Give yourself a little more credit, man! :-p
When I was doing a lot of drum building and shell repairs, I often had
to correct shells on which the two bearing edges weren't parallel.
The bearing edges are the open ends of the drum shell (wood cylinder) on
which the drums heads sit. If they aren't parallel the drum can end up
producing some pretty ugly overtones. Add to that, the fact that the
bearing edges must be perpendicular to the sides of the shell. We call
it having "square edges." This must be achieved on repairs, newly built
shells, or shells that are being cut down for custom work.
Imagine trying to figure out a tool, jig, any kind of setup that will
produce that on a cylinder 10" to 24" wide by 5" to 20" deep. I'm not
sure there's a planer or surface sander that would handle it. :-)
After many experiments, I ended up doing the most simple thing. I
adhered sandpaper to my flat table, which was a piece of granite large
enough to hold a 22" bass drum shell.
Then I would set the shell down with the bearing edge sitting flat
against the sandpaper. I would just rotate the shell in an arc, either
spinning in a circle or back and forth, depending on what I needed to
correct.
I would check for square using a set-up table that had a registration
stick which was square to the table surface and had another sliding
piece on it to set as a height gauge. With the shell set on the table,
against the square, I could rotate the drum to see if the shell stayed
square against the stick. If there was light between them, at any
point, that told be where I needed to sand the bearing edge down to
correct it. Same thing with the height marker.
The most efficient way I found to do this was by hand on the sanding
table. I would just put a bit more pressure on my hands on the areas
that needed sanded down. I got very quick with this technique and I
produced dead-nuts-on results doing it.
Bigger drums shops use motorized sanding tables, but they are flipped up
90degrees, so you can hold the shell against it while standing next to
it. Better dust collection that way, too.