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Micrometers, height gauge and depth gauge

Short answer

External micrometer has a 0.5 mm pitch screw and 50-division thimble (LC 0.01 mm). Internal micrometer measures bores with extension rods; depth micrometer measures depths; vernier height gauge does layout work; vernier depth gauge measures hole depths. Common errors: zero error, wear, non-parallelism, pitch error, variable force, temperature, parallax.

Describe the construction and working of an external micrometer. Briefly describe the internal micrometer, depth micrometer, vernier height gauge and vernier depth gauge with one application each. List the common errors of a micrometer.

External micrometer

It works on the principle of a screw and nut. A precision screw (spindle) of pitch 0.5 mm turns inside a fixed nut in the barrel (sleeve). A thimble attached to the spindle is divided into 50 parts, so one division moves the spindle 0.5/50=0.010.5/50 = 0.01 mm.

   anvil  spindle   sleeve  thimble  ratchet
  |==|   |=========|-----|=|||||||||=[R]
  |  |___|         |
  |__________frame__________|

Parts: U-shaped frame, fixed anvil, spindle, locking nut, sleeve with datum line and the main scale (0.5 mm marks), thimble, and a ratchet stop that gives constant measuring force (about 5 to 10 N) to avoid error due to excessive pressure.

Reading: sleeve reading + thimble division on datum line ×\times 0.01 mm. Used for outside diameters and thicknesses to 0.01 mm (vernier micrometers read 0.001 mm).

Other instruments

InstrumentPrinciple / use
Internal micrometerSame screw principle, with jaws or rods (extension rods) that touch the bore wall; measures hole diameters, e.g. cylinder bores
Depth micrometerA base (bridge) rests on the work face and a rod projects from the spindle; measures depth of slots, holes, steps
Vernier height gaugeA vernier caliper mounted vertically on a heavy base with a scriber; marks and measures heights on a surface plate, used in layout work
Vernier depth gaugeA beam with a vernier scale and a base; measures depth of holes and recesses with least count 0.02 mm

Common errors of a micrometer

  1. Zero error (anvil and spindle in contact do not read zero).
  2. Wear of anvil and spindle faces and of the screw thread; backlash.
  3. Non-parallelism or flatness error of measuring faces.
  4. Error in the pitch of the screw and thimble graduation.
  5. Variable measuring force if the ratchet is not used; deflection of the frame.
  6. Temperature effects, e.g. holding the frame in the hand, and parallax in reading.

✓ Answer checked by Claude Opus 5.5 (high effort)

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