The Excellon and Sieb & Meyer drill formats are designed to drive CNC drilling and routing machines. They are broadly
similar, differing only in minor details.

Each drill file requires a separate tool-file giving the diameter of the tool ( in some cases the tool-file is embedded in the header of the drill file).
Your drill file should always show the finished hole-size you require.

A drill file without embedded tool sizes looks like this:

M48
%
T01
X-001375Y-008500
X-002125Y-008750
T02
X-006625Y+018250
X-007875Y+019500

A drill file with embedded tool sizes:

M48
INCH
T01C00.020
T02C00.024
T03C00.035
%
M70
T01
X07292Y04884
X07292Y05071
X07380Y08123

Where:

  • INCH/METRIC defines the unit
  • T01 is the tool number
  • C indiates that the text numbers are the drill sizes :
    00.020 = drill size 0.020″ or 20 mil or 0.50 mm.

More information on the Excellon format is available on Wikipedia.

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