Countersink Inside a tube

[Tuomas]

There are tools made to countersink the otherwise inaccessible back side of a hole but such tools would have been expensive overkill for your problem.

Your solution of constructing the needed tool inside the obstruction is an ingenious technique that should be in everyone's mental quiver of unusual solutions. Good on you.

At my workplace, we use tungsten carbide cutters and die grinder.
Like this:



Also ones that are shaped like car valve.
Those are cheap and easy solution.

[Toolmaker51]

What Marv comes closest to describing in commercial version are a notched shaft corresponding to hole diameter, and a cutter with a female opposite of the notch; basically a bayonet fitting. They are called back countersinks, in concert with back spotfacers and counterbores. Very common in certain castings, and lots of aircraft forgings. These are important when a fastener needs a flat seat close to a wall, impeded by a fillet or radius.

They run easily in a sturdy drill press or milling machine, and a quill stop makes depth control very simple.