By default Eagle generates the NC drill files with 2.3 file format (i.e. 3 digits after
the decimal point) in some cases if your board is designed in Imperial units (while default
Eagle database units are metric) and the board is high density there is small error due to the
rounding when the drill XY coordinates are generated. This leads to small displacement of the
holes inside the pad area i.e. your holes are not in the center of your pads. You can increase
the NC drill precision by following procedure:
Go in your EAGLE BIN folder and make copy of EAGLE.DEF file, then open it and change [EXCELLON]
section as follows:
Type = DrillStation
Long = "Excellon drill station"
Init = "%%\n"
Reset = "M30\n"
ResX = 100000
ResY = 100000
;Rack = ""
Select = "%s\n" ; (Drill code)
Drill = "X%6.0fY%6.0f\n" ; (x, y)
Info = "Drill File Info:\n"\
" Data Mode : Absolute\n"\
" Units : 1/1000 Inch\n"\
" End Of Block : CR/LF\n"\