Our Commstones come in 6 grits for commutator or brass/copper slip rings: EC EX tra Coarse-36 Grit, C Coarse-46 Grit, M Medium-90 Grit, F Fine-120 Grit, P Polishing-220 Grit, EP EX tra Polish-320 GritWe also have 3 grits for cast iron and steel slip rings: SRC Coarse-36S Grit, SRM Medium-60S Grit, SRF Finish-90S GritCommutator stones are used for grinding commutators and slip rings into concentricity for maintenance and repair. When compared to turning tools commstones are easier to use, more time effective, and remove less copper. Commstones should be twice as long as the width of the longest flat spot and as long as will work freely between adjacent brush sets.