Measuring ToolsMeasuring instrument
Outside Micrometer
A screw-driven spindle in a rigid frame with a friction thimble, measuring one dimension over a narrow range to a thousandth of a millimetre.
What it does
- Measuring a diameter where the tolerance matters
- Checking wire, sheet and small turned parts
- Confirming a caliper reading on a soft or thin part
What to look forthe judgement calls, before the spec sheet
- One frame covers a 25 mm range; a larger dimension needs either a larger frame or a different instrument. Buying a set covers a range that one micrometer cannot.
- The friction thimble or ratchet stop is what makes it repeatable. Without consistent measuring force, precision is meaningless.
- For threads, a thread micrometer with the correct anvil is the correct instrument: a plain micrometer measures across the crests, not across the pitch diameter that actually controls fit.
Misuse and safetythe ways this tool causes rework
- Check zero before every session and let the frame reach the same temperature as the work. A micrometer held in a warm hand measures a slightly smaller part than the same part measured cold.
- Store it with the measuring faces apart. Faces left closed trap moisture, and a rusted face is no longer a reference surface.
Projects that call for itand what it is doing on each one
All projectsFrequently asked questions
Micrometer or caliper for identification?
A caliper. Identification needs a reading good to about 0.1 mm, and the caliper's jaws reach over a thread crest more easily than a micrometer's flat faces. Use the micrometer once you know what the part is meant to be.
What is pitch diameter and why does it matter?
The diameter on the thread flank where the width of the thread equals the width of the gap. It is the dimension that decides whether two threads fit, and it is what thread gauges and tolerance classes are written against.