why does brushes arc on Flymo motor?

There will always be some arcing on a commutator - the thing the brushes press against - sometimes it's possible to clean it up to make the interference between the pieces run more smoothly - when I do it I use very fine sandpaper and make the commutator shine - but I still get a few tiny sparks.

https://www.youtube.com/watch?v=MkBLwUG_AdU

https://www.youtube.com/watch?v=gc4l1eooPKM

What could be the cause?
It might be a dirty or damaged commutator.

Good Luck...
April 2016