Wings 7, Total 470 - Testing countersink depth and Tech Counselor Visit

So I thought that I would be all set to start countersinking the nut plate holes last night - right up till I reviewed several more VAF posts discussing the method that should be used to do this. I originally thought that I had this all figured out and I was ready to go ahead and use my method of setting the countersink depth so that the dimple in the skin would sit inside the countersink hole and the skin would rest properly over the flange.

I paused after reading several posts that started mentioning that Vans no longer recommended using the test dimple/countersink method because too many people were over-countersinking the holes in the flange. So instead of using the dimple test method, Vans started recommendng a range for the sze of the diameter of the countersink, which should be somewhere between .365 and .375 inches. Apparently they mentioned this in an old RVator article, which is the monthly magazine that Van's was creating until they stopped in 2010. Most folks are apparently still adhering to this recommendation from Vans and they split the difference of the range and settle for countersink diameter of .370 inches. The argument being presented against this approach was that with this diameter, the skin/dimple does not quite sit flush, and there is a small amount of spring back.

Apparently Vans says that this is OK, and several other builders said that this works out just fine. Then came the other glaring comments about making test pieces to determine what the correct countersink depth should be. This ended up hitting me like a ton of bricks - how stupid would I be to conduct "experiments" on the actual parts of the airplane. Have I not learned anything yet?

So after a deep sigh, and a realization that I might not get too much actual hole drilling done on the wing spar, I put the idea of experimenting on the actual wing spar completely out of my mind, and got out a couple of pieces of scrap aluminum from my trim bundle that I received with the empennage kit. One of them was a piece of flat 1/8 inch thick aluminum plate, and the other was a piece of .032 inch thick aluminum which is supposed to be the thickness of the Wing tank skins, although I thought that they were a bit thicker at .040. I drilled a series of #19 holes in the thick aluminum. This was my countersink test piece. Then I drilled a similar hole in the .032 inch piece, deburred the hole, and set a #8 dimple in it. Then I took my microstop countersink tool and started countersinking the hole in the thicker aluminum with a shallow depth at first, and increasing the depth a little at a time in between measurements with the digital caliper.

I performed a couple of different tests. The first was to countersink until the dimple and the skin would just sit flush in the countersunk hole with little or no movement. This seemed to occur at a dimater of about .396, or just shy of a .040 diameter. This is almost 3/100s larger than the recommended size of .370. SO then I countersunk another hole to the recommended .370 diameter, and I could detect just a small amount of spring back of the dimple, but I could still almost squeeze the dimpled skin flush with the thicker skin containing the countersunk hole. Since this is the dimension that recent builders seem to be going with, I decided to do the same.

Now for some really crappy pics to take you through the process - they came out blurry for some reason.

First is the diameter of the #8 dimple in the .032 thick test piece, as shown from my digital caliper it is .375 inches wide, or just slightly larger than the .370 dimension of the countersunk hole.

Here you see the math that I used to meaure the dimater of the base of the #8 dimple

And here are the test countersunk holes, one of them too large, and the other set to the .370 recommended diameter as shown:

And the top side of the #8 dimple

And here are the #8 screw dimple dies:

And here is the #19 Countersink bit that I used:

I got 2 of the 3 drawer knobs that I ordered for the new tool drawer - very cool!

Reflections are reaking havoc on this next pic, but this is the first countersunk hole that I drilled on the spar flange. One down, many mnay more to go:

And finally this the actual method/process that I used to drill the holes and countersink it:

1. Started with a # 21 drill bit to drill completely through the center hole and the aluminum angle on the back side
2. This creates a hole that can then be easily drilled out with the #19 drill bit, which is the final size hole you want for the #19 countersink bit. So you switch bits and final drill the hole to #19.
3. With the correct depth set on the microstop countersink tool based on the work on the test pieces, insert the pilot of the #19 countersink into the hole and countersink the hole.
4. Check the countersink for even diameter all the way around the hole. This does leave the bottom edge of the hole rather razor sharp, which raises concerns from some builders, but Vans apparently dismisses this and indicates that this is the way that it should be.
5. Wash, rinse, repeat for the remaining holes. Oh yeah, forgot to mention that you need to remove the clecoes and reposition the clamps as necessary to ensure that the countersink tool can sit flush to the spar flange.

So now that the first one is done, the rest of them should go fairly quickly I think. I need to get oe more shot that I did not take tonight of the screw head sitting inside the countersunk hole and also inside the dimple. It sits pretty flush inside the dimple, but sits noticeably deeper inside the countersink than level with the top of the flange.

And finally, My EAA tech counselor visit this evening. John Linz, a fellow EAA chapter 301 member, was kind enough to look over my my mostly completed empennage parts. Apparently everything is good enough to press onward, even with all the dings and scratches noted. John is a great guy, and we talked a lot about building and flying while he was here. He also told me some of the horror stories he had encountered or heard about from various other builders over the years. Pretty crazy stuff. I am glad to report that I don't think I fell into that category!

It is always a good feeling to have other experienced builders critique your work at various stages. You must have built at least one airplane in order to receive the designation as an EAA technical counselor. Nothing beats first hand experience. After the visit was done, some paperwork was filled out, and he was on his way after visiting/reviewing everything for about an hour or so. Thanks for looking everything over John!

Now back to the wings!