Universal motor (spindle) brush wear

This may be OT. Please reply to me directly so as to not clutter the
list.

I am using a Porter Cable laminate trimmer as a CNC router spindle.
I have seen this and similar tools often used in CNC routers. AFAIK,
all such tools use universal motors, and are subject to brush wear.

My questions is: How long should I expect the universal motor
brushes to last? What sort of PM cycle is necessary to prevent
running out of brushes and damaging the motor?

I have about 50-100 hours of operation so far, mostly cutting
hardwood, and about 10 hours cutting aluminum.

If noone has guidelines, I'll make the measurements and report my
experience.

-- Carl
cmm@...
http://www.foxkid.net