The pictures end up reflecting so much light from the countersunk holes that they did not turn out very clear when shot from straight on, but the idea was to try to capture how close to the edge of the spar flange you are when your inspection plate holes are countersunk to the prescribed depth. In shot, the edge of the countersink is extremely close to the edge of the spar flange, but the hole for the screw on the back side, as well as the nut plate that will be attached there, all have plenty of surface to adhere to. I must admit that it was a little nerve racking trying to set a countersunk hole so close to the edge, but if you want your inspection plates to sit as flush as possible against the spar, you need to countersink it as close as possible without going over. Anywhere from .310 to .3110 is probably just fine.
After the holes were done I deburred them on both sides of each flange. True to what most people say, when you are done countersinking them the inside edges are rather razor sharp and rather burred as well. Using my deburring tool on both sides with just a quick turn on each side removes the sharp edge and leaves everything quite smooth. I will need to countersink the inspection plate holes on the left wing spar just a bit more, since I was very conservative on those initially because they were the first ones and I was not sure about how far I should take them at that time.
Countersinking the rivet holes with a #40 countersink bit is next - two of those for every one large hole that was done - what fun that will be. Actually that should go rather quickly, since the countersinks are not that deep for the rivet heads, which sit flush on top of the spar. After that I need to prime all the holes, since the anodized coating has now been removed.
Then I am going to have to get up on my garage shelf and dig out the fuel tank skins and ribs, since I will be working on all those very shortly.
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