After the fact information is one step above useless. But I will throw out some thoughts so "next time" might not cause so much guessing. The DuMore motor is a series-universal motor, so mentioned above. The same as a sewing machine motor. Among other things, speed is determined by the number of functional windings on the armature. The number of commutator segments will generally give an idea of a high speed, medium speed, etc. Not a specific speed, there are many other factors involved.
In the first place, sand paper should not be applied to the commutator. Period. There are commutator dressing "stones" that are a rubberized abrasive. But on small motors are difficult to properly address the surface. They may be ground to a slim profile to make dressing a little easier.
You mentioned a "black stain" on the copper. That in itself is a sign of a loose connection. The black stain is actually a very dark brown, the sign of high temperature. Each commutator segment is attached to a part of the windings. There are several different winding types, also a connection to speed. In all probability, your sanding had a great deal to do with the loose "whiskers" becoming loose.
If you are competent with a small soldering iron, they can (theoretically) be reconnected. That sort of work is a young eyes job. But it can be done. A small torch works as well. But it must be a very small flame, far smaller than most acetylene flames. If the torch is larger than your thumb, it's too big.
The commutator segments are "undercut" some small amount. The dividing spacers are usually mica, considerably harder than the brushes. Undercutting is a tedious job, requiring meticulous workmanship. A hacksaw blade ground to a hook shape is about the best tool there is. There are tools specifically for the job, but most repairmen use a hacksaw blade. A whisker bridging the gap is deadly, any such must be removed. The copper segments are tapered at the edges, that is part of the undercutting process.
Brushes are a carbon compound. They can be filed, although sanding is far smoother. They are dressed to match the curvature of the commutator. For small motors, a generally "almost" fit is sufficient. A large rat tail file with sandpaper wrapped around it will form the curve, which should line up with the commutator.
The brushes should move easily, but not sloppy, in the brush holders. Dressing the sides with sandpaper is usually sufficient. Dressing a larger brush down to size is easily accomplished, albeit rather messy for the carbon dust. Most brushes have "pigtails" attached for more direct electrical connection. The spring usually is for tension only. There are exceptions, of course, but usually a sign of poor quality.
If you do intend to replace the armature as a single piece, and exchange is not required, it will provide an opportunity to learn a little about armatures. There is a test instrument called a "growler" that is used to test armatures. I saw one on eBay a few days back, but the seller was right proud of it. All that is required is a hacksaw blade and sufficient experiencs to understand the results. Oh, by the way, replace the bearings as you replace the rotor.
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