To make the most of more upcoming good spray painting weather, decided to continue preparing parts for primer and top coating as necessary. Next up was the F-1203A Bulkhead which required quite a bit of attention. There are three side by side holes at the center bottom of the bulkhead that require being enlarged to 7/16” and two adjacent holes that required being enlarged to 3/4". Step drills are a great choice in this situation, so the drill press was outfitted accordingly and the drilling commenced.
The plans call for lots of machine countersinking on the forward side of the F-1203A bulkhead … a total of 46 rivet holes for AN426AD3 rivets to secure nutplates and 6 #30 holes along the top edge for 120 degree flush pop rivets. In the photo bellow, the areas that require work are outlined in red … the 46 #40 rivet holes are circled in red and the 6 #30 holes requiring 120 degree machine countersinking are marked in green.
The F-1203A bulkhead is made from .040 aluminum … this is near the upper suggested limit for dimpling. Thicker materials are usually machine countersunk but the decision has been made to dimple the bottom of the bulkhead for the flush rivets that are being used on the RV-12 project. The bottom flange of the bulkhead is not quite 90 degrees so knowing thicker material tends to flatten out a little when dimpled, an angle gage was used to measure the angle before dimpling so that angle can be reset after dimpling.
The dimpling process relaxed the bend of the flange only a few degrees so it was tapped back into place with a soft hammer. Next the flange angle was checked to verify it is now at the same angle as it was prior to dimpling.
Even though the autopilot will not be installed until much later, the RV-12 forum consensus is to install the autopilot mounting brackets now while building the fuselage as opposed to installing the brackets afterwards. The autopilot bag was opened and the brackets were removed, deburred and prepped for primer.
