Micrometer

A micrometer (also called a screw gauge) is a precision measuring instrument that uses a finely threaded screw to measure distances with extreme accuracy — typically to 0.01 mm or 0.001 mm. It is more precise than a caliper but can only measure outside dimensions.

How it works

The micrometer has an anvil (fixed) and a spindle (movable) that move toward each other as you turn the thimble. The spindle is attached to a screw with a known pitch — usually 0.5 mm per revolution. As you rotate the thimble, the spindle advances by exactly that distance per full turn. The scale on the sleeve shows whole millimetres, and the thimble scale subdivides each millimetre into 50 divisions, giving a reading resolution of 0.01 mm.

Types

• Outside micrometer: Measures outside dimensions — diameter of a wire, thickness of a sheet. The most common type.
• Inside micrometer: Measures inside dimensions — bore diameter, slot width. Uses extension rods or a different jaw configuration.
• Depth micrometer: Measures the depth of slots, recesses, and counterbores.
• Digital micrometer: Swaps the analog scale for an electronic display. Faster to read but requires batteries.

When to use a micrometer vs. a caliper

A caliper is versatile — it measures outside, inside, and depth dimensions — but typically reads to 0.02 mm. A micrometer is less versatile (outside dimensions only on standard models) but reads to 0.01 mm or better. For precision machining, bearing clearances, and wire diameters, the micrometer is the tool of choice. For general workshop measurements, the caliper is usually sufficient.