as have the right tool later on to dimple the rest of the holes on the spar.

And here is the finished result. Once you have the right tool, the job gets done quickly. The rear spars are now ready for priming and riveting.

But that will have to wait for tomorrow. I'm done for this session. Whew!

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