Before finishing the countersinks on the first Spar, and looking forward to the number of them to go, I order from Cleveland Tools a single fluted 3/8" #40, #8 and #6 cutters. Below the two are side by side, note the single fluted cutter edge verses the 3 fluted cutter. After using each, I prefer the single cutter as it makes for a much smoother countersink.
With my new #8 single fluted cutter, the countersinks along the Spar flange look very smooth and the cutter when it bites, really takes off, much different than the three fluted cutter.
There are four holes in three different areas along the bottom flange that gets #6 countersinks that will hold the three access plates down. Again the single cutter and the temporary alumimum angle was used and all holes turned out very good.
The picture below shows the how close the finished countersink holes are to the edge. The manual tells you to use #30 cutter to countersink the holes but I believe that by using the 3/8" single fluted cutter that is made for the #6 screw, the edge distance works out better.
As with any hole drilled in alumimum, each one will need to be deburred. The thickness of the #8 and #6 holes are thinner and care is needed when deburring them as not not to enlarge the holes anymore.
All countersink holes will need to be spot primed. I took my rattle can 7220 primer sparyed some in a small containier and useing a Q-Tip dabed each hole with the primer. Next step will be fastening the Nut Plate to the flanges.
Since the holes for the 6 access plate were countersank, I went ahead and preped the 6 access corver plates. Be sure that the four holes along the top are drilled and dimpled for #6 screws, while the remaining holes are dimpled for #8 screws.
Total time this session, 10 hours.
Build Meter: 305 hours.