External, internal and depth micrometers

The micrometer is one of the most precise instruments in workshop metrology, able to give dimensional measurements with resolutions down to 0.001 mm thanks to the micrometer screw principle. It is available in numerous variants dedicated to measuring external dimensions (lengths, diameters), internal ones (bores and internal diameters), depths, threads and special applications (tubes, discs, gear teeth). Its robustness, ease of use and great reliability make it the instrument of choice for high-precision dimensional inspection on turned, ground and precision-machined parts.

Operating principle

The micrometer is based on a precision screw which, turned through one complete revolution, advances axially by a constant pitch (0.5 mm on standard metric models). Rotation of the thimble, divided into 50 graduations, allows the fraction of a pitch to be read with a resolution of 0.01 mm; an additional vernier or a digital display brings the resolution to 0.001 mm. A ratchet (or friction drive) ensures a constant, repeatable measuring force, reducing dependence on the operator’s touch.

Types

External micrometers

The most widespread measure lengths and diameters with a range of 0-25 mm, and upwards through 25-50 and 50-75 to over 1000 mm in a U-frame. The measuring faces are of carbide for durability; there are versions with knife-edge anvils for grooves, ball anvils for curved walls, discs for sheet and laminates, and models for threads with V-thread inserts.

Internal micrometers

Available in three-point versions (self-centering, for bores from 6 mm upwards) and two-jaw rod versions, they measure internal diameter with better accuracy than a vernier caliper. The three-point series are particularly suited to ground through-bores.

Depth micrometers

Consisting of a base and a micrometer screw with interchangeable rods, they measure grooves, blind holes and steps down to depths of 300 mm by changing the rod.

Metrological characteristics

The typical accuracy of a 0-25 mm analog micrometer is ±0.002-0.003 mm in accordance with ISO 3611 and DIN 863; digital models add dynamic zeroing, memory, USB/Bluetooth data output and a final resolution of 0.001 mm. Periodic calibration uses reference gauge blocks and setting bars, following ISO 17025 catalogues. The measuring force is standardized and must stay within the prescribed values, to avoid elastic deformation of the workpiece.

Applications and industries

Micrometers are widely used in the precision machine shop, in the construction of molds and tooling, in precision turning, grinding and milling, in the dimensional quality control of technical components, in the manufacture of shafts, gears, pistons and piston rings, and in the automotive, aerospace, railway, hydraulics, and valve and actuator sectors.

Standards and selection criteria

The reference standards are ISO 3611 (external micrometers), DIN 863-1, ISO 9493 (three-point internal) and ISO 8512 (depth). The choice must take into account the measuring range, the resolution required, the geometry of the measuring faces, the presence of data output for integration with acquisition and SPC software, and the environmental conditions. Digital models with a Bluetooth interface are ideal for Industry 4.0 environments; mechanical models remain irreplaceable in harsh ones.