As most builders have found, or will find, supplied tube and rod of whatever material (carbon, aluminium or steel), come in a nominal size only. There is always, or mostly, a size discrepancy or variation from the specified size.
I now make my own receiver tubes to suit any supplied rods or joiner tubes, for this very reason.
However, there comes a time when a hole needs to be drilled in a bellcrank (for example), to fit a 3 or 4 mm factory made rod. Without a full set of numbered drills sitting in the shed on hand, it can be frustrating to drill/file/ream a hole out that will be a good fit with near zero slop.
One way around this, is to reduce the size of an off the shelf, hardware store twist drill. I use a disc sander as per the video. For final polishing, a grind stone works well too.

Reducing twist drill diameter (0 min 28 sec)