Modern solid carbide drills produce holes in a tolerance zone of H8-H10; in which case the hole axis is accurate in half this tolerance. For eg:- A dia 10 hole drilled hole is between 22-45 microns which means the axis is controlled within 11-22 microns.
Reamed holes are accurate having a tolerance of H6-H7 i.e. for dia 10 :-11 to 16 microns, so it is obvious that to produce such an accurate hole , the hole axis has to be more accurate than 11-22 microns which is possible with a modern solid carbide drill, hence for such accurate holes it is a preferred process to Hole-mill & then Ream.
A Hole-mill is normally an undersized reamer with a boring geometry i.e. the size of the hole-mill is normally 0.2-0.6mm more than the size of the drill so that there are no drill marks on the hole plus the hole axis is corrected for subsequent reaming operation.