The horizontal stabilizer forward spar needs four rivet holes in the spar web dimpled to take AN426D4-6 rivets, with corresponding countersinks machined into the reinforcing angles that sit on the forward side of the spar.
I marked the correct side of the spar web and angles to ensure I didn't carelessly dimple or c/s the wrong side (which would be bad...), and then used a machine countersink cage on the air drill to take material off the angles, taking a little off at a time on the first hole - checking depth with a rivet as I went until I'd got it just right.
The corresponding holes on the spar were dimpled with dimple dies so that the dimpled material sat nicely in the countersinks underneath.
Not a lot of work, but since it was the first time for real I took an hour to make absolutely sure that I didn't do something daft.
Spar and angles marked up with a spirit pen
3 of the 4 holes countersunk, with rivet in one for depth.
