[rotorwrench]

The brushes have a hard time with the mica between the segments if the segments wear down to flush or lower. It's funny how the copper will wear faster than the mica. The mica should be undercut a bit on all armatures whether it be a generator or a motor or the brushes will start to bounce around on there and not make as good a contact as they should. They will wear faster the more rough the surface is. When you have a starter that uses copper based brushes is when you can get away without undercutting but those type generally have micarta between the segments so that it will wear along with the copper.

Electrical shops have a neat little tiny motor driven cutter that is part of a small armature lathe set up to just cut straight down between each segment but they can be cut by hand with a real thin coping saw blade or similar thin saw blade. They only need a small amount of mica removed.

The good growler sets usually have a test lamp set up built in to check the commutator segments for continuity and shorting as well as the inductor set up to check for armature winding shorts that cause magnetism in the core segments. Generally, a hack saw blade is used for best results.