Originally Posted by rapid_ascent
You need to have a backup piece of some sort because the pilot of the countersink bit needs to be supported in some way.
I built this jig for the center 9 + 9 holes that you have to countersink into the flanges and caps of the HS front spar on a -10.
This worked really well to support the countersink cage along the edge. There's a spar cap there so elongating the hole wasn't a concern, but seems this approach could make countersinking flanges in other areas a little less hit or miss and would provide a pilot behind thiner material too.
Definitely some extra time clecoing from hole to hole but it worked. Eventually only used 2 clecos for each set up and it stayed put.
(Ignore the extra holes. They don't do anything. This was scrap from a botched doubler