Yes, Manticore is correct. A worn comm will not actually be round. It may look to be round but in actual fact the comm between the different copper segments would have worn down further than the rest of the comm. If you sand the comm all you do is clean up the burnt marks but the comm is still not round. The important part of what the lathe does is make your comm round again so the brushes don't bounce as the motor spins. Bouncing leads to sparks which causes much quicker burning of the comm!
A good secondhand lathe should not set you back too much and in the long run it will work out cheaper!