DC Motor reversing

Even with unbiased (radially aligned and positioned) brush placement,
some brushes can have escessive looseness in their guides, resulting
in brush tilt. This can expose a sharp brush edge to commutator
slots, and possible chipping or worse of the edge. Pieces of brush
material floating around inside a motor can cause things to melt or
burn.

While not so much of a problem on a mill or lathe axis (where motion
is split evenly between forward and reverse directions), it can still
cause problems. I've seen a Sherline DC motor suffer a major arcing
event during a (rare) deliberate reversal...

Also, I've had a treadmill motor on my Emco CNC5 lathe for some
time, now - because it isn't working terribly hard, oxide / dust
buildup can fool the DC speed control. (Were it working harder,
I believe the problem would be lessened, as the dust would be
vaporized...). My solution - bang the motor with a screwdriver
handle or similar, to let the brushes get a new "electrical grip"...

/mark