actually, it's the closest 64th of an inch... hence the 2315 is World Archery (9.3mm) legal, the 2317 ISN'T, but the 2318 (the new one) IS.for aluminium, the first 2 digits of the 4 digit spine is the arrow diameter in 64ths of an inch
ie 1816: shaft diameter 18/64", tube wall thickness 16/1000"
I am not aware of a 'standard' table for shaft diameters for all materials. Apart from individual manufacturers producing their own for each arrow, I doubt there is one as each construction method varies. ie a thin walled large dia. pure carbon arrow can have the same stiffness as a thick walled skinny pure carbon arrow.Is there a table of standard shaft diameters for all materials -