Make sure the armature face is clean, the surface smooth and flat, and that the segment isolation material is relieved below the face of the copper by at least .010" After double checking the relief depth, I usually run the unit, brushes removed, and using a fine grit sand paper (1,000), and a thin smooth block of wood, lightly sand the armature face, to make sure there are no high spots. The assembly is then blown off with compressed air. I then wrap the armature with sandpaper starting with 300 grit, with the grit side away from the copper. The brushes are installed, and the sandpaper is run back and forth around the armature circumference, under each brush, until the brushes conform to the curvature of the armature. Next I use the finest grit paper I can find (1,000), grit side away from the copper, and repeat the procedure for a few seconds. The brushes are loosened in their holders, and the sand paper is removed. The brushes are then removed, one at a time, and the faces wiped down with a white cotton rag, soaked with electrical cleaner. When the brushes wipe clean, with no more residue getting on the cotton, then they are replaced loosely in their respective holders, and the entire assembly is blown off with compressed air. there must be no residue of carbon dust or abrasive left in the area. DO NOT USE EMERY ABRASIVES
Emery will embed in the brushes and the armature, and will cause excessive wear to both componants!
Now install the springs on the brushes to the proper pre=load. The same procedure applies to the AC slip rings as well. After sanding to diameter, clean up and setting the springs, start the unit and let it run, at speed, but with no load for several minutes. After 3 or 4 minutes, load moderately, and then increase load to maximum after about 10 minutes of operation. Watch the armature for arcing and segment jumping, that may indicate electrical issues. If all is well, you are good to go!