Micrometer

Micrometers are precision instruments designed for measurements requiring higher accuracy and resolution than calipers can provide. The measurement is made by the precise rotation of a screw thread, where each revolution advances the spindle by the thread pitch (typically 0.5 mm or 0.025 in). Outside micrometers measure external dimensions, inside micrometers measure bore diameters, and depth micrometers measure hole depths or step features. Digital micrometers add an electronic encoder for easy reading and data output.

Calibration of micrometers involves verifying accuracy at multiple points using gage blocks, checking zero setting, evaluating spindle flatness and parallelism (for outside micrometers), verifying the ratchet or friction thimble mechanism, and confirming the lock clamp does not shift the reading. Temperature control is especially important for micrometer calibration because the screw mechanism and anvils are subject to thermal expansion. Calibration is typically performed at 20°C (68°F) reference temperature.

In calibration management, micrometers demand careful handling and storage. The precision screw mechanism can be damaged by dropping, over-tightening, or contamination. Micrometers should be stored with the spindle slightly open (never clamped) to prevent thermal-expansion-induced stress on the frame. Calibration intervals are typically 6 to 12 months, with more frequent intervals for micrometers used in harsh environments or for critical measurements. Proper use of the ratchet stop or friction thimble to ensure consistent measuring force is essential for reliable results.