Sanding a commutator out of the machine may dress it up. But as many have chimed in, a commutator stone is used while the motor is running. If a motor locks up after a clog and left buzzing, sometimes the armature will heat up and have two high spots 180 degrees apart where the carbons have heated the commutator and raised the individual segments. Armatures can also get out of round. If you can imagine, if the commutator is not on the same axis as the shaft and bearings. The carbons will move in and out when the motor runs and will cause the carbons to arc. Using a stone while the motor is running will true it to the axis of rotation. When the arc will not calm down, spin the motor by hand. The dark spots will be the high spots and the copper segments will be the low spots. Sometimes you have to put a lot of pressure on the stone to get it back in round. On convertibles and Eureka (sanitaires) I have used a die grinder to make a hole in the upper housing just below the cooling fan, and have stoned an armature that way. If a motor is left to arc for awhile, the carbon brush will get very hot. When carbons get hot the carbon material can expand and the brush can get stuck in the tube. You can use a jewelers file to clean the tube. You can also file the carbon, again because it swells, usually at the tip, so that it slides in the tube freely. Sometimes the spring on the brush gets so hot that it looses its tension, all of the coils of the spring remain compressed and will not spring back. When that happens, you have no choice but to replace the carbon brush. The only way I know of to check an armature for a short is to use a growler. You can test two ways with a growler, a direct short, and for spark between segments (dead segment), but that is a whole topic in itself. I don't want to come off as a know it all, but I have been in the repair side for over 30 years, from everything from convertibles to propane to autoscrubbers. Was just trying to give some insight to the general collector, not to experienced vac shops.