Unfortunately, I do not have a 'before' picture, but I have taken shots of the repair process.
The first picture here is the final result.
Kind of a tease, no?
Step 1: Rebuiding A DC Motor Commutator: Clean up the Commutator Shaft
I secured the output shaft end in the lathe-chuck, and to ensure true turning, I places the commutator end of the shaft in a bushing held in the tail-stock chuck.
It took some very careful cutting, but I was able to remove very little of the insulating core material, which appeared to be phenolic.
Note that the 'lugs' on the left of the phenolic are the wiring attachment points, and the 'ring of dots' on the right are the sad remains of the commutator sectors. In most commutators, the sectors are molded into the core, with tabs or under-cuts. The 'beads' on the right are well embedded into the phenolic core.
The 'lugs' are beveled to help with soldering adhesion of the new sectors.
BTW, the phenolic diameter is just under 1/2"! (1.2 cm)
Step 2: Rebuiding A DC Motor Commutator: The new Commutator Sectors!
The copper ring was selected to have a very close fit (almost press-fit!) to the phenolic core.
The ring was cut from a copper-pipe reducer, intended to receive a 1/2" nominal Cu pipe.
Note the bevel inside of the copper ring. It is a soldering thing!
Step 4: Rebuiding A DC Motor Commutator: Prepation for Soldering....
Note on the solder!
Lead free, high-melting point stuff.
Stronger than tin-lead, by a bit.
After tinning, I used solder-wick to remove excess solder, as too much would not allow parts to fit properly.
Step 5: Rebuiding A DC Motor Commutator: Soldering!
Use -lots- of non-corrosive paste flux! This will help prevent solder bridging at the lugs.
Step 6: Rebuiding A DC Motor Commutator: Truing up the ring...
Then a bit of Scotch-Brite red for a nice finish.
Also turned down the right end of the ring, to expose just the edge of the 'beads'. See, told you they were soldered!
Step 7: Rebuiding A DC Motor Commutator: Slotting.
Ended up using Xacto-style razor-saw. Slow, tedious, but accurate cuts.
After cutting, drizzled in some cyanoacrylate cement, which wicked under the sectors.
Solder should hold them, gluing can't hurt.
Back on the lathe to de-burr the sector edges, and job is done!
Big NOTE: This motor is being used as a light-duty servo. Any high RPM or heavy current applications should use hard silver-solder or brazing instead of electrical solder. The phenolic core can tolerate brief high-heat.