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The comm is probably in bad shape, if it's not more serious like open windings or shorted lands. I'm not an expert but on drills I have sanded the commutator. Inspect it and I think you'll find it is damaged (excessive sparking causes damage). You have to be careful not to damage the wires attached to the segments. I've used a woman's emory board because it's fine and flat. Particles removed can go between the landings and could be conductive. A toothbrush and/or air might help. Hold the emory against it and turn it. Routers run way more quickly than drills etc, not sure if this might be too crude of a fix for them but it worked several times for me on slower motors. You only need to clean where the brushes ride.
Years ago When I worked on electric tools... I used commutator stones purchased from Eurton Electric in California to resurface commutators. Quick and easy to remove the brush's then insert the brush stones gently while spinning the armature to resurface them. They were available in different grits and worked quite well. Most of the damage to commutators is caused by heat from using too light of a cord. The heat generated causes the tension springs on the brushes to anneal... thus loosing tension, which leads to less contact, which causes more heat. The heat generated will eventually destroy the mica insulators that seperate the commutators and will eventually short out the armature. Using a cord that has been over flexed (where the strands break) or too light of a cord, is the biggest enemy to an electric motor because it will no longer provide enough power to the motor... Kind of like starving a gas engine of oil.
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