Types of micrometer

Outside micrometers

The common micrometer measures shafts, pins and thicknesses between a fixed anvil and the spindle. Each one covers 25 mm (0–25, 25–50 mm and so on); ranges above zero are set on a setting standard. Variants with blade, disc or gear-tooth anvils reach grooves, soft materials and gear teeth (NPL Good Practice Guide 40).

Depth micrometers

A depth micrometer measures the depth of holes, slots and steps. Its base is a lapped reference face held on the top surface; the rod runs down to the bottom. The sleeve and thimble are read as on an outside micrometer, but the graduations run in the reverse direction. Interchangeable rods in 25 mm steps extend the range, typically to 150 mm and up to 300 mm (NPL GPG 40). The Chinese product standard is GB/T 1218-2018.

Jaw-type inside micrometers

An inside micrometer with two jaws (GB: 内测千分尺, JB/T 10006-2018) reads like an outside micrometer but measures bores and groove widths between its jaws. Open the jaws in the hole until the ratchet turns idly for two or three clicks, moving one jaw slightly round the bore to find the largest reading. Its zero is set with gauge blocks fitted with jaws, a setting ring, or by measuring across its jaws with an outside micrometer. Because its jaws are offset from the screw axis, too much force gives large errors: measure with the same force as when setting (NPL GPG 40).

Two-point (tubular) inside micrometers

A two-point inside micrometer (GB/T 8177-2024) has spherical measuring faces at both ends and extension rods for large bores. It has to be rocked across and along the bore until it no longer rocks, which takes skill and time; the warmth of the hands lengthens it and makes the reading small, so wear gloves and keep the measuring time short (NPL GPG 40).

Three-point bore micrometers

A three-point inside micrometer (GB/T 6314-2018) has three evenly spaced contacts that a tapered cone (or tapered threads) moves outwards. Three contacts align it with the hole axis, so it is easier to use than two-point instruments (NPL GPG 40). For a deep bore an extension rod goes between the measuring head and the micrometer head.

Which one to use

In KELIBRON

The lab has outside micrometers from 0–25 to 75–100 mm, a jaw-type inside micrometer for 5–30 mm (it reads the 28 mm bushing bore) and a three-point micrometer for 62–75 mm that reads in steps of 0.005 mm (50 thimble divisions, 0.25 mm of diameter per turn). It comes with a Ø62 setting ring: on the ring, press Zero (in a real instrument, the sleeve is turned with a spanner) so that it reads 62, then measure the Ø68 housing bore. A depth micrometer and a two-point inside micrometer are not in the lab yet.