Armature Maintenance (for really big motors) so as to prevent the conditions you describe, include "stoning and undercut". This trues the armature, and removes sufficient material between the windings such that carbon dust is dispersed. Otherwise, it can accumulate and cause all sorts of problems. And if you have a raised edge the brushes will chatter - and that is a serious sparking cause. So, attached is a YouTube for smaller motors that accomplishes what you may need.

https://www.youtube.com/watch?v=oB-h-vZc6FY

Peter Wieck
Melrose Park, PA