Millwright Level 1 Practice Exam 2025 – All-in-One Guide to Master Your Certification!

The datum line on a micrometer is found on the sleeve, which is the stationary part of the micrometer. This line is crucial for ensuring accurate measurements, as it serves as a reference point to help align the measurement scale and the thimble scale. When taking a measurement, the thimble rotates to move the spindle toward the anvil, and the position of the thimble is read against the datum line on the sleeve to determine the precise measurement.

The other components of the micrometer serve different functions. The barrel, generally, does not have measurement indicators like the sleeve does. The thimble is the part you turn to advance the measuring spindle but does not contain the reference scale for direct readings. The anvil is the part that provides a fixed surface for the object being measured, but it does not serve as a reference for the measurement scale. Thus, the sleeve is the correct answer since it houses the datum line that is essential for accurate measurement readings.