There’s a great post over on This Old House by Norm Abrams covering the proper care of common power tools, including a great description of how to clean motor brushes. An excerpt:
“Most small power tools have two “brushes,” solid blocks of carbon graphite that conduct electricity to the motor’s spinning armature. Friction gradually wears these brushes away, and if they’re not replaced, the motor loses power and eventually quits. You can tell it’s time for new brushes when you see lots of arcing — small, harmelss sparks inside the motor housing as the tool is running.”
Easily accessible brushes are a prime feature on high-end power tools as the manufacturer expects the tool to see a long and productive life. If you’re not already familiar with this kind of maintenance, be sure and give this a read. It’s complete with drawings explaining the whole process.
TLC for Power Tools [This Old House]