The trueness of a commutator is held to about what tolerance?

• A. .001 inch
• B. .002 inch
• C. .003 inch
• D. .004 inch

Answer: (B)

Trueness refers to how concentric and flat the face of the commutator runs as it turns. To keep brushes seating evenly and transferring current smoothly, the commutator face is held true to about two thousandths of an inch. This tight runout minimizes variation in brush contact as the rotor spins, which reduces sparking, uneven wear, and heat buildup, helping the machine run reliably.

If the runout were looser, like three or four thousandths, you’d see more uneven contact and faster wear of both brushes and bars. A much tighter standard, such as one thousandth, is possible but not typically necessary for most applications and would be more demanding to achieve.