February 3, 2021
I was really wondering about the order in which the manual talked about attaching all the platenuts on the wing spars. It seemed to me that you would want to cut the big countersink BEFORE you mount the platenuts so that you could get it all primed at once, but I decided to trust the book and do it the way the manual says to do it.
The book is right. What happens is the pilot of the countersink bit will find it's way down into the platenut and it helps stabilize the cut. It chatters like crazy until the last little bit and then it cleans up nicely. I started using my cordless drill here because I wanted a slow turning drill, but I hate holding that big heavy beast up for hours at a time and the next day I had forgotten to plug the battery in. So I reverted to my trusty air drill that turns a little faster but it is lighter. Well as it turns out the faster speed seemed to do a better job of cutting those countersinks. Full speed on the air drill is too fast but I put it on the regulator and dialed it down to about 40 - 50 psi or so and it worked nicely.
If I had to do it all over again I might change it a little. I saw a suggestion on somebody's website where the used a block of aluminum that they had drilled a #30 hole in to help guide the pilot of the countersink bit. I wish I could remember where I saw that. They may have had to put some kind of threads in it to hole it to the spar too. I bet that would work a lot better and you could prime the cuts before you assemble the platenuts to the spar.
By the way you will enlarge that hole in the spar quite a bit. It made me a little uneasy at first (after all it's only the main wing spar, not like it's structural or anything :-) but that is the way it's supposed to be.
A viewer showed me where I had seen that little Aluminum block for guiding the pilot. See Here.
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