Okay, so I needed to drill the F-715 seat ribs for the bolts going through the F-704. The tricky part here is that holes in F-704 are somewhat valuable. Even though they are not "close tolerance" holes, they still are in the part that is kind of expensive.
And on the opposite side you have a sheet metal rib, which is not friends with regular drill bits. So you cannot really run a full-size drill bit (seems like the holes in F-704 are already #10 holes). The drill will grab and tear the hole in the sheet metal and in the process of doing so, it might also damage the F-704 with its side cutting edges.
It might be a less of an issue if used in an air drill as these turn fast, but I have a slow electric drill.
So, what would a reasonable person do?
Well, one advice I saw was to use brass tubes as bushings so you can drill a small hole first. Then you can upsize the hole using unibit or carefully with the twist drill bit.
Another option is to use a transfer punch to locate the hole on the rib, remove the F-704 then drill.
There are probably some other reasonable choices as well.
What did I do? I made my own tool, of course! Another D-bit. Possibly, less controversial than the one I used for the wing spars.
Same idea, the cutting edge is at the tapered part, but with taper going all the way to a sharp tip (so it can "drill"). This time I did harden it.
Of course I broke the tip on the second hole, which I kind of expected, but since the tip is not an important part of the tool (it is only used to punch the hole, the most important cutting is done by the wide taper), it still worked fine. I probably should have made a small flat "spade" end from the get-go. I smoothed and sharpened the new "cutting edge" to help it go through the aluminum.
The tool (with the broken tip).
The hole after some deburring. The tool leaves a lot of burrs on exit (I think, it's because it has a neutral rake angle / no relief angle, so it pushes the chips rather than cuts them), but the hole looks good (technically, this tool is also a reamer, cutting very round holes).
![IMG_6791.jpeg IMG_6791.jpeg](https://vansairforce.net/data/attachments/21/21666-66b4fb908ebc0a5d3a0bd03aa717c016.jpg)