All performed by CNC machines. The machines can
either be programmed optically or directly from the CNC outputs from the
modern cad systems. Or if necessary CNC Data can often be extracted from
the Gerber photoplot Data files. If both plated and unplated
holes are required say for mounting holes or similar please advise separately
from the data files as this information isn't always carried through from
the CAD systems automatically.
Our favorite format for the data is
imperial Excellon 2-3 format but we are quite used to most variants and
have written software to convert other file formats.
The drills we stock are all metric and start off
with 0.05mm size increments which raise to 0.1mm increments for the larger
sizes. All imperial sizes from the CAD systems are converted to the
nearest larger size. To achieve the correct final size for PTH or Multi
Layer Boards we by default increment drill sizes by 0.1mm.
Small point to note is that holes tend to be a
little undersize. This is due mainly to the heat generated during
drilling. This causes a small amount of hole shrinkage. Another factor is
that drills are often made to a minus tolerance. The net result is that a
1.00mm hole could well measure 0.98mm. Sometimes it can be demonstrated in
extreme cases by the fact that a drill will not easily fit back into the
hole it has just drilled.
Full documentation of the Std Excellon machine
language can be found at
http://www.excellon.com/applicationengineering/manuals/program.htm
