Hitachi C10FC sparking commutator
 

Inherited this Hitachi C10FC dropsaw, and noticed excessive sparking
around the commutator. Pulled out the brushes and noticed they were
worn down to nothing. Replaced them with new brushes , used saw for a
short time and noticed sparking!. Inspected new brushes and noticed
serious erosion on one side of the brushes. What could be happening
here ?, Note that there are two windings on the field and the finer one
is open circuit - is this a braking winding, and could its failure
affect the normal running of the motor ?. Any ideas appreciated.
Cheers
Mike