Optional lightening and cover ribs^4h

Today I continued working on the central bulkhead.

The first thing I did was to do an optional cutout on the control column mounts. I did not think of a better procedure than to drill a line of holes, then use a 1/4" endmill to machine the straight line. Then I used a hacksaw to do the diagonal slits and a file to smooth the part. Finally, some finish deburring with a Scotch-Brite wheel.

This is what the process looked like.

And the final product.

Then I switched to cover support ribs, F-782B / F-783B. You are supposed to match drill them to the central section, but since all the holes in the central section bars are already final-sized, I did not want to put any cutting tool through them. Instead, I used a transfer punch to transfer the location of the holes. The punch was a bit too wobbly in the hole, so I wrapped it with painter's tape.

Lightly tapping the punch leaves a small notch in the part. It's almost invisible.

However, it is big enough for a center punch to center on it and make it big enough for the drill.

The procedure is accurate enough. After reaming with a #12 reamer (which I think is slightly smaller than the holes in the central section bars) the bolts fit snugly.

Then I drilled the holes for the bushings, and this is the final result of the day.

Then I was looking at the F-704 parts and I noticed that two open holes in the top part of the forward section are not symmetric. After looking at the plans and the images I found on the internet, I found that most likely the factory was supposed to install rivets in these holes but missed them. I'll reach Vans support next week to see what can I do here (I expect that AN3 bolts would be an option).