Which tool would provide the most accurate measurement of the diameter of a shaft?

The outside micrometer is the most suitable tool for accurately measuring the diameter of a shaft because it is specifically designed for precise measurements of small dimensions. This tool employs a calibrated screw mechanism, which allows for very fine adjustments and readings, typically to a precision of 0.001 inches (or 0.01 mm).

When measuring the diameter of shafts, micrometers have a much narrower measuring face compared to other tools, ensuring that the measurement acccurately reflects the diameter rather than any surface irregularities. The tool is often equipped with a thimble and ratchet for consistent application of pressure, which also enhances measurement accuracy and repeatability.

In contrast, while calipers and dial indicators can provide good measurements, they generally lack the same level of precision that an outside micrometer offers. A steel ruler, though useful for more general measurements, does not provide the fine resolution needed for precise work like the diameter of a shaft. Thus, for tasks requiring high accuracy, the outside micrometer is the ideal choice.