The shaft that goes through the field windings on the inside of the case is the armature. It has a set of windings that rotate inside the field windings. On the back end of it is a commutator. That is where the brushes ride and make contact between the windings. It has several copper strips separated by a softer insulation. It must be smooth and round or the brushes will not contact properly. If it has scratches, lines ,or is out of round it can probably be turned down slightly. The insulated slots can be deepened with a scraper or piece of hacksaw blade.

Check the brushes for smoothness and proper length. They wear out. The brushes need to move easily in their holder. The spring tension keeps them in contact yet allows them to move a little. If the commutator is too much out of round it will throw the brushes out too far to work. Clean all of the wire connections.

This is an easy rebuild. Do the best you can but it doesn't have to be perfect. Get all the dirt and grease out. These old things are pretty forgiving, that's why we love them.

"I wonder if God created man because he was disappointed in the monkey?" Mark Twain