When to use 'Support' option?

[curious aardvark]

if you wanted to make that completely without supports.

print the shaft and bottom cylinder as one piece.
then have a hole through the second cylinder so that it will fit over the shaft.
And nhave the same size hole partially through the top cylinder.
And just press and glue the three parts together :-)

you'll also find that $fn=50;
is usually round enough, the printer and slicer will round it off even more.

I always used to use fn=100
these days I rarely go higher than 75 and normally nearer 50. And It makes no obvious difference.

uhu all purpose glue is the best thing I've found for pla. Pretty sure whatever solvent it has does a partial dissolve on the pla.
I do know when i've deliberately stuck a couple pieces together and then broken them - they never break at the glue line.

have you bought a set of micro files yet ?
I prefer to print holes tight and then file to the fit I want.

Put it this way - if you make the hole too large, it can't be reduced. But if you make it too small - you can easily enlarge it.