Taunton, MA · Used Gages 508-343-1219
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Buying guide · Micrometers

Buying Used Micrometers: The Bench Check, Optical Flats and When Re-Cal Isn't Worth It

Used outside micrometers from Mitutoyo, Starrett, Brown & Sharpe and the rest turn up everywhere: shop cleanouts, auctions, eBay, a retiring machinist's box. Many are fine for years more. Some have a worn screw, rolled faces or a sprung frame that a clean case hides. This is the bench check to run before you pay, plus how to tell when a calibration would cost more than the mic is worth.

Ask about a mic: 508-343-1219 Open the full gage checklist

1. What to bring

  • A lint-free wipe and a little solvent to clean the measuring faces.
  • A few gage blocks you trust (see the gage block guide), chosen so the readings land at different spots on the thimble, for example .105, .210, .315, .420 and .500 in on a 0-1 in mic.
  • An optical flat if you have one, and a bright, even light.
  • A loupe or phone macro lens for the faces and graduations.
  • For a listing you can't hold: ask for the photos in section 6 before you make an offer.

2. Look it over

  • Faces: anvil and spindle faces clean, with no chips on carbide faces and no rust pits or bright rolled edges on steel ones.
  • Frame: no cracks, deep dings or signs of being dropped or used as a clamp. Heat insulators (the plastic grips on the frame) present on larger mics.
  • Graduations: sleeve and thimble lines readable, vernier lines (on .0001 in mics) not worn off, no burrs on the thimble edge.
  • Markings: maker, range and resolution legible, and the serial matches any cert or case.

3. Spindle, ratchet and lock

  • Run the spindle over the full range. It should turn smoothly with no tight spots, grit or side play when you wiggle the thimble.
  • Close on a block with the ratchet or friction thimble several times. It should slip the same way every time. A ratchet that skips or grabs gives readings that wander.
  • Set the lock and check that the reading doesn't move when it clamps, and that the spindle frees up again when you release it.
  • A spindle that is loose at one end of its travel and tight at the other usually means a worn screw or nut. That's a repair, not a cleaning.

4. Zero and readings across the range

  • Clean the faces, close them gently with the ratchet and check zero. On mics over 1 in (25 mm) use the setting standard, and handle it by its insulator so your hand doesn't warm it.
  • A zero that's a little off can usually be adjusted with the spanner wrench. A zero that changes each time you close is a bigger problem.
  • Measure your blocks at several points across the range, not only at whole turns. A screw can be fine at .500 and off at .315.
  • Repeat each reading a few times. Good mics repeat; a spread between readings points to the ratchet, the spindle or dirty faces.
  • A mic missing its setting standard can still be good, but the standard is a price point and you'll need one to set it.

5. Anvil flatness with an optical flat

An optical flat is a polished glass disc. Wring it lightly onto the anvil (then the spindle face) under a good light and you'll see colored or dark bands. Each band is roughly half a wavelength of the light, about 0.3 micrometers or a bit over 10 millionths of an inch.

  • Few, straight, evenly spaced bands mean a flat face.
  • Many curved bands or tight rings near the edge mean a rolled or worn face.
  • Checking parallelism between the two faces takes optical parallels or blocks at several spindle positions; a calibration lab will do it if you need it documented.
No optical flat? Clean the faces, close them on a thin strip of clean paper and pull it out. Drag that feels the same at the center and edges is a rough sign the faces are parallel. It's not a measurement, just a screen.

6. Sets, digital mics and the photos to ask for

  • Sets (0-6 in, 0-12 in and so on): every frame and every setting standard present and from the same maker. A mixed set sold as complete is worth less than a matched one.
  • Digital mics: it powers up, the display is clean, origin holds after you switch it off and on, and the data output works if you need it. It still has to pass every mechanical check above.
  • Buy big-brand digital mics from a source you can check. Look-alike knockoffs of well-known brands are sold online.
  • Photos to ask for: the closed mic showing the zero reading, both measuring faces close up, the whole frame, the sleeve and thimble markings, the case and any setting standard or cert.

7. Re-cal, repair or walk away

  • If the mic will be used to accept parts under a quality system, plan to have it calibrated before you rely on it, and get that quote before you agree on a price.
  • For a common small mic, a calibration plus any repair can cost a large share of what a good used one sells for. A cheap mic that needs work is rarely a bargain.
  • Larger mics, sets and specialty mics (blade, disc, point, tube) are where a calibration usually pays for itself.
  • Old certs are history. A cert shows the mic was in tolerance on that date, not today.

Walk away from: chipped or pitted faces, a spindle that binds or rocks, a zero that won't repeat, a sprung or cracked frame, or a price based on the brand with no way to show it still measures.

Used micrometer questions

Is it worth buying used micrometers?

Often, yes. A well-kept name-brand outside mic can work for decades. The ones to pass on have chipped or worn faces, a spindle that binds or wobbles, a sprung frame, or readings that drift across the range. Treat any old cert as history and check it yourself.

How do I check a used micrometer quickly?

Clean the faces, close them with the ratchet or friction thimble and check zero (use the setting standard on mics over 1 inch or 25 mm). Run the spindle through the whole range to feel for tight spots or play, check that the lock holds without moving the reading, and measure a few gage blocks at different points in the range.

How do you check micrometer anvil flatness?

With an optical flat under good light. Wring the flat lightly onto the anvil or spindle face and count the interference bands. Fewer, straighter bands mean a flatter face. Each band is roughly half a wavelength of light, about 0.3 micrometers or a bit over 10 millionths of an inch. Curved bands or many rings mean a worn or rolled face.

Why check a micrometer at several points and not only at whole turns?

A micrometer screw can be off in one part of its travel and fine in another. Blocks that land at different positions on the thimble, not just at .100, .200 and so on, show errors that a whole-turn check would miss.

Should I buy a used digital micrometer?

It can be fine if the display is clean, it powers up and holds origin, the data port (if you need one) works and it passes the same mechanical checks as a vernier mic. Buy big-brand digital mics from a source you can check, because look-alike knockoffs are sold online.

When is recalibrating a used micrometer not worth it?

When the faces are chipped or badly worn, the frame is sprung, or the spindle binds. Get a calibration quote before you buy. For a common small mic, a calibration and any repair can cost a large share of what a good used one costs, so a cheap bad mic is rarely a bargain.

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