NC Drill Files

Excellon is a manufacturer of CNC control systems for both drillers and routers. This company has been in the business of CNC control for many years. The Excellon Company can be best described as one of the pioneers Printed Circuit board drillers and routers.

The term "Excellon Drill File" is a bit of a misnomer, as the company's name has been attached to what should be simply described as an NC (numeric control) drill file. The most popular format of this data is Excellon's and is described in the following section. Although the term "Excellon" has been coined into the CAD environment don't let it confuse the issue of NC drill data.

There are many manufactures of drilling and routing equipment today. Most are fully compatible with Excellon's control codes. Some PCB vendors still require an older format known as EIA-Binary because their drill systems use an older paper tape reader system. Modern drilling equipment now uses the preferred ASCII drill formats and a PC or similar control computer interpret the ASCII files.


Maximum drill block length is 100 characters, English (inch) or Metric (mm), Leading or Trailing Zeros, Incremental or Absolute modes are supported. Maximum m.n format supported is 2.4
NC Drilling File and Gerber file Similarities

An NC drilling file is very similar in nature to that of the Gerber file. The main differences are the absence of control codes in the NC drill file. The drill assumes that each X/Y pair is a hole location and the drill will plunge at each X/Y coordinate that is listed in the file. The NC drill file contains delineators that identify groups of X/Y coordinates to be drilled with a specific Tool Size.

The delineator is "T"+#, the ‘#’ is cross referenced to a customer supplied list. There is no sorting necessary or specific sequence required when identifying the T code.

T1 for instance could be a .2550" while T2 could be a .0200" etc. There is provision for header comments in the drill file as well.

Many CAD systems such as Tango, Orcad, and Protel for Windows place the T code sizes in the header of the NC drill file just before the Start of Data marker "%".